Chandra X-ray Observatory
Chandra, previously known as the AXAF (Advanced X-ray Astrophysics Facility), is a Flagship-class space observatory of NASA. In 1976, the mission was proposed to NASA by Riccardo Giacconi and Harvey Tananbaum. Preliminary work began the following year at MSFC (Marshall Space Flight Center), Huntsville, AL and SAO (Smithsonian Astrophysical Observatory), Cambridge, MA. 1)
Prior to launch in 1999, NASA's AXAF spacecraft mission was renamed to 'Chandra X-ray Observatory' in honor of the late Indian-American Nobel laureate, Subrahmanyan Chandrasekhar. Chandrasekhar, known to the world as Chandra, which means ``moon" or ``luminous" in Sanskrit, was a popular entry in a recent NASA contest to name the spacecraft. The contest drew more than six thousand entries from fifty states and sixty-one countries. The co-winners were a tenth grade student in Laclede, Idaho, and a high school teacher in Camarillo, CA. ”Chandrasekhar made fundamental contributions to the theory of black holes and other phenomena that the Chandra X-ray Observatory will study. His life and work exemplify the excellence that we can hope to achieve with this great observatory," said NASA Administrator Dan Goldin. Widely regarded as one of the foremost astrophysicists of the 20th century, Chandrasekhar won the Nobel Prize in 1983 for his theoretical studies of physical processes important to the structure and evolution of stars. He and his wife immigrated from India to the U.S. in 1935. Chandrasekhar served on the faculty of the University of Chicago until his death in 1995.
The CXO (Chandra X-ray Observatory) is part of NASA's fleet of "Great Observatories" along with the Hubble Space Telescope, the Spitzer Space Telescope and the now deorbited Compton Gamma Ray Observatory. Chandra allows scientists from around the world to obtain X-ray images of exotic environments to help understand the structure and evolution of the universe. The CXO, which was launched by Space Shuttle Columbia in 1999, can better define the hot, turbulent regions of space. This increased clarity can help scientists answer fundamental questions about the origin, evolution, and destiny of the universe. 2)
NASA's Chandra X-ray Observatory is a telescope specially designed to detect X-ray emission from very hot regions of the Universe such as exploded stars, clusters of galaxies, and matter around black holes. Because X-rays are absorbed by Earth's atmosphere, Chandra must orbit above it, up to an altitude of 139,000 km (86,500 mi, apogee) in space. The Smithsonian's Astrophysical Observatory in Cambridge, MA, hosts the Chandra X-ray Center (CXC) which operates the satellite, processes the data, and distributes it to scientists around the world for analysis. The Center maintains an extensive public web site about the science results and an education program. 3) 4)
The Chandra X-Ray Observatory provides information on the nature of objects ranging from comets in our Solar System to quasars at the edge of the observable universe. Some of the major questions addressed by Chandra are: 5)
• What and where is the "dark matter" in our universe? The largest and most massive objects in the universe are galaxy clusters — enormous collections of galaxies that include some like our own. These galaxies are bound together into a cluster by gravity. Much of the mass in the cluster is in the form of an incredibly hot, X-ray emitting gas that fills the entire space between the galaxies. Yet, neither the mass of the galaxies, nor the mass of the hot X-ray gas is enough to provide the gravity that we know holds the cluster together. Additional mass, due to a mysterious substance called dark matter, is required. X-ray observations with Chandra are mapping the location of the dark matter and helping us to identify it.
• What is the powerhouse driving the explosive activity in many distant galaxies? The centers of many distant galaxies are incredible sources of energy and radiation — especially X-rays. Scientists theorize that massive black holes are at the center of these active galaxies, gobbling up any material — even whole stars — that pass too closely. Detailed studies with Chandra are probing the faintest of these active galaxies. The research shows not only how their energy output changes with time, but also how these objects produce their intense energy emissions in the first place.
• Does the Universe contain “dark energy” and if so, how important is it? Because galaxy clusters are the largest bound structures in the Universe, they likely represent a fair sample of the matter content in the Universe. If so, the ratio of dark matter to hot gas would be the same for every cluster. Scientists used this assumption and Chandra data from galaxy clusters to show that the rate of expansion of the Universe began to accelerate about six billion years ago. This is an important confirmation of results from optical observations of supernovae in distant galaxies. Many scientists attribute the driving force behind cosmic acceleration to “dark energy,” which would make it the dominant form of energy in the Universe. Although there is no accepted explanation for dark energy, most astrophysicists think that dark energy may be intimately connected with the nature of space-time itself.
The CXO (Chandra X-ray Observatory) program is managed by the Marshall Center for the Science Mission Directorate, NASA Headquarters, Washington, D.C. Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime development contractor that assembled and tested the observatory for NASA. 6)
The Chandra X-ray Observatory has three major elements:
1) the spacecraft system,
2) the telescope system containing the X-ray optics, two X-ray transmission gratings1 that can be inserted into the X-ray path, and a 10 m long optical bench and
3) the science instruments. The SIM (Science Instrument Module) holding two focal-plane cameras, ACIS (Advanced CCD Imaging Spectrometer) and HRC (High Resolution Camera).
The spacecraft module contains computers, communication antennas and data recorders to transmit and receive information between the observatory and ground stations. The onboard computers and sensors, with ground-based control center assistance, command and control the vehicle and monitor its health.
The spacecraft module also provides reaction wheels to aim the entire observatory, a set of small momentum unloading system thrusters to control momentum buildup, an aspect camera that tells the observatory its position relative to the stars, and a Sun sensor that protects it from excessive light. Electrical power is provided by solar arrays that also charge three nickel-hydrogen batteries that provide backup power.
Figure 1: The Chandra X-ray Observatory is the world's most powerful X-ray telescope. It has eight-times greater resolution and is able to detect sources more than 20-times fainter than any previous X-ray telescope (image credit: NASA/CXC & J. Vaughan)
At the heart of the telescope system is the HRMA (High Resolution Mirror Assembly). Since high-energy X-rays would penetrate a normal mirror, special cylindrical mirrors were created. The two sets of four nested mirrors resemble tubes within tubes. Incoming X-rays graze off the highly polished mirror surfaces and are funneled to the instrument section for detection and study. 7)
The mirrors of the Chandra X-ray Observatory are the largest of their kind and the smoothest ever created. If the surface of the state of Colorado were as relatively smooth, Pike’s Peak would be less than 1 inch tall. The largest of the eight mirrors is almost 4 feet in diameter and 3 feet long. Assembled, the mirror group has a mass of >1 ton.
The optical design is based on the principal that X-rays reflect efficiently only if the grazing angle between the incident ray and the reflecting surface is less than the critical angle. This angle, typically of order 0.017 radians (one degree), is approximately 10-2 (2ρ)1/2 /E, where ρ is the density of the reflecting material in g-cm-3 and E is the photon energy in keV. The X-ray optical elements for Chandra resemble shallow angle cones, and two reflections are required to provide good imaging over a useful field of view; the first reflecting surface is a paraboloid of revolution and the second a hyperboloid — the Wolter 1 configuration. The collecting area is increased by nesting concentric mirror pairs, all having the same focal plane.
Figure 2: Schematic of grazing incidence, X-ray telescope. This cross section through four nested pairs of mirrors illustrates the principle of grazing incidence reflection and focusing of X-rays. Two reflections are required to make an image. The grazing angles range from about 3.5 degrees for the outer pair to about 2 degrees for the inner pair (image credit: NASA/CXC, S. Lee)
The optical elements have four paraboloid-hyperboloid pairs with a common 10 m focal length. The element lengths are about 0.83 m, the diameters approximately 0.63, 0.85, 0.97, and 1.2 m, and wall thickness range from 16 mm for the smaller elements to 24 mm for the outer ones. Zerodur from Schott is the optical element material chosen because of its low coefficient of thermal expansion and demonstrated capability of permitting very smooth polished surfaces.
Fabrication: The major fabrication phases included coarse and fine grinding, polishing, and final smoothing. The grinding and polishing operations were done with relatively small tools under computer control. Cycles were iterative; a mirror element would be measured to yield an error map, appropriate tools selected to reduce the errors, and a polishing control file for the next cycle generated. The next cycle would cause more material removal in the high areas. The residual errors would be smaller than previously, and so the process converged to the required accuracies.
Coating: The mirror elements were coated at Optical Coating Laboratories, Inc. (OCLI) in Santa Rosa, CA, by sputtering with iridium over a binding layer of chromium. Before each coating optical witness samples were used to show that the thickness would be uniform and that the surface smoothness was not degraded. The x-ray reflectivity of these witness flats was also measured to verify coating density. Witness samples coated simultaneously with the flight elements were also tested. The final cleaning occurred at OCLI prior to coating, and subsequently stringent contamination controls were put into place to minimize particulate and molecular contamination.
Figure 3: Schematic of Grazing Incidence, X-Ray Mirror. This cutaway illustrates the design and functioning of the HRMA (High Resolution Mirror Assembly) on Chandra (image credit: NASA/CXC, D. Berry)
Assembly and Alignment: The final alignment and assembly of the mirror elements into a telescope was done by the Eastman Kodak Company (EKC). The mirror element support structure, prior to inserting the reflecting elements, is shown in Figure 4. Each mirror element was bonded near its middle to flexures attached to carbon fiber composite support sleeves. The four support sleeves and associated flexures for the paraboloids can be seen near the top of the figure. The flexures produce only very small radial forces on the mirrors, and therefore reduce support-induced axial slope errors. The thin mirror shells are also susceptible to a deformation mode in which both ends become oval, but with perpendicular major axes. Supporting the mirror elements near their centers minimized the coupling of support errors into this mode.
Calibration: The telescope was taken to NASA/MSFC for end-to-end ground X-ray calibration beginning in December 1996. The calibration was performed over a 6 month period at the Center’s X-ray Calibration Facility. Previously, the largest paraboloid-hyperboloid pair, uncoated and uncut to its final length, had been x-ray tested at the Facility during an earlier phase of the development program. This early test showed a measured angular resolution of 0.22 arcsec (FWHM). During the activities in 1996-1997 all the flight instrumentation (ACIS, HRC, the LETG and HETG) were tested with the telescope. Moreover, additional calibrations were performed using the telescope and synchrotron-facility-calibrated non-flight detectors. The on-ground calibration results demonstrated, in advance of launch, that the Chandra X-Ray Observatory would provide the required science capabilities: high-resolution (sub-arcsec) imaging, high resolution spectrometric imaging, and high-resolution dispersive spectroscopy.
An outline drawing of the Observatory was shown in Figure 6. The spacecraft equipment panels are mounted to, and supported by, a central cylindrical structure. The rear of the spacecraft attaches to the telescope system. The spacecraft includes six subsystems :
1) Structures and Mechanical Subsystem . This subsystem includes all spacecraft structures, mechanisms (both mechanical and electro-mechanical), and structural interfaces with the Space Shuttle. Mechanisms, such as those required for the sunshade door, are also part of this subsystem.
2) TCS (Thermal Control Subsystem) . Thermal control is primarily passive, using thermal coatings and multi-layer insulation blankets. On-board-computer-controlled electrical heaters augment these passive elements to maintain sensitive items such as the HRMA at nearly constant temperature.
3) Electrical and Power Subsystem . This subsystem includes all hardware necessary to generate, condition, and store electrical energy. Power is generated by solar cells mounted on two solar array wings (three panels each), sized to provide a 15% end-of-life power margin. Electrical power is stored in three, NiH2, 30-Ampere-hour batteries. These batteries provide spacecraft power during times when either the Earth or Moon partially or completely blocks the Sun. Even so, the battery capacity requires that certain non-critical items, including science instruments, be powered down during eclipses. These eclipses occur infrequently due to the particular nature of the Chandra orbit.
4) CCDM (Communication, Command, and Data Management) subsystem . This subsystem includes all the equipment necessary to provide ranging, modulation, and demodulation of radio frequency transmission of commands and data to and from the DSN (Deep Space Network) NASA Communication System (NASCOM). The CCDM includes two low gain antennas, providing omnidirectional communications, an on-board computer (OBC), a serial digital data bus for communication with other spacecraft components, the spacecraft clock, and a telemetry formatter which provides several different formats.
5) PCAD (Pointing Control and Aspect Determination) subsystem . This subsystem includes the hardware and control algorithms for attitude determination and for attitude and solar array control. The solar arrays can be rotated about one axis. The PCAD subsystem also includes hardware for safing the observatory.
The PCAD system controls the pointing and dithering of the observatory and provides the data from which both the relative and absolute aspect are determined. Dithering is imposed to spread the instantaneous image over many different pixels of the focal-plane detector to smooth out pixel-to-pixel variations. The dither pattern is a Lissajous figure. The amplitude, phase, and velocity depend on which instrument (ACIS or HRC) is in the focal plane.
Key elements of the PCAD system are the set of redundant gyroscopes, momentum wheels, and an ACA (Aspect Camera Assembly) consisting of a four inch optical telescope with (redundant) CCD detector. The aspect camera simultaneously images a fiducial light pattern produced by light emitting diodes placed around the focal-plane instruments along with the flux from up to five bright stars that may be in the aspect camera's field-of-view. An interesting consequence is that the user may request that one of the targets of the aspect camera be at the location of the X-ray target. For bright optical counterparts, this option allows real-time optical monitoring albeit at the price of a reduced-accuracy aspect solution.
Figure 5: Photo of the aspect camera (image credit: NASA/CXC)
6) Propulsion Subsystem. This subsystem consists of the IPS (Integral Propulsion System) and the MUPS (Momentum Unloading Propulsion Subsystem ). The IPS contains the thrusters and fuel for control of the orbit and spacecraft orientation during orbit transfer. This system was disabled once the final orbit was achieved for observatory safety reasons. The MUPS provides momentum unloading during normal on-orbit operations. Given current usage rates there would be sufficient MUPS fuel to support ~50 further years of operation.
Table 1: Chandra X-ray Observatory technical parameters (Ref. 5)
Launch: The Chandra spacecraft was launched on 23 July 1999 on the Space Shuttle Columbia (STS-93) from the Kennedy Space Center, LC-39B. Use of Boeing’s IUS (Inertial Upper Stage), and Chandra’s own liquid propulsion system. Two burns of the IUS took place an hour after Chandra was released.
Figure 7: Photo of the Chandra spacecraft launch on STS-93 (image credit: NASA)
Orbit: HEO (Highly Elliptical Orbit) with a perigee of ~10,000 km and an apogee of ~140,000 km, inclination = 76.72°, period of ~64 hours.
Figure 8: Animation of Chandra X-ray Observatory's orbit around Earth from August 7, 1999, to March 8, 2019 (image credit: NASA)
Chandra: Some imagery and mission status
Note: For a more detailed and complete coverage of the Chandra X-ray Observatory mission imagery and status reports, the reader is referred to the CXC (Chandra X-ray Center) homepage at http://chandra.harvard.edu/
• June 22, 2020: By detecting an X-ray flare from a very young star using NASA's Chandra X-ray Observatory, researchers have reset the timeline for when stars like the Sun start blasting high-energy radiation into space, as reported in our latest press release. This is significant because it may help answer some questions about our Sun's earliest days as well as some about the Solar System today. 8) 9)
Figure 9: This artist's illustration depicts the object where astronomers discovered the X-ray flare. HOPS 383 is called a young "protostar" because it is in the earliest phase of stellar evolution that occurs right after a large cloud of gas and dust has started to collapse. Once it has matured HOPS 383, which is located about 1,400 light years from Earth, will have a mass about half that of the Sun (video credits: X-ray: NASA/CXC/Aix-Marseille University/N. Grosso et al.; Illustration: NASA/CXC/M. Weiss)
- Chandra observations in December 2017 revealed the X-ray flare, which lasted for about 3 hours and 20 minutes. The flare is shown as a continuous loop in the inset box of the illustration. The rapid increase and slow decrease in the amount of X-rays is similar to the behavior of X-ray flares from young stars more evolved than HOPS 383. No X-rays were detected from the protostar outside this flaring period, implying that during these times HOPS 383 was at least ten times fainter, on average, than the flare at its maximum. It is also 2,000 times more powerful than the brightest X-ray flare observed from the Sun, a middle-aged star of relatively low mass.
- As material from the cocoon falls inward toward the disk, there is also an exodus of gas and dust. This "outflow" removes angular momentum from the system, allowing material to fall from the disk onto the growing young protostar. Astronomers have seen such an outflow from HOPS 383 and think powerful X-ray flares like the one observed by Chandra could strip electrons from atoms at the base of it. This may be important for driving the outflow by magnetic forces.
- Furthermore, when the star erupted in X-rays, it would have also likely driven energetic flows of particles that collided with dust grains located at the inner edge of the disk of material swirling around the protostar. Assuming something similar happened in our Sun, the nuclear reactions caused by this collision could explain unusual abundances of elements in certain types of meteorites found on Earth.
- No other flares from HOPS 383 were detected over the course of three Chandra observations with a total exposure of just under a day. Astronomers will need longer X-ray observations to determine how frequent such flares are during this very early phase of development for stars like our Sun.
- A paper describing these results appeared in the journal of Astronomy & Astrophysics and is available online at https://arxiv.org/abs/2006.02676. The authors of the paper are Nicolas Grosso (Astrophysics Laboratory of Marseille at Aix-Marseille University in France), Kenji Hamaguchi (Center for Research and Exploration in Space Science & Technology and NASA's Goddard Space Flight Center in Greenbelt, MD), David Principe (Massachusetts Institute of Technology), and Joel Kastner (Rochester Institute of Technology). 10)
Figure 10: The illustration shows HOPS 383 surrounded by a donut-shaped cocoon of material (dark brown) — containing about half of the protostar's mass — that is falling in towards the central star. Much of the light from the infant star in HOPS 383 is unable to pierce through this cocoon, but X-rays from the flare (blue) are powerful enough to do so. Infrared light emitted by HOPS 383 is scattered off the inside of the cocoon (white and yellow). A version of the illustration with a region of the cocoon cut out shows the bright X-ray flare from HOPS 383 and a disk of material falling towards the protostar (image credit: NASA/CXC/M. Weiss)
• June 02, 2020: By combining data from telescopes with supercomputer simulations and virtual reality (VR), a new visualization allows you to experience 500 years of cosmic evolution around the supermassive black hole at the center of the Milky Way. 11)
- This visualization, called "Galactic Center VR", is the latest in a series from astrophysicists, and is based on data from NASA's Chandra X-ray Observatory and other telescopes. This new installment features their NASA supercomputer simulations of material streaming toward the Milky Way's four-million-solar-mass black hole known as Sagittarius A* (Sgr A*). The visualization has been loaded into a VR environment as a novel method of exploring these simulations, and is available for free at both the Steam and Viveport VR stores.
Figure 11: Combining data from Chandra and other telescopes with supercomputer simulations and virtual reality, a new visualization allows users to experience 500 years of cosmic evolution around the Milky Way's supermassive black hole called Sgr A*. Each color represents different phenomena including Wolf-Rayet stars (white), their orbits (grey), and hot gas due to the supersonic wind collisions observed by Chandra (blue and cyan). There are also regions where cooler material (red and yellow) overlaps with the hot gas (purple). The visualization covers about 3 light years, or about 18 trillion miles, around Sgr A*. For more information, visit: https://chandra.si.edu/photo/2020/gcenter/ (video credits: NASA/CXC/Pontifical Catholic Univ. of Chile /C.Russell et al.)
- The researchers modeled winds from 25 very bright and massive objects known as Wolf-Rayet stars, which permeate the central few light years of the galaxy as they orbit Sgr A*. Wolf-Rayet stars produce so much light that they blow off their outer layers into space to create supersonic winds. Watch as some of this material is captured by the black hole's gravity and plummets toward it.
- When the winds from the Wolf-Rayet stars collide, the material is heated to millions of degrees by shocks — similar to sonic booms — and produce copious amounts of X-rays. The center of the galaxy is too distant for Chandra to detect individual examples of these collisions, but the overall X-ray glow of this hot gas is detectable with Chandra's sharp X-ray vision.
- In the visualization, different colors represent assorted objects and phenomena. The white twinkling crosses are the Wolf-Rayet stars, and their orbits are in grey (which can be toggled on and off). The blue and cyan colors show the simulation's X-ray emission from hot gas due to the supersonic wind collisions observed by Chandra, while the red and yellow show all of the wind material, which is dominated by cooler gas and seen infrared and other telescopes. The purple is where the red and blue overlap.
- The visualization spans the full simulation size, which covers about 3 light years, or about 18 trillion miles, centered on Sgr A*. Due to this large scale, the astronomers increased the Sgr A* marker by about 10,000 times. Without this enlargement, the actual size of Sgr A* would render it to be much smaller than a single pixel.
- The visualization also delivers a 3D perspective through the use of VR goggles such as the HTC Vive. Each element of the simulation is loaded into the VR environment, creating a data-based simulation. By providing a six-degrees-of-freedom VR experience, the user can look and move in any direction they choose. The user can also play the simulation at different speeds and choose between seeing all 25 winds or just one wind to observe how the individual elements affect each other in this environment.
- Dr. Christopher Russell of Pontificia Universidad Católica de Chile (PUC), who is now at Catholic University of America and NASA Goddard Space Flight Center, presented this VR experience on behalf of himself and his colleagues of the Instituto de Astrofísica VR Lab at the 236th meeting of the American Astronomical Society meeting that is being held virtually for the first time. The other team members are Baltasar Luco (PUC), Prof. Jorge Cuadra (PUC and Universidad Adolfo Ibáñez), and Miguel Sepúlveda (Universidad de Chile). Their simulations for this VR experience were run on a NASA High End Computing (HEC) supercomputer located at NASA's Ames Research Center.
- NASA's Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science and flight operations from Cambridge and Burlington, Massachusetts.
• May 28, 2020: Astronomers have caught a black hole hurling hot material into space at close to the speed of light. This flare-up was captured in a new movie from NASA's Chandra X-ray Observatory. 12)
Figure 12: The black hole and its companion star make up a system called MAXI J1820+070, located in our Galaxy about 10,000 light years from Earth. The black hole in MAXI J1820+070 has a mass about eight times that of the Sun, identifying it as a so-called stellar-mass black hole, formed by the destruction of a massive star. [(This is in contrast to supermassive black holes that contain millions or billions of times the Sun's mass) image credit: X-ray: NASA/CXC/Université de Paris/M. Espinasse et al.; Optical/IR:PanSTARRS]
- The companion star orbiting the black hole has about half the mass of the Sun. The black hole's strong gravity pulls material away from the companion star into an X-ray emitting disk surrounding the black hole.
- While some of the hot gas in the disk will cross the "event horizon" (the point of no return) and fall into the black hole, some of it is instead blasted away from the black hole in a pair of short beams of material, or jets. These jets are pointed in opposite directions, launched from outside the event horizon along magnetic field lines. The new footage of this black hole's behavior is based on four observations obtained with Chandra in November 2018 and February, May, and June of 2019, and reported in a paper led by Mathilde Espinasse of the Université de Paris.
- The main panel of the graphic (Figure 12) is a large optical and infrared image of the Milky Way galaxy from the PanSTARRS optical telescope in Hawaii, with the location of MAXI J1820+070 above the plane of the galaxy marked by a cross. The inset shows a movie that cycles through the four Chandra observations, where "day 0" corresponds to the first observation on November 13th, 2018, about four months after the jet's launch. MAXI J1820+070 is the bright X-ray source in the middle of the image and sources of X-rays can be seen moving away from the black hole in jets to the north and south. MAXI J1820+070 is a point source of X-rays, although it appears to be larger than a point source because it is much brighter than the jet sources. The southern jet is too faint to be detected in the May and June 2019 observations.
- Just how fast are the jets of material moving away from the black hole? From Earth's perspective, it looks as if the northern jet is moving at 60% the speed of light, while the southern one is traveling at an impossible-sounding 160% of light speed!
- This is an example of superluminal motion, a phenomenon that occurs when something travels towards us near the speed of light, along a direction close to our line of sight. This means the object travels almost as quickly towards us as the light it generates, giving the illusion that the jet's motion is more rapid than the speed of light. In the case of MAXI J1820+070, the southern jet is pointing towards us and the northern jet is pointing away from us, so the southern jet appears to be moving faster than the northern one. The actual velocity of the particles in both jets is greater than 80% of the speed of light.
- Only two other examples of such high-speed expulsions have been seen in X-rays from stellar-mass black holes.
Figure 13: Illustration of a black hole accreting matter from a companion star as jets blast away from the black hole (image credit: NASA/CXC/M. Weiss)
- MAXI J1820+070 has also been observed at radio wavelengths by a team led by Joe Bright from the University of Oxford, who previously reported the detection of superluminal motion of compact sources based on radio data alone that extended from the launch of the jets on July 7, 2018 to the end of 2018.
- Because the Chandra observations approximately doubled the length of time the jets were followed, a combined analysis of the radio data and the new Chandra data by Espinasse and her team gave more information about the jets. This included evidence that the jets are decelerating as they travel away from the black hole.
- Most of the energy in the jets is not converted into radiation, but is instead released when particles in the jets interact with surrounding material. These interactions might be the cause of the jets' deceleration. When the jets collide with surrounding material in interstellar space, shock waves — akin to the sonic booms caused by supersonic aircraft — occur. This process generates particle energies that are higher than that of the Large Hadron Collider.
- The researchers estimate that about 400 million billion pounds of material was blown away from the black hole in these two jets launched in July 2018. This amount of mass is comparable to what could be accumulated on the disk around the black hole in the space of a few hours, and is equivalent to about a thousand Halley's Comets or about 500 million times the mass of the Empire State Building.
- Studies of MAXI J1820+070 and similar systems promise to teach us more about the jets produced by stellar-mass black holes and how they release their energy once their jets interact with their surroundings.
- Radio observations conducted with the Karl G. Jansky Very Large Array and the MeerKAT array were also used to study MAXI J1820+070's jets.
• April 23, 2020: Astronomers may have discovered a new kind of survival story: a star that had a brush with a giant black hole and lived to tell the tale through exclamations of X-rays. 14)
Figure 14: Astronomers may have discovered a new kind of survival story: a star that had a brush with a giant black hole and lived to tell the tale through exclamations of X-rays. Data from NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton uncovered the account that began with a red giant star wandering too close to a supermassive black hole in a galaxy about 250 million light years from Earth. The black hole, located in a galaxy called GSN 069, has a mass about 400,000 times that of the Sun, putting it on the small end of the scale for supermassive black holes (image credit: X-ray: NASA/CXO/CSIC-INTA/G. Miniutti et al.; Illustration: NASA/CXC/M. Weiss)
- Data from NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton uncovered the account that began with a red giant star wandering too close to a supermassive black hole in a galaxy about 250 million light years from Earth. The black hole, located in a galaxy called GSN 069, has a mass about 400,000 times that of the Sun, putting it on the small end of the scale for supermassive black holes.
- Once the red giant was captured by the black hole’s gravity, the outer layers of the star containing hydrogen were stripped off and careened toward the black hole, leaving the core of the star – known as a white dwarf – behind.
- “In my interpretation of the X-ray data the white dwarf survived, but it did not escape,” said Andrew King of the University of Leicester in the UK, who performed this study. “It is now caught in an elliptical orbit around the black hole, making one trip around about once every nine hours.”
- As the white dwarf makes its nearly thrice-daily orbit, the black hole pulls material off at its closest approach (no more than 15 times the radius of the event horizon – the point of no return – away from the black hole). The stellar detritus enters into a disk surrounding the black hole and releases a burst of X-rays that Chandra and XMM-Newton can detect. In addition, King predicts gravitational waves will be emitted by the black hole and white dwarf pair, especially at their nearest point.
- What would be the future of the star and its orbit? The combined effect of gravitational waves and an increase in the star’s size as it loses mass should cause the orbit to become more circular and grow in size. In this case, the rate of mass loss steadily slows down, and the white dwarf slowly spirals away from the black hole.
- “It will try hard to get away, but there is no escape. The black hole will eat it more and more slowly, but never stop,” said King. “In principle, this loss of mass would continue until and even after the white dwarf became a planet, with a mass similar to Jupiter, in about a trillion years. This would be a remarkably slow and convoluted way for the universe to make a planet!”
- Astronomers have found many stars that have been completely torn apart by encounters with black holes (so-called tidal disruption events), but there are very few reported cases of near misses, where the star likely survived.
- Grazing encounters like this should be more common than direct collisions given the statistics of cosmic traffic patterns, but they could easily be missed for a couple of reasons. First, it can take a more massive, surviving star too long to complete an orbit around a black hole for astronomers to see repeated bursts. Another issue is that supermassive black holes that are much more massive than the one in GSN 069 may directly swallow a star rather than the star falling into orbits where they periodically lose mass. In these cases, astronomers wouldn’t observe anything.
- “In astronomical terms, this event is only visible to our current telescopes for a short time – about 2,000 years,” said King. “So unless we were extraordinarily lucky to have caught this one, there may be many more that we are missing. Such encounters could be one of the main ways for black holes the size of the one in GSN 069 to grow.”
- King predicts that the white dwarf has a mass of only two tenths the mass of the Sun. If the white dwarf was the core of the red giant that was completely stripped of its hydrogen, then it should be rich in helium. The helium would have been created by the fusion of hydrogen atoms during the evolution of the red giant.
- “It’s remarkable to think that the orbit, mass and composition of a tiny star 250 million light years away could be inferred,” said King.
- King made a prediction based on his scenario. Because the white dwarf is so close to the black hole, effects from the Theory of General Relativity mean that the direction of the orbit's axis should wobble, or “precess.” This wobble should repeat every two days and may be detectable with sufficiently long observations.
- A paper describing these results appears in the March 2020 issue of the Monthly Notices of the Royal Astronomical Society, and is available online. NASA's Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science and flight operations from Cambridge and Burlington, Massachusetts.
• February 27, 2020: The biggest explosion seen in the universe has been found. This record-breaking, gargantuan eruption came from a black hole in a distant galaxy cluster hundreds of millions of light years away. 15)
- "In some ways, this blast is similar to how the eruption of Mt. St. Helens in 1980 ripped off the top of the mountain," said Simona Giacintucci of the Naval Research Laboratory in Washington, DC, and lead author of the study. "A key difference is that you could fit fifteen Milky Way galaxies in a row into the crater this eruption punched into the cluster's hot gas."
- Astronomers made this discovery using X-ray data from NASA's Chandra X-ray Observatory and ESA's XMM-Newton, and radio data from the Murchison Widefield Array (MWA) in Australia and the Giant Metrewave Radio Telescope (GMRT) in India.
- The unrivaled outburst was detected in the Ophiuchus galaxy cluster, which is about 390 million light years from Earth. Galaxy clusters are the largest structures in the Universe held together by gravity, containing thousands of individual galaxies, dark matter, and hot gas.
- The unrivaled outburst was detected in the Ophiuchus galaxy cluster, which is about 390 million light years from Earth. Galaxy clusters are the largest structures in the Universe held together by gravity, containing thousands of individual galaxies, dark matter, and hot gas.
- Although black holes are famous for pulling material toward them, they often expel prodigious amounts of material and energy. This happens when matter falling toward the black hole is redirected into jets, or beams, that blast outward into space and slam into any surrounding material.
- Chandra observations reported in 2016 first revealed hints of the giant explosion in the Ophiuchus galaxy cluster. Norbert Werner and colleagues reported the discovery of an unusual curved edge in the Chandra image of the cluster. They considered whether this represented part of the wall of a cavity in the hot gas created by jets from the supermassive black hole. However, they discounted this possibility, in part because a huge amount of energy would have been required for the black hole to create a cavity this large.
- The latest study by Giacintucci and her colleagues show that an enormous explosion did, in fact, occur. First, they showed that the curved edge is also detected by XMM-Newton, thus confirming the Chandra observation. Their crucial advance was the use of new radio data from the MWA and data from the GMRT archives to show the curved edge is indeed part of the wall of a cavity, because it borders a region filled with radio emission. This emission is from electrons accelerated to nearly the speed of light. The acceleration likely originated from the supermassive black hole.
- "The radio data fit inside the X-rays like a hand in a glove," said co-author Maxim Markevitch of NASA's Goddard Space Flight Center in Greenbelt, Maryland. "This is the clincher that tells us an eruption of unprecedented size occurred here."
- The amount of energy required to create the cavity in Ophiuchus is about five times greater than the previous record holder, MS 0735+74, and hundreds and thousands of times greater than typical clusters.
- The black hole eruption must have finished because the researchers do not see any evidence for current jets in the radio data. This shutdown can be explained by the Chandra data, which show that the densest and coolest gas seen in X-rays is currently located at a different position from the central galaxy. If this gas shifted away from the galaxy it will have deprived the black hole of fuel for its growth, turning off the jets.
Figure 15: Composite image of the Ophiuchus Galaxy Cluster: Record-Breaking Explosion by Black Hole Spotted (image credit: X-ray: Chandra: NASA/CXC/NRL/S. Giacintucci, et al., XMM-Newton: ESA/XMM-Newton; Radio: NCRA/TIFR/GMRT; Infrared: 2MASS/UMass/IPAC-Caltech/NASA/NSF) 16) 17)
Table 2: Some descriptive text to Figure 15
• January 6, 2020: Using NASA’s Chandra X-ray Observatory, astronomers have seen that the famous giant black hole in Messier 87 is propelling particles at speeds greater than 99% of the speed of light. 18)
- The Event Horizon Telescope Collaboration released the first image of a black hole with observations of the massive, dark object at the center of galaxy Messier 87, or M87, last April. This black hole has a mass of about 6.5 billion times that of the sun and is located about 55 million light years from Earth. The black hole has been called M87* by astronomers and has recently been given the Hawaiian name of “Powehi.”
- For years, astronomers have observed radiation from a jet of high energy particles – powered by the black hole – blasting out of the center of M87. They have studied the jet in radio, optical, and X-ray light, including with Chandra. And now by using Chandra observations, researchers have seen that sections of the jet are moving at nearly the speed of light.
- “This is the first time such extreme speeds by a black hole’s jet have been recorded using X-ray data,” said Ralph Kraft of the Center of Astrophysics | Harvard & Smithsonian (CfA) in Cambridge, Mass., who presented the study at the American Astronomical Society meeting in Honolulu, Hawaii. “We needed the sharp X-ray vision of Chandra to make these measurements.”
- When matter gets close enough to a black hole, it enters into a swirling pattern called an accretion disk. Some material from the inner part of the accretion disk falls onto the black hole and some of it is redirected away from the black hole in the form of narrow beams, or jets, of material along magnetic field lines. Because this infall process is irregular, the jets are made of clumps or knots that can sometimes be identified with Chandra and other telescopes.
- The researchers used Chandra observations from 2012 and 2017 to track the motion of two X-ray knots located within the jet about 900 and 2,500 light years away from the black hole. The X-ray data show motion with apparent speeds of 6.3 times the speed of light for the X-ray knot closer to the black hole and 2.4 times the speed of light for the other.
- “One of the unbreakable laws of physics is that nothing can move faster than the speed of light,” said co-author Brad Snios, also of the CfA. “We haven’t broken physics, but we have found an example of an amazing phenomenon called superluminal motion.”
- Superluminal motion occurs when objects are traveling close to the speed of light along a direction that is close to our line of sight. The jet travels almost as quickly towards us as the light it generates, giving the illusion that the jet’s motion is much more rapid than the speed of light. In the case of M87*, the jet is pointing close to our direction, resulting in these exotic apparent speeds.
- Astronomers have previously seen such motion in M87*’s jet at radio and optical wavelengths, but they have not been able to definitively show that matter in the jet is moving at very close to the speed of light. For example, the moving features could be a wave or a shock, similar to a sonic boom from a supersonic plane, rather than tracing the motions of matter.
- This latest result shows the ability of X-rays to act as an accurate cosmic speed gun. The team observed that the feature moving with an apparent speed of 6.3 times the speed of light also faded by over 70% between 2012 and 2017. This fading was likely caused by particles’ loss of energy due to the radiation produced as they spiral around a magnetic field. For this to occur the team must be seeing X-rays from the same particles at both times, and not a moving wave.
- “Our work gives the strongest evidence yet that particles in M87*’s jet are actually traveling at close to the cosmic speed limit”, said Snios.
Figure 16: Using NASA’s Chandra X-ray Observatory, astronomers have seen that the famous giant black hole in Messier 87 is propelling particles at speeds greater than 99% of the speed of light (image credit: NASA/CXC/SAO/B, Brad Snios, et al.)
- The Chandra data are an excellent complement to the EHT (Event Horizon Telescope) data. The size of the ring around the black hole seen with the Event Horizon Telescope is about a hundred million times smaller than the size of the jet seen with Chandra.
- Another difference is that the EHT observed M87 over six days in April 2017, giving a recent snapshot of the black hole. The Chandra observations investigate ejected material within the jet that was launched from the black hole hundreds and thousands of years earlier.
- “It’s like the Event Horizon Telescope is giving a close-up view of a rocket launcher," said the CfA’s Paul Nulsen, another co-author of the study, “and Chandra is showing us the rockets in flight.”
- In addition to being presented at the AAS meeting, these results are also described in a paper in The Astrophysical Journal led by Brad Snios that is available online. NASA's Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science and flight operations from Cambridge and Burlington, Massachusetts. 19)
• January 5, 2020: Astronomers and visualization specialists from NASA's Universe of Learning program have combined the visible, infrared and X-ray vision of NASA's Great Observatories to create a three-dimensional representation of the dynamic Crab Nebula, the tattered remains of an exploded star. - The multiwavelength computer graphics visualization is based on images from the Chandra X-ray Observatory and the Hubble and Spitzer space telescopes. 20)
Figure 17: This new multiwavelength image of the Crab Nebula combines X-ray light from the Chandra X-ray Observatory (in blue) with visible light from the Hubble Space Telescope (in yellow) and infrared light seen by the Spitzer Space Telescope (in red). This particular combination of light from across the electromagnetic spectrum highlights the nested structure of the pulsar wind nebula. The X-rays reveal the beating heart of the Crab, the neutron-star remnant from the supernova explosion seen almost a thousand years ago. This neutron star is the super-dense collapsed core of an exploded star and is now a pulsar that rotates at a blistering rate of 30 times per second. A disk of X-ray-emitting material, spewing jets of high-energy particles perpendicular to the disk, surrounds the pulsar. The infrared light in this image shows synchrotron radiation, formed from streams of charged particles spiraling around the pulsar's strong magnetic fields. The visible light is emission from oxygen that has been heated by higher-energy (ultraviolet and X-ray) synchrotron radiation. The delicate tendrils seen in visible light form what astronomers call a "cage" around the rich tapestry of synchrotron radiation, which in turn encompasses the energetic fury of the X-ray disk and jets. These multiwavelength interconnected structures illustrate that the pulsar is the main energy source for the emission seen by all three telescopes. The Crab Nebula resides 6,500 light-years from Earth in the constellation Taurus [image credits: NASA, ESA and J. DePasquale (STScI) and R. Hurt (Caltech/IPAC)]
- The approximately four-minute video dissects the intricate nested structure that makes up this stellar corpse, giving viewers a better understanding of the extreme and complex physical processes powering the nebula. The powerhouse "engine" energizing the entire system is a pulsar, a rapidly spinning neutron star, the super-dense crushed core of the exploded star. The tiny dynamo is blasting out blistering pulses of radiation 30 times a second with unbelievable clockwork precision.
- The visualization was produced by a team at the Space Telescope Science Institute (STScI) in Baltimore, Maryland; the Caltech/IPAC in Pasadena, California; and the Center for Astrophysics, Harvard & Smithsonian (CfA) in Cambridge. Massachusetts. It will debut at the American Astronomical Society meeting in Honolulu, Hawaii. The movie is available to planetariums and other centers of informal learning worldwide.
- "Seeing two-dimensional images of an object, especially of a complex structure like the Crab Nebula, doesn't give you a good idea of its three-dimensional nature," explained STScI's visualization scientist Frank Summers, who led the team that developed the movie. "With this scientific interpretation, we want to help people understand the Crab Nebula's nested and interconnected geometry. The interplay of the multiwavelength observations illuminate all of these structures. Without combining X-ray, infrared and visible light, you don't get the full picture."
- Certain structures and processes, driven by the pulsar engine at the heart of the nebula, are best seen at particular wavelengths.
Figure 18: This visualization features a three-dimensional multiwavelength representation of the Crab Nebula, a pulsar wind nebula that is the remains of an exploded star. The movie is based on images from NASA’s three Great Observatories: the Chandra X-ray Observatory and the Hubble and Spitzer space telescopes [video credit: NASA, ESA, F. Summers, J. Olmsted, L. Hustak, J. DePasquale, G. Bacon (STScI), N. Wolk (CfA|H&S/CXC), R. Hurt (Caltech/IPAC)]
- The movie begins by showing the Crab Nebula in context, pinpointing its location in the constellation Taurus. This view zooms in to present the Hubble, Spitzer and Chandra images of the Crab Nebula, each highlighting one of the nested structures in the system. The video then begins a slow buildup of the three-dimensional X-ray structure, showing the pulsar and a ringed disk of energized material, and adding jets of particles firing off from opposite sides of the energetic dynamo.
- Appearing next is a rotating infrared view of a cloud enveloping the pulsar system, and glowing from synchrotron radiation. This distinctive form of radiation occurs when streams of charged particles spiral around magnetic field lines. There is also infrared emission from dust and gas.
- The visible-light outer shell of the Crab Nebula appears next. Looking like a cage around the entire system, this shell of glowing gas consists of tentacle-shaped filaments of ionized oxygen (oxygen missing one or more electrons). The tsunami of particles unleashed by the pulsar is pushing on this expanding debris cloud like an animal rattling its cage.
- The X-ray, infrared and visible-light models are combined at the end of the movie to reveal both a rotating three-dimensional multiwavelength view and the corresponding two-dimensional multiwavelength image of the Crab Nebula.
- The three-dimensional structures serve as scientifically informed approximations for imagining the nebula. "The three-dimensional views of each nested structure give you an idea of its true dimensions," Summers said. "To enable viewers to develop a complete mental model, we wanted to show each structure separately, from the ringed disk and jets in stark relief, to the synchrotron radiation as a cloud around that, and then the visible light as a cage structure surrounding the entire system."
- These nested structures are particular to the Crab Nebula. They reveal that the nebula is not a classic supernova remnant as once commonly thought. Instead, the system is better classified as a pulsar wind nebula. A traditional supernova remnant consists of a blast wave, and debris from the supernova that has been heated to millions of degrees. In a pulsar wind nebula, the system's inner region consists of lower-temperature gas that is heated up to thousands of degrees by the high-energy synchrotron radiation.
- "It is truly via the multiwavelength structure that you can more cleanly comprehend that it's a pulsar wind nebula," Summers said. "This is an important learning objective. You can understand the energy from the pulsar at the core moving out to the synchrotron cloud, and then further out to the filaments of the cage."
- Summers and the STScI visualization team worked with Robert Hurt, lead visualization scientist at IPAC, on the Spitzer images, and Nancy Wolk, imaging processing specialist at the Chandra X-ray Center at the CfA, on the Chandra images. Their initial step was reviewing past research on the Crab Nebula, an intensely studied object that formed from a supernova seen in 1054 by Chinese astronomers.
- Starting with the two-dimensional Hubble, Spitzer and Chandra images, the team worked with experts to analyze the complex nested structures comprising the nebula and identify the best wavelength to represent each component. The three-dimensional interpretation is guided by scientific data, knowledge and intuition, with artistic features filling out the structures.
- The visualization is one of a new generation of products and experiences being developed by the NASA's Universe of Learning program. The effort combines a direct connection to the science and scientists of NASA's Astrophysics missions with attention to audience needs to enable youth, families and lifelong learners to explore fundamental questions in science, experience how science is done, and discover the universe for themselves.
- This video demonstrates the power of multiwavelength astronomy. It helps audiences understand how and why astronomers use multiple regions of the electromagnetic spectrum to explore and learn about our universe.
- NASA's Universe of Learning materials are based upon work supported by NASA under cooperative agreement award number NNX16AC65A to the Space Telescope Science Institute, working in partnership with Caltech/IPAC, Jet Propulsion Laboratory, CfA and Sonoma State University.
• October 25, 2019: Astronomers using data from NASA’s Chandra X-ray Observatory and other telescopes have put together a detailed map of a rare collision between four galaxy clusters. Eventually all four clusters – each with a mass of at least several hundred trillion times that of the Sun — will merge to form one of the most massive objects in the universe. 21)
- Galaxy clusters are the largest structures in the cosmos that are held together by gravity. Clusters consist of hundreds or even thousands of galaxies embedded in hot gas, and contain an even larger amount of invisible dark matter. Sometimes two galaxy clusters collide, as in the case of the Bullet Cluster, and occasionally more than two will collide at the same time.
- The new observations show a mega-structure being assembled in a system called Abell 1758, located about 3 billion light-years from Earth. It contains two pairs of colliding galaxy clusters that are heading toward one another. Scientists first recognized Abell 1758 as a quadruple galaxy cluster system in 2004 using data from Chandra and XMM-Newton, a satellite operated by the European Space Agency (ESA).
- X-rays from Chandra are shown as blue and white, depicting fainter and brighter diffuse emission, respectively. This new composite image also includes an optical image from the SDSS (Sloan Digital Sky Survey). The Chandra data revealed for the first time a shock wave — similar to the sonic boom from a supersonic aircraft — in hot gas visible with Chandra in the northern pair's collision. From this shock wave, researchers estimate two clusters are moving about 2 million to 3 million miles per hour (3 million to 5 million km/hour), relative to each other.
- Chandra data also provide information about how elements heavier than helium, the "heavy elements," in galaxy clusters get mixed up and redistributed after the clusters collide and merge. Because this process depends on how far a merger has progressed, Abell 1758 offers a valuable case study, since the northern and the southern pairs of clusters are at different stages of merging.
- In the southern pair, the heavy elements are most abundant in the centers of the two colliding clusters, showing that the original location of the elements has not been strongly impacted by the ongoing collision. By contrast, in the northern pair, where the collision and merger has progressed further, the location of the heavy elements has been strongly influenced by the collision. The highest abundances are found between the two cluster centers and to the left side of the cluster pair, while the lowest abundances are in the center of the cluster on the left side of the image.
- Collisions between clusters affect their component galaxies as well as the hot gas that surrounds them. Data from the 6.5-meter MMT (Multiple Mirror Telescope) telescope in Arizona, obtained as part of the Arizona Cluster Redshift Survey, show that some galaxies are moving much faster than others, probably because they have been thrown away from the other galaxies in their cluster by gravitational forces imparted by the collision.
- The team also used radio data from the GMRT (Giant Meterwave Radio Telescope), and X-ray data from ESA’s XMM-Newton mission.
- A paper describing these latest results by Gerrit Schellenberger, Larry David, Ewan O’Sullivan, Jan Vrtilek (all from Center for Astrophysics | Harvard & Smithsonian) and Christopher Haines (Universidad de Atacama, Chile) was published in the September 1st, 2019 issue of The Astrophysical Journal, and is available online. 22)
- NASA's Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center controls science and flight operations from Cambridge, Massachusetts.
Figure 19: Each pair in the system contains two galaxy clusters that are well on their way to merging. In the northern (top) pair seen in the composite image, the centers of each cluster have already passed by each other once, about 300 to 400 million years ago, and will eventually swing back around. The southern pair at the bottom of the image has two clusters that are close to approaching each other for the first time (image credit: X-ray: NASA/CXC/SAO/G. Schellenberger et al.; Optical:SDSS)
Figure 20: Labeled image of Abell 1758 system (image credit: X-ray: NASA/CXC/SAO/G. Schellenberger et al.; Optical:SDSS) 23)
• August 26, 2019: NASA’s Chandra X-ray Observatory has captured many spectacular images of cosmic phenomena over its two decades of operations, but perhaps its most iconic is the supernova remnant Cassiopeia A. 24)
- Located about 11,000 light-years from Earth, Cas A (as it’s nicknamed) is the glowing debris field left behind after a massive star exploded. When the star ran out of fuel, it collapsed onto itself and blew up as a supernova, possibly briefly becoming one of the brightest objects in the sky. (Although astronomers think that this happened around the year 1680, there are no verifiable historical records to confirm this.)
- The shock waves generated by this blast supercharged the stellar wreckage and its environment, making the debris glow brightly in many types of light, particularly X-rays. Shortly after Chandra was launched aboard the Space Shuttle Columbia on July 23, 1999, astronomers directed the observatory to point toward Cas A. It was featured in Chandra's official “First Light” image, released Aug. 26, 1999, and marked a seminal moment not just for the observatory, but for the field of X-ray astronomy. Near the center of the intricate pattern of the expanding debris from the shattered star, the image revealed, for the first time, a dense object called a neutron star that the supernova left behind.
- Since then, Chandra has repeatedly returned to Cas A to learn more about this important object. A new video shows the evolution of Cas A over time, enabling viewers to watch as incredibly hot gas – about 20 million degrees Fahrenheit – in the remnant expands outward. These X-ray data have been combined with data from another of NASA’s “Great Observatories,” the Hubble Space Telescope, showing delicate filamentary structures of cooler gases with temperatures of about 20,000 degrees Fahrenheit. Hubble data from a single time period are shown to emphasize the changes in the Chandra data.
Figure 21: The Latest Look at "First Light" from Chandra. NASA’s Chandra X-ray Observatory has captured many spectacular images of cosmic phenomena over its two decades of operations, but perhaps its most iconic is the supernova remnant Cassiopeia A (image credit: X-ray: NASA/CXC/RIKEN/T. Sato et al.; Optical: NASA/STScI)
Figure 22: This video shows Chandra observations from 2000 to 2013, or about the time it takes for a child to enter kindergarten and then graduate from high school. This gives astronomers a rare chance to watch as a cosmic object changes on human timescales, giving them new insight into the physics involved. For example, particles in the blue outer shock wave carry more energy than those produced by the most powerful particle accelerators on Earth. As this blast wave hits material in its path it slows down, sending a shock wave backwards at speeds of millions of miles per hour (video credit: Chandra X-ray Observatory, Published on 26 August 2019)
- The blue, outer region of Cas A shows the expanding blast wave of the explosion. The blast wave is composed of shock waves, similar to the sonic booms generated by a supersonic aircraft. These expanding shock waves produce X-ray emission and are sites where particles are being accelerated to energies that reach about two times higher than the most powerful accelerator on Earth, the LHC (Large Hadron Collider). As the blast wave travels outwards at speeds of about 11 million miles per hour, it encounters surrounding material and slows down, generating a second shock wave – called a “reverse shock” – that travels backwards, similar to how a traffic jam travels backwards from the scene of an accident on a highway.
- These reverse shocks are usually observed to be faint and much slower moving than the blast wave. However, a team of astronomers led by Toshiki Sato from RIKEN in Saitama, Japan, and NASA’s Goddard Space Flight Center, have reported reverse shocks in Cas A that appear bright and fast moving, with speeds between about 5 and 9 million miles per hour. These unusual reverse shocks are likely caused by the blast wave encountering clumps of material surrounding the remnant, as Sato and team discuss in their 2018 study. This causes the blast wave to slow down more quickly, which re-energizes the reverse shock, making it brighter and faster. Particles are also accelerated to colossal energies by these inward moving shocks, reaching about 30 times the energies of the LHC.
- This recent study of Cas A adds to a long collection of Chandra discoveries over the course of the telescope’s 20 years. In addition to finding the central neutron star, Chandra data have revealed the distribution of elements essential for life ejected by the explosion, have constructed a remarkable three dimensional model of the supernova remnant, and much more.
- Scientists also created a historical record in optical light of Cas A using photographic plates from the Palomar Observatory in California from 1951 and 1989 that had been digitized by the Digitized Access to a Sky Century @ Harvard (DASCH) program, located at the Center for Astrophysics | Harvard & Smithsonian (CfA). These were combined with images taken by the Hubble Space Telescope between 2000 and 2011. This long-term look at Cas A allowed astronomers Dan Patnaude of CfA and Robert Fesen of Dartmouth College to learn more about the physics of the explosion and the resulting remnant from both the X-ray and optical data.
- This recent study of Cas A adds to a long collection of Chandra discoveries over the course of the telescope’s 20 years. In addition to finding the central neutron star, Chandra data have revealed the distribution of elements essential for life ejected by the explosion, clues about the details of how the star exploded, and much more.
- NASA's Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center controls science and flight operations from Cambridge, Massachusetts.
• July 29, 2019: Recent testing has shown that super-thin, lightweight X-ray mirrors made of a material commonly used to make computer chips can meet the stringent imaging requirements of next-generation X-ray observatories. 25)
- As a result, the X-ray mirror technology being developed by Will Zhang and his team at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, has been baselined for the Design Reference Mission of the conceptual Lynx X-ray Observatory — one of four potential missions that scientists have vetted as worthy pursuits under the 2020 Decadal Survey for Astrophysics.
- If selected and ultimately launched in the 2030s, Lynx could literally carry tens of thousands of Zhang’s mirror segments, which would offer a two orders-of-magnitude leap in sensitivity over NASA’s flagship Chandra X-ray Observatory and the European Space Agency’s ATHENA (Advanced Telescope for High-Energy Astrophysics). Chandra itself offered a significant leap in capability when it launched in 1999. It can observe X-ray sources — exploded stars, clusters of galaxies, and matter around black holes —100 times fainter than those observed by previous X-ray telescopes.
- In another development, Zhang and his team have secured a nearer-term flight opportunity aboard a sounding rocket mission scheduled for 2021. This would represent the technology’s first demonstration in space.
Figure 23: Goddard scientist Will Zhang holds mirror segments made of silicon. These X-ray optics have been baselined for the proposed Lynx X-ray Observatory (image credit: NASA, Chris Gunn)
Seven-Year Development Effort
- The effort to develop the new optic began seven years ago when Zhang began experimenting with mono-crystalline — a single-crystal silicon that had never before been used to create X-ray mirrors. These specially fabricated optics must be curved and nested inside a cylindrically shaped container so that highly energetic X-ray photons graze their surfaces and deflect into an observatory’s instruments rather than passing through them.
- His goal — given the cost of building space observatories, which only increase in price as they get larger and heavier — was to develop easily reproducible, lightweight, super-thin mirrors, without sacrificing quality.
- His goal — given the cost of building space observatories, which only increase in price as they get larger and heavier — was to develop easily reproducible, lightweight, super-thin mirrors, without sacrificing quality.
- Reviews conducted by a NASA-commissioned panel of 40 experts deemed that Zhang’s optics made of the brittle, highly stable silicon material are capable of the same image quality as the four pairs of larger and heavier mirrors flying on Chandra. The panel also deemed two other technologies – full-shell mirrors and adjustable optics – as being able to fulfill the requirements of the conceptual Lynx Observatory.
- Not only could Zhang’s mirrors provide 0.5 arcsec resolution — comparable to the image quality afforded by ultra-high-definition television — they also met Zhang’s low-mass requirements. They are 50 times lighter and thinner than Chandra’s, Zhang said. This means future observatories could carry far more mirrors, creating a larger collection area for snagging X-rays emanating from high-energy phenomena in the universe.
Now the Hard Part Begins
- But Zhang said he and his team are still “far, far away from flying our optics.”
- He and his engineering team now have to figure out how to bond these fragile mirror segments inside the canister, which protects the entire mirror assembly during a rocket launch and maintains their nested alignment.
- “We have a lot to do, and not a lot of time to do it,” Zhang said. “This is now an engineering challenge.”
- Time is of the essence, he added. Just two years from now, Zhang’s team must deliver a 288-segment mirror assembly to Randall McEntaffer, a professor at Pennsylvania State University in State College who is developing a sounding rocket mission called the OGRE( Off-plane Grating Rocket Experiment), expected to launch from the Wallops Flight Facility in 2021. In addition to the mirrors, OGRE will carry a university-developed spectrograph equipped with next-generation X-ray diffraction gratings used to split X-ray light into its component colors or wavelengths to reveal an object’s temperature, chemical makeup, and other physical properties.
- OGRE will do much to advance the mirror assembly, Zhang added. The mission will help determine whether the team’s design can protect the fragile nest of mirrors from extreme launch forces experienced during liftoff and ascent through Earth’s atmosphere.
Figure 24: X-ray observatories like Chandra give us a new view of our universe beyond what we can see with our eyes. Now a team at NASA’s Goddard Space Flight Center is working to take X-ray astronomy to the next level. Goddard astrophysicist Dr. William Zhang believes his mirrors can show us what’s happening near supermassive black holes like the one in the center of our Milky Way galaxy. Here’s how the mirrors come together (video credit: NASA, Uploaded on 26 July 2019)
Other Opportunities Available
- Zhang envisions a bright future for the team’s optics. Even if Lynx isn’t chosen for development by the 2020 Decadal Survey, other proposed missions could benefit, Zhang said. These include a couple X-ray observatories now being investigated as potential astrophysics Probe-class missions and another now being considered by the Japanese.
- “Five years ago, people said it couldn’t be done, but we proved our ideas,” Zhang said. “My team is grateful to Goddard’s Internal Research and Development program for giving us the seed money. We couldn’t have achieved this without it.”
• On 23 July 1999, the Space Shuttle Columbia blasted off from the Kennedy Space Center carrying the Chandra X-ray Observatory. In the two decades that have passed, Chandra’s powerful and unique X-ray eyes have contributed to a revolution in our understanding of the cosmos. 26)
- “In this year of exceptional anniversaries – 50 years after Apollo 11 and 100 years after the solar eclipse that proved Einstein’s General Theory of Relativity – we should not lose sight of one more,” said Paul Hertz, Director of Astrophysics at NASA. “Chandra was launched 20 years ago, and it continues to deliver amazing science discoveries year after year.”
- To commemorate Chandra’s 20th anniversary of science operations, NASA has released new images representing the breadth of Chandra’s exploration, demonstrating the variety of objects it studies as well as how X-rays complement the data collected in other types of light. From the colossal grandeur of a galaxy cluster to the light from infant stars, these new images are a sample of Chandra’s spectacular X-ray vision.
- Chandra is one of NASA’s “Great Observatories” (along with the Hubble Space Telescope, Spitzer Space Telescope, and Compton Gamma Ray Observatory), and has the sharpest vision of any X-ray telescope ever built. It is often used in conjunction with telescopes like Hubble and Spitzer that observe in different parts of the electromagnetic spectrum, and with other high-energy missions like the European Space Agency’s XMM-Newton and NASA’s NuSTAR.
- Chandra’s discoveries have impacted virtually every aspect of astrophysics. For example, Chandra was involved in a direct proof of dark matter’s existence. It has witnessed powerful eruptions from supermassive black holes. Astronomers have also used Chandra to map how the elements essential to life are spread from supernova explosions.
- Many of the phenomena Chandra now investigates were not even known when the telescope was being developed and built. For example, astronomers now use Chandra to study the effects of dark energy, test the impact of stellar radiation on exoplanets, and observe the outcomes of gravitational wave events.
- “Chandra remains peerless in its ability to find and study X-ray sources,” said Chandra X-ray Center Director Belinda Wilkes. “Since virtually every astronomical source emits X-rays, we need a telescope like Chandra to fully view and understand our Universe.”
- Chandra was originally proposed to NASA in 1976 by Riccardo Giacconi, recipient of the 2002 Nobel Prize for Physics based on his contributions to X-ray astronomy, and Harvey Tananbaum, who would become the first director of the Chandra X-ray Center. It took decades of collaboration – between scientists and engineers, private companies and government agencies, and more – to make Chandra a reality.
- “The building and operation of Chandra has always been and continues to be a team effort,” said Martin Weisskopf, Chandra Project Scientist of NASA’s Marshall Space Flight Center. “It’s been an honor and a privilege to be involved with this scientific powerhouse.”
- In 2018, NASA awarded a contract extension to continue operation and science support of Chandra through 2024, with the possibility of two three-year options.
- The Chandra X-ray Observatory was named in honor of the late Nobel laureate Subrahmanyan Chandrasekhar. NASA's Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center controls science and flight operations from Cambridge, Mass.
Figure 25: NASA’s Chandra X-ray Observatory is commemorating its 20th anniversary with an assembly of new images. These images represent the breadth of Chandra’s exploration, demonstrating the variety of objects it studies as well as how X-rays complement the data collected in other types of light (image credit: NASA/CXC)
• June 10, 2019: A new state-of-the-art facility that will operate NASA's Chandra X-ray Observatory has opened. This new OCC (Operations Control Center) will help Chandra continue its highly efficient performance as NASA's premier X-ray observatory. 27)
- As the name suggests, the OCC controls the operation of the Chandra spacecraft while it is in orbit, as scientists and engineers design plans for efficiently and safely observing its targets.
- Two years before Chandra's launch into space in 1999, NASA awarded the Smithsonian Astrophysical Observatory a contract to establish the first OCC as part of the Chandra X-ray Center, under the direction of NASA's Marshall Space Flight Center. Northrup Grumman was and continues to be a prime contractor for Chandra, employing many staff members at the OCC.
- The first OCC was in a Draper Laboratories building in Cambridge, Mass. Draper decided to expand and not renew the OCC lease due to their own company growth. A location for a new OCC was found in Burlington, Mass., about 10 miles north of Cambridge.
- "It is not a simple move since the activities of the OCC are very complex," said Scott Wolk, one of Chandra's Flight Directors. "We have to consider security, technical and infrastructure requirements, and ultra-high-speed communications."
- The OCC is in contact with the Chandra spacecraft (which is in a highly elliptical orbit around the Earth) about three times per day via the Deep Space Network, or DSN. The DSN consists of three dishes located around the globe that allows NASA to communicate with spacecraft throughout their orbits.
- The new OCC offers an opportunity to modernize certain aspects of the Chandra mission as it celebrates its 20th anniversary in space in 2019, with the design taking two decades of experience operating Chandra into account. The new OCC brings all of the operational teams into one space to facilitate collaboration and situational awareness, but uses glass walls and physical separation to manage sound so individual team members can still effectively perform focused technical work. They also created a purpose-built area for our spacecraft simulator, which was an important upgrade that will serve the mission well going forward.
- "Chandra has always been at the forefront of exploring the Universe in X-rays," said Chandra X-ray Center Director Belinda Wilkes. "This is a chance to enhance our 20th century, but still unique, state-of-the-art mission in space."
- In order to move from the original OCC to the new one, several reviews had to be conducted and passed. In early May, NASA granted approval to operate the new OCC on an interim basis while the original OCC served as a back-up. On May 30th, the final step — called an Operations Readiness Review — occurred and the new OCC became fully operational.
- "This is an exciting day for the Chandra mission," said Christopher Eagan, Manager of the OCC. "We are all looking forward to what Chandra will discover next."
- While Chandra originally had a nominal 5-year mission, a new contract was signed with NASA to potentially extend its extremely successful operations through 2027.
- "The new OCC is a critical accomplishment that reflects tremendous creativity and rigor by the Chandra team," said Helen Cole, Chandra Project Manager at the Marshall Space Flight Center. "Looking forward, it will serve as a cornerstone capability for future scientific discovery by the Chandra mission."
- NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations. The Chandra Flight Operations Team is provided by Northrop Grumman Aerospace Systems.
• May 31, 2019: A group of researchers has identified and characterized for the first time in a complete way a powerful eruption in the atmosphere of the active star HR 9024, marked by an intense flash of X-rays followed by the emission of a giant bubble of plasma, ie hot gas containing charged particles. This is the first time a CME (Coronal Mass Ejection) has been seen in a star other than our Sun. The corona is the outer atmosphere of a star. 28)
- The work, appearing in an article in the latest issue of the journal Nature Astronomy, used data collected by NASA's Chandra X-ray Observatory. The results confirm that CMEs are produced in magnetically active stars and are relevant to stellar physics, and they also open the opportunity to systematically study such dramatic events in stars other than the Sun.
- "The technique we used is based on monitoring the velocity of plasmas during a stellar flare," said Costanza Argiroffi (University of Palermo in Italy and associate researcher at the National Institute for Astrophysics in Italy) who led the study. "This is because, in analogy with the solar environment, it is expected that, during a flare, the plasma confined in the coronal loop where the flare takes place moves first upward, and then downwards reaching the lower layers of the stellar atmosphere. Moreover, there is also expected to be an additional motion, always directed upwards, due to the CME associated with the flare".
- The team analyzed a particularly favorable flare, which took place on the active star HR 9024, about 450 light-years away from us. The HETGS (High-Energy Transmission Grating Spectrometer) aboard Chandra is the only instrument that allows measurements of the motions of coronal plasmas with speeds of just a few tens of thousands of miles per hour.
- The results of this observation clearly show that, during the flare, very hot material (between 18 to 45 million degrees Fahrenheit) first rises and then drops with speeds between 225,000 to 900,000 miles per hour. This is in excellent agreement with the expected behavior for the material linked to the stellar flare.
- "This result, never achieved before, confirms that our understanding of the main phenomena that occur in flares is solid," said Argiroffi. "We were not so confident that our predictions could match in such a way with observations, because our understanding of flares is based almost completely on observations of the solar environment, where the most extreme flares are even a hundred thousand times less intense in the X-radiation emitted".
Figure 26: A giant stellar eruption detected for the first time. This artist's illustration depicts a CME from a star. These events involve a large-scale expulsion of material, and have frequently been observed on the Sun (image credit: NASA/CXC/INAF/Argiroffi, C. et al. Illustration: NASA/GSFC/S. Wiessinger)
- "The most important point of our work, however, is another: we found, after the flare, that the coldest plasma — at a temperature of 'only' seven million degrees Fahrenheit — rose from the star, with a constant speed of about 185,000 miles per hour," said Argiroffi. "And these data are exactly what one would have expected for the CME associated with the flare."
- The Chandra data allowed, in addition to the speed, the mass of the studied CME to be obtained, equal to two billion billion pounds, about ten thousand times greater than the most massive CMEs launched into interplanetary space by the Sun, in agreement with the idea that the CMEs in active stars are larger-scale versions of solar CMEs. The observed speed of the CME, however, is significantly lower than expected. This suggests that the magnetic field in the active stars is probably less efficient in accelerating CMEs than the solar magnetic field.
- These results have been published in the most recent issue of Nature Astronomy and are also available on the arXiv. The co-authors of the paper are Fabio Reale from the University of Palermo in Palermo, Italy, Jeremy Drake from the Center for Astrophysics | Harvard and Smithsonian (CfA), Angela Ciaravella from the National Institute for Astrophysics (INAF) in Palermo, Paola Testa from CfA, Rosaria Bonito from INAF in Palermo, Marco Miceli from the University of Palermo, Salvatore Orlando from INAF in Palermo and Giovanni Peres from the University of Palermo.
• April 16,2019: A bright burst of X-rays has been discovered by NASA's Chandra X-ray Observatory in a galaxy 6.6 billion light years from Earth. This event likely signaled the merger of two neutron stars and could give astronomers fresh insight into how neutron stars — dense stellar objects packed mainly with neutrons — are built. 29)
- When two neutron stars merge they produce jets of high energy particles and radiation fired in opposite directions. If the jet is pointed along the line of sight to the Earth, a flash, or burst, of gamma rays can be detected. If the jet is not pointed in our direction, a different signal is needed to identify the merger.
Figure 27: These images show the location of an event, discovered by NASA's Chandra X-ray Observatory, that likely signals the merger of two neutron stars. A bright burst of X-rays in this source, dubbed XT2, could give astronomers fresh insight into how neutron stars — dense stellar objects packed mainly with neutrons — are built (image credit: X-ray: NASA/CXC/Uni. of Science and Technology of China/Y. Xue et al; Optical: NASA/STScI)
- The detection of gravitational waves — ripples in spacetime — is one such signal. Now, with the observation of a bright flare of X-rays, astronomers have found another signal, and discovered that two neutron stars likely merged to form a new, heavier and fast-spinning neutron star with an extraordinarily strong magnetic field.
- "We've found a completely new way to spot a neutron star merger," said Yongquan Xue of the University of Science and Technology of China and lead author of a paper appearing in Nature. "The behavior of this X-ray source matches what one of our team members predicted for these events." 30)
- Chandra observed the source, dubbed XT2, as it suddenly appeared and then faded away after about seven hours. The source is located in the Chandra Deep Field-South, the deepest X-ray image ever taken that contains almost 12 weeks of Chandra observing time, taken at various intervals over several years. The source appeared on March 22nd, 2015 and was discovered later in analysis of archival data.
- "The serendipitous discovery of XT2 makes another strong case that nature's fecundity repeatedly transcends human imagination,"said co-author Niel Brandt of the Pennsylvania State University and principal investigator of the relevant Chandra Deep Field-South.
- The researchers identified the likely origin of XT2 by studying how its X-ray light varied with time, and comparing this behavior with predictions made in 2013 by Bing Zhang from the University of Nevada in Las Vegas, one of the corresponding authors of the paper. The X-rays showed a characteristic signature that matched those predicted for a newly-formed magnetar — a neutron star spinning around hundreds of times per second and possessing a tremendously strong magnetic field about a quadrillion times that of Earth's.
- The team think that the magnetar lost energy in the form of an X-ray-emitting wind, slowing down its rate of spin as the source faded. The amount of X-ray emission stayed roughly constant in X-ray brightness for about 30 minutes, then decreased in brightness by more than a factor of 300 over 6.5 hours before becoming undetectable. This showed that the neutron star merger produced a new, larger neutron star and not a black hole.
- This result is important because it gives astronomers a chance to learn about the interior of neutron stars, objects that are so dense that their properties could never be replicated on Earth.
- "We can't throw neutron stars together in a lab to see what happens, so we have to wait until the Universe does it for us," said Zhang. "If two neutron stars can collide and a heavy neutron star survives, then this tells us that their structure is relatively stiff and resilient."
- Neutron star mergers have been prominent in the news since the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves from one in 2017. That source, known as GW170817, produced a burst of gamma rays and an afterglow in light detected by many other telescopes, including Chandra. Xue's team think that XT2 would also have been a source of gravitational waves, however it occurred before Advanced LIGO started its first observing run, and it was too distant to have been detected in any case.
- Xue's team also considered whether the collapse of a massive star could have caused XT2, rather than a neutron star merger. The source is in the outskirts of its host galaxy, which aligns with the idea that supernova explosions that left behind the neutron stars kicked them out of the center a few billion years earlier. The galaxy itself also has certain properties — including a low rate of star formation compared to other galaxies of a similar mass — that are much more consistent with the type of galaxy where the merger of two neutron stars is expected to occur. Massive stars are young and are associated with high rates of star formation.
- "The host-galaxy properties of XT2 indeed boost our confidence in explaining its origin,"said co-author Ye Li from Peking University.
- The team estimated the rate at which events like XT2 should occur, and found that it agrees with the rate deduced from the detection of GW170817. However, both estimates are highly uncertain because they depend on the detection of just one object each, so more examples are needed.
- "We've started looking at other Chandra data to see if similar sources are present", said co-author Xuechen Cheng, also of the University of Science and Technology of China. "Just as with this source, the data sitting in archives might contain some unexpected treasures."
• March 21, 2019: Want to take a trip to the center of the Milky Way? Check out a new immersive, ultra-high-definition visualization. This 360º-movie offers an unparalleled opportunity to look around the center of the galaxy, from the vantage point of the central supermassive black hole, in any direction the user chooses. 31)
Figure 28: A new immersive, 360º, ultra-high-definition visualization allows viewers to view the center of our Galaxy as if they were sitting in the position of the Milky Way’s supermassive black hole (Sgr A*). By combining supercomputer simulations with Chandra data, the visualization shows the effects of dozens of massive stellar giants with fierce winds blowing off their surfaces in the region covering a few light years surrounding Sgr A*. Blue and cyan represent X-ray emission from hot gas with temperatures of tens of millions of degrees, while the red emission shows ultraviolet emission from moderately dense regions of cooler gas with temperatures of tens of thousands of degrees, and yellow shows the cooler gas with the highest densities (video credit: NASA/CXC/Pontifical Catholic Univ. of Chile /C. Russell et al.)
- By combining NASA Ames supercomputer simulations with data from NASA's Chandra X-ray Observatory, this visualization provides a new perspective of what is happening in and around the center of the Milky Way. It shows the effects of dozens of massive stellar giants with fierce winds blowing off their surfaces in the region a few light years away from the supermassive black hole known as Sagittarius A* (Sgr A* for short).
- These winds provide a buffet of material for the supermassive black hole to potentially feed upon. As in a previous visualization, the viewer can observe dense clumps of material streaming toward Sgr A*. These clumps formed when winds from the massive stars near Sgr A* collide. Along with watching the motion of these clumps, viewers can watch as relatively low-density gas falls toward Sgr A*. In this new visualization, the blue and cyan colors represent X-ray emission from hot gas, with temperatures of tens of millions of degrees; red shows ultraviolet emission from moderately dense regions of cooler gas, with temperatures of tens of thousands of degrees; and yellow shows of the cooler gas with the highest densities.
- A collection of X-ray-emitting gas is seen to move slowly when it is far away from Sgr A*, and then pick up speed and whip around the viewer as it comes inwards. Sometimes clumps of gas will collide with gas ejected by other stars, resulting in a flash of X-rays when the gas is heated up, and then it quickly cools down. Farther away from the viewer, the movie also shows collisions of fast stellar winds producing X-rays. These collisions are thought to provide the dominant source of hot gas that is seen by Chandra.
- When an outburst occurs from gas very near the black hole, the ejected gas collides with material flowing away from the massive stars in winds, pushing this material backwards and causing it to glow in X-rays. When the outburst dies down the winds return to normal and the X-rays fade.
- The 360-degree video of the Galactic Center is ideally viewed through virtual reality (VR) goggles, such as Samsung Gear VR or Google Cardboard. The video can also be viewed on smartphones using the YouTube app. Moving the phone around reveals a different portion of the movie, mimicking the effect in the VR goggles. Finally, most browsers on a computer also allow 360-degree videos to be shown on YouTube. To look around, either click and drag the video, or click the direction pad in the corner.
- Dr. Christopher Russell of the Pontificia Universidad Católica de Chile (Pontifical Catholic University) presented the new visualization at the 17th meeting of the High-Energy Astrophysics (HEAD) of the American Astronomical Society held in Monterey, Calif. NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.
- The source of the cosmic squall is a supermassive black hole buried at the center of the galaxy, officially known as SDSS 1430+1339. As matter in the central regions of the galaxy is pulled toward the black hole, it is energized by the strong gravity and magnetic fields near the black hole. The infalling material produces more radiation than all the stars in the host galaxy. This kind of actively growing black hole is known as a quasar.
- Located about 1.1 billion light years from Earth, the Teacup's host galaxy was originally discovered in visible light images by citizen scientists in 2007 as part of the Galaxy Zoo project, using data from the Sloan Digital Sky Survey. Since then, professional astronomers using space-based telescopes have gathered clues about the history of this galaxy with an eye toward forecasting how stormy it will be in the future. This new composite image contains X-ray data from Chandra (blue) along with an optical view from NASA's Hubble Space Telescope (red and green).
- Located about 1.1 billion light years from Earth, the Teacup's host galaxy was originally discovered in visible light images by citizen scientists in 2007 as part of the Galaxy Zoo project, using data from the Sloan Digital Sky Survey. Since then, professional astronomers using space-based telescopes have gathered clues about the history of this galaxy with an eye toward forecasting how stormy it will be in the future. This new composite image contains X-ray data from Chandra (blue) along with an optical view from NASA's Hubble Space Telescope (red and green).
- The "handle" of the Teacup is a ring of optical and X-ray light surrounding a giant bubble. This handle-shaped feature, which is located about 30,000 light-years from the supermassive black hole, was likely formed by one or more eruptions powered by the black hole. Radio emission — shown in a separate composite image with the optical data — also outlines this bubble, and a bubble about the same size on the other side of the black hole.
- Previously, optical telescope observations showed that atoms in the handle of the Teacup were ionized, that is, these particles became charged when some of their electrons were stripped off, presumably by the quasar's strong radiation in the past. The amount of radiation required to ionize the atoms was compared with that inferred from optical observations of the quasar. This comparison suggested that the quasar's radiation production had diminished by a factor of somewhere between 50 and 600 over the last 40,000 to 100,000 years. This inferred sharp decline led researchers to conclude that the quasar in the Teacup was fading or dying.
- New data from Chandra and ESA's XMM-Newton mission are giving astronomers an improved understanding of the history of this galactic storm. The X-ray spectra (that is, the amount of X-rays over a range of energies) show that the quasar is heavily obscured by gas. This implies that the quasar is producing much more ionizing radiation than indicated by the estimates based on the optical data alone, and that rumors of the quasar's death may have been exaggerated. Instead the quasar has dimmed by only a factor of 25 or less over the past 100,000 years.
- The Chandra data also show evidence for hotter gas within the bubble, which may imply that a wind of material is blowing away from the black hole. Such a wind, which was driven by radiation from the quasar, may have created the bubbles found in the Teacup.
- Astronomers have previously observed bubbles of various sizes in elliptical galaxies, galaxy groups and galaxy clusters that were generated by narrow jets containing particles traveling near the speed of light, that shoot away from the supermassive black holes. The energy of the jets dominates the power output of these black holes, rather than radiation.
- In these jet-driven systems, astronomers have found that the power required to generate the bubbles is proportional to their X-ray brightness. Surprisingly, the radiation-driven Teacup quasar follows this pattern. This suggests radiation-dominated quasar systems and their jet-dominated cousins can have similar effects on their galactic surroundings.
- A study describing these results was published in the March 20, 2018 issue of The Astrophysical Journal Letters and is available online. The authors are George Lansbury from the University of Cambridge in Cambridge, UK; Miranda E. Jarvis from the Max-Planck Institut für Astrophysik in Garching, Germany; Chris M. Harrison from the European Southern Observatory in Garching, Germany; David M. Alexander from Durham University in Durham, UK; Agnese Del Moro from the Max-Planck-Institut für Extraterrestrische Physik in Garching, Germany; Alastair Edge from Durham University in Durham, UK; James R. Mullaney from The University of Sheffield in Sheffield, UK and Alasdair Thomson from the University of Manchester, Manchester, UK. 34)
Figure 29: Composite optical/X-ray image of a storm raging in a cosmic tea cup(image credit: NASA/CXC/Univ. of Cambridge/G. Lansbury et al; Optical: NASA/STScI/W. Keel et al.)
Figure 30: Animation of a quasar – nicknamed the Teacup because of its shape – is causing a storm in galaxy about 1.1 billion light-years from Earth. The power source of the quasar, astronomers say, is a supermassive black hole at the galaxy’s center (video credit: X-ray: NASA/CXC/Univ. of Cambridge/G. Lansbury et al; Optical: NASA/STScI/W. Keel et al.)
• February 14, 2019: New results from NASA's Chandra X-ray Observatory may have helped solve the Universe's "missing mass" problem, as reported in our latest press release. Astronomers cannot account for about a third of the normal matter — that is, hydrogen, helium, and other elements — that were created in the first billion years or so after the Big Bang. 35)
- Scientists have proposed that the missing mass could be hidden in gigantic strands or filaments of warm (temperature less than 100,000 Kelvin) and hot (temperature greater than 100,000 K) gas in intergalactic space. These filaments are known by astronomers as the WHIM (Warm-Hot Intergalactic Medium). They are invisible to optical light telescopes, but some of the warm gas in filaments has been detected in ultraviolet light. The main part of this graphic is from the Millennium simulation, which uses supercomputers to formulate how the key components of the Universe, including the WHIM, would have evolved over cosmic time.
- If these filaments exist, they could absorb certain types of light such as X-rays that pass through them. The inset in this graphic represents some of the X-ray data collected by Chandra from a distant, rapidly-growing supermassive black hole known as a quasar. The plot is a spectrum — the amount of X-rays over a range of wavelengths — from a new study of the quasar H1821+643 that is located about 3.4 billion light years from Earth.
- The latest result uses a new technique that both hones the search for the WHIM carefully and boosts the relatively weak absorption signature by combining different parts of the spectrum to find a valid signal. With this technique, researchers identified 17 possible filaments lying between the quasar and Earth, and obtained their distances.
- For each filament the spectrum was shifted in wavelength to remove the effects of cosmic expansion, and then the spectra of all the filaments were added together so that the resulting spectrum has a much stronger signal from absorption by the WHIM than in the individual spectra.
- Indeed, the team did not find absorption in the individual spectra. But by adding them together, they turned a 5.5-day-long observation into the equivalent of almost 100 days' worth (about 8 million seconds) of data. This revealed an absorption line from oxygen expected to be present in a gas with a temperature of about one million Kelvin.
- By extrapolating from these observations of oxygen to the full set of elements, and from the observed region to the local Universe, the researchers report they can account for the complete amount of missing matter. A paper describing these results was published in The Astrophysical Journal on February 13, 2019. 36)
- NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.
Figure 31: A quick look at where is the Universe hiding its Missing Mass? (video credit: produced by CXC; animations: ESO/M. Kornmesser; NASA/GSFC/D. Berry)
Figure 32: The Universe's "missing mass" may have been found, according to a new study using Chandra data. About a third of the "normal" matter (ie, hydrogen, helium, and other elements) created shortly after the Big Bang is not seen in the present-day Universe. One idea is that this missing mass is today in filaments of warm and hot gas known as the WHIM. Researchers suggest evidence for the WHIM is seen in absorption features in X-rays collected from a quasar billions of light years away (image credit: Illustration: Springel et al. (2005); Spectrum: NASA/CXC/CfA/Kovács et al.)
• January 29, 2019: A new study using data from NASA's Chandra X-ray Observatory and ESA's XMM-Newton suggests that dark energy may have varied over cosmic time, as reported in our latest press release. This artist's illustration helps explain how astronomers tracked the effects of dark energy to about one billion years after the Big Bang by determining the distances to quasars, rapidly growing black holes that shine extremely brightly. 37)
Figure 33: Dark energy, a proposed force or energy that permeates all space and accelerates the Universe's expansion, may vary over time. A new study combining X-rays from Chandra and XMM-Newton and ultraviolet and optical data from SDSS (Sloan Digital Sky Survey) provides distances to quasars. These quasars are observed back to times about a billion years after the Big Bang. The new result showed that dark energy's effect on the expansion rate in the early Universe may have been different from today (image credit: NASA/CXC/M.Weiss; X-ray: NASA/CXC/Univ. of Florence/G.Risaliti & E.Lusso)
- First discovered about 20 years ago by measuring the distances to exploded stars called supernovas, dark energy is a proposed type of force, or energy, that permeates all space and causes the expansion of the Universe to accelerate. Using this method, scientists tracked the effects of dark energy out to about 9 billion years ago.
- The latest result stems from the development of a new method to determine distances to about 1,598 quasars, which allows the researchers to measure dark energy's effects from the early Universe through to the present day. Two of the most distant quasars studied are shown in Chandra images in the insets.
- The new technique uses ultraviolet (UV) and X-ray data to estimate the quasar distances. In quasars, a disk of matter around the supermassive black hole in the center of a galaxy produces UV light (shown in the illustration in blue). Some of the UV photons collide with electrons in a cloud of hot gas (shown in yellow) above and below the disk, and these collisions can boost the energy of the UV light up to X-ray energies. This interaction causes a correlation between the amounts of observed UV and X-ray radiation. This correlation depends on the luminosity of the quasar, which is the amount of radiation it produces.
- Using this technique the quasars become standard candles, as implied by the artist's illustration, showing quasars with the same luminosity at different distances from Earth. Once the luminosity is known, the distance to the quasars can be calculated. This is because the observed amount of radiation from quasars converted into standard candles depend on their distance from Earth in a predictable way.
- The researchers compiled UV data for 1,598 quasars to derive a relationship between UV and X-ray fluxes, and the distances to the quasars. They then used this information to study the expansion rate of the universe back to very early times, and found evidence that the amount of dark energy is growing with time.
- Since this is a new technique, the astronomers took extra steps to show that this method gives reliable results. They showed that results from their technique match up with those from supernova measurements over the last 9 billion years, giving them confidence that their results are reliable at even earlier times. The researchers also took great care in how their quasars were selected, to minimize statistical errors and to avoid systematic errors that might depend on the distance from Earth to the object. 38)
• January 9, 2019: Astronomers have discovered behavior by a jet from a giant black hole that has never been seen before. Using NASA's Chandra X-ray Observatory they have observed a jet that bounced off a wall of gas and then punched a hole in a cloud of energetic particles. This behavior can tell scientists more about how jets from black holes interact with their surroundings. 39) 40)
- The discovery was made in Cygnus A, a large galaxy in the middle of a cluster of galaxies about 760 million light years from Earth. Chandra data show powerful jets of particles and electromagnetic energy blasting away from a rapidly growing black hole at the center of Cygnus A. After traveling more than 200,000 light years on either side of the black hole, the jets have slowed down via its interaction with multimillion-degree intergalactic gas that envelopes Cygnus A. This interaction has produced enormous clouds of energetic particles that emits X-rays and radio waves.
- In a deep observation that lasted 23 days, scientists used Chandra to create a highly detailed map of both the jet and the intergalactic gas, which they used to track the path of the jets from the black hole. The jet on the left expanded after ricocheting and created a hole in the surrounding cloud of particles that is between 50,000 and 100,000 light years deep and only 26,000 light years wide.
- "Not only did we see this black hole jet rebound off intergalactic gas, in much the same way that a pebble would skip or ricochet off the surface of a pond, it then punched a hole in a cloud of energetic particles," said Amalya Johnson of Columbia University in New York, who led the new study. "This is the first time we've seen such a cosmic hole punch."
- Chandra's sharp imaging was crucial for this discovery. "It's remarkable that Chandra can capture intricate details in X-rays about what this black hole is doing more than a billion trillion miles away from Earth," said co-author Paul Nulsen of the Center for Astrophysics Harvard and Smithsonian (CfA) in Cambridge, Mass. "Thanks to the Chandra data we can see where the jet ricocheted and follow its path before striking the gas a second time."
- While best known for pulling things toward it, black holes are also adept at blasting material away from themselves. As the black hole spins, it can produce a rotating, tightly-wound vertical tower of powerful magnetic fields. This allows the black hole to redirect some of the energy released by gas spiraling toward it, creating an energetic jet traveling at very high speeds away from the black hole. The Cygnus A jet is one of the largest and most powerful ever observed.
- The scientists are working to determine what forms of energy — kinetic energy, heat or radiation — the jet carries. The composition of the jet and the types of energy determine how the jet behaves when it ricochets, such as the size of the hole it creates. Theoretical models of the jet and its interactions with surrounding gas are needed to make conclusions about the jet's properties.
- Energy produced by jets from black holes can heat intergalactic gas in galaxy clusters and prevent it from cooling and forming large numbers of stars in a central galaxy like Cygnus A. "We know that rapidly growing black holes have a large effect on their environment," said co-author Bradford Snios, also of the CfA. "By studying Cygnus A we hope to learn more about how giant black holes affect their host galaxy over time."
Figure 34: X-ray and optical composite. A ricocheting jet blasting from a giant black hole has been captured by Chandra. These images of Cygnus A show X-rays from Chandra and an optical view from Hubble of the galaxies and stars in the same field of view. Chandra's data reveal the presence of a powerful jet of particles and electromagnetic energy that has shot out from the black hole and slammed into a wall of hot gas, then ricocheted to punch a hole in a cloud of energetic particles, before it collides with another part of the gas wall (image credit: X-ray: NASA/CXC/Columbia Univ./A. Johnson et al.; Optical: NASA/STScI)
- Figure 34 shows the location of the supermassive black hole, the jets, the point that the jet on the left ricocheted off a wall of intergalactic gas ("hotspot E"), and the point where the jet then struck the intergalactic gas a second time ("hotspot D"). The inset contains a close-up view of the hotspots on the left and the hole punched by the rebounding jet, which surrounds hotspot E. The image in the inset combines X-rays from all three energy ranges to give the greatest sensitivity to show fine structures such as the hole.
- The hole is visible because the path of the rebounding jet between hotspots E and D is almost directly along the line of sight to Earth, as shown by the schematic figure depicting the view of Cygnus A from above. A similar rebounding of the jet likely occurred between hotspots A and B but the hole is not visible because the path is not along the Earth's line of sight.
Figure 35: Black holes are notorious for pulling things toward them. But in some cases, black holes can act as powerful engines to blast material away. One of those black holes is found in Cygnus A, a large galaxy embedded within in a cluster of galaxies. Cygnus A's black hole is blasting a jet — a tightly-wound column of material — away from it at extremely high speeds. Astronomers found that his jet ricocheted off a wall of hot gas, then punched a hole in a cloud of energetic particles, leaving behind a gigantic hole (video credit: Chandra X-ray Observatory, Published on Jan 9, 2019)
• October 24, 2018: On the evening of October 21, Chandra returned to science observations after the team successfully carried out a procedure to enable a new gyroscope configuration for the spacecraft. The team initiated a set of maneuvers to change the pointing and orientation of the spacecraft to confirm that the gyroscopes were behaving as expected. During the coming week, scientists will collect spacecraft data to fine-tune the performance for the new gyroscope configuration. As a final step, the team will uplink a software patch to apply any necessary adjustments to the on-board computer. 41)
- October 15 update: The cause of Chandra's safe mode on October 10 has now been understood and the Operations team has successfully returned the spacecraft to its normal pointing mode. The safe mode was caused by a glitch in one of Chandra's gyroscopes resulting in a 3-second period of bad data that in turn led the on-board computer to calculate an incorrect value for the spacecraft momentum. The erroneous momentum indication then triggered the safe mode.
• October 16, 2018: About a year ago, astronomers excitedly reported the first detection of electromagnetic waves, or light, from a gravitational wave source. Now, a year later, researchers are announcing the existence of a cosmic relative to that historic event. 42)
- The discovery was made using data from telescopes including NASA’s Chandra X-ray Observatory, Fermi Gamma-ray Space Telescope, Neil Gehrels Swift Observatory, the NASA Hubble Space Telescope (HST), and the Discovery Channel Telescope (DCT).
- The object of the new study, called GRB 150101B, was first reported as a gamma-ray burst detected by Fermi in January 2015. This detection and follow-up observations at other wavelengths show GRB 150101B shares remarkable similarities to the neutron star merger and gravitational wave source discovered by Advanced Laser Interferometer Gravitational Wave Observatory (LIGO) and its European counterpart Virgo in 2017 known as GW170817. The latest study concludes that these two separate objects may, in fact, be related.
- "It's a big step to go from one detected object to two," said Eleonora Troja, lead author of the study from NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and the University of Maryland at College Park (UMCP). "Our discovery tells us that events like GW170817 and GRB 150101B could represent a whole new class of erupting objects that turn on and off in X-rays and might actually be relatively common."
- Troja and her colleagues think both GRB 150101B and GW170817 were most likely produced by the same type of event: the merger of two neutron stars, a catastrophic coalescence that generated a narrow jet, or beam, of high-energy particles. The jet produced a short, intense burst of gamma rays (known as a short GRB), a high-energy flash that can last only seconds. GW170817 proved that these events may also create ripples in space-time itself called gravitational waves.
- The apparent match between GRB 150101B and GW170817 is striking: both produced an unusually faint and short-lived gamma ray burst, and both were a source of bright, blue optical light lasting a few days, and X-ray emission lasted much longer. The host galaxies are also remarkably similar, based on Hubble Space Telescope and DCT observations. Both are bright elliptical galaxies with a population of stars a few billion years old and displaying no evidence for new stars forming.
- "We have a case of cosmic lookalikes," said co-author Geoffrey Ryan of UMCP. "They look the same, act the same and come from similar neighborhoods, so the simplest explanation is that they are from the same family of objects."
Figure 36: A distant cosmic relative to the first source that astronomers detected in both gravitational waves and light may have been discovered, as reported in our latest press release. This object, called GRB 150101B, was first detected by identified as a gamma ray burst (GRB) by NASA's Fermi Gamma-ray Space Telescope in January 2015. This image shows data from NASA’s Chandra X-ray Observatory (purple in the inset boxes) in context with an optical image of GRB 150101B from the Hubble Space Telescope (image credit: X-ray: NASA/CXC/GSFC/UMC/E. Troja et al.; Optical and infrared: NASA/STScI)
- In the cases of both GRB 150101B and GW170817, the slow rise in the X-ray emission compared to most GRBs implies that the explosion was likely viewed "off-axis," that is, with the jet not pointing directly towards the Earth. The discovery of GRB150101 represents only the second time astronomers have ever detected an off-axis short GRB.
- The latest study concludes that these two separate objects may, in fact, be related. The discovery suggests that events like GW170817 and GRB 150101B could represent a whole new class of erupting objects that turn on and off in X-rays and might actually be relatively common.
- The researchers think both GRB 150101B and GW170817 were most likely produced by the same type of event: the merger of two neutron stars, a catastrophic coalescence that generated a narrow jet, or beam, of high-energy particles. The jet produced a short, intense burst of gamma rays (known as a short GRB), a high-energy flash that can last only seconds. GW170817 proved that these events may also create ripples in space-time itself called gravitational waves.
- While there are many commonalities between GRB 150101B and GW170817, there are two very important differences. One is their location. GW170817 is about 130 million light years from Earth, while GRB 150101B lies about 1.7 billion light years away. Even if Advanced LIGO had been operating in early 2015, it would very likely not have detected gravitational waves from GRB 150101B because of its greater distance.
- It is possible that a few mergers like the ones seen in GW170817 and GRB 150101B had been detected as short GRBs before but had not been identified with other telescopes. Without detections at longer wavelengths like X-rays or optical light, GRB positions are not accurate enough to determine what galaxy they are located in.
- In the case of GRB 150101B, astronomers thought at first that the counterpart was an X-ray source detected by Swift in the center of the galaxy, likely from material falling into a supermassive black hole. However, follow-up observations with Chandra, with its sharp X-ray resolution, detected the true counterpart away from the center of the host galaxy. This can be seen in the Chandra images. Not only has the source dimmed dramatically, it is clearly outside the center of the galaxy, which appears as the constant brighter source to the upper right. 43)
• October 12, 2018: At approximately 13:55 GMT on 10 October 2018, NASA’s Chandra X-ray Observatory entered Safe Mode, where the telescope’s instruments are put into a safe configuration, critical hardware is swapped to back-up units, the spacecraft points so that the solar panels get maximum sunlight, and the mirrors point away from the Sun. Analysis of available data indicates the transition to safe mode was nominal, i.e., consistent with normal behavior for such an event. All systems functioned as expected and the scientific instruments are safe. The cause of the Safe Mode transition is currently under investigation, and we will post more information when it becomes available. 44)
- Chandra is 19 years old, which is well beyond the original design lifetime of 5 years. In 2001, NASA extended its lifetime to 10 years. It is now well into its extended mission and it is expected to continue carrying out forefront science for many years to come.
• September 25, 2018: NASA has awarded a contract extension to the Smithsonian Astrophysical Observatory (SAO) in Cambridge, Massachusetts, to continue operations and science support for the agency's Chandra X-ray Observatory. 45)
- The contract extends the agreement between NASA and SAO through Sept. 30, 2024, followed by two three-year options that would extend the contract through Sept. 30, 2030. The total potential value of the contract extension is $563.5 million.
- This contract covers continued operation of the Chandra X-ray Center (CXC) in Cambridge, which conducts key aspects of Chandra's observation, operations and research program. Core functions of the CXC include system engineering, ground system development and maintenance, mission operations, science and operations planning, science research and dissemination, and outreach support.
- Since its launch on July 23, 1999, Chandra has been NASA's flagship mission for X-ray astronomy and is among NASA's fleet of Great Observatories. The agency's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for the agency's Science Mission Directorate in Washington.
• June 21, 2018: A new study using data from NASA's Chandra X-ray Observatory indicates that black holes have squelched star formation in small, yet massive galaxies known as "red nuggets", as reported in our latest press release. The results suggest some red nugget galaxies may have used some of the untapped stellar fuel to grow their central supermassive black holes to unusually massive proportions. 46) 47) 48)
- Red nuggets are relics of the first massive galaxies that formed within only one billion years after the Big Bang. While most red nuggets merged with other galaxies over billions of years, a small number remained solitary. These relatively pristine red nuggets allow astronomers to study how the galaxies — and the supermassive black hole at their centers — act over billions of years of isolation.
- In the latest research, astronomers used Chandra to study the hot gas in two of these isolated red nuggets, Mrk 1216, and PGC 032673. (The Chandra data, colored red, of Mrk 1216 is shown in the inset.) These two galaxies are located only 295 million and 344 million light years from Earth, respectively, rather than billions of light years for the first known red nuggets, allowing for a more detailed look. The gas in the galaxy is heated to such high temperatures that it emits brightly in X-ray light, which Chandra detects. This hot gas contains the imprint of activity generated by the supermassive black holes in each of the two galaxies.
Figure 37: An artist's illustration (main panel) shows how material falling towards black holes can be redirected outward at high speeds due to intense gravitational and magnetic fields. These high-speed jets can tamp down the formation of stars. This happens because the blasts from the vicinity of the black hole provide a powerful source of heat, preventing the galaxy's hot interstellar gas from cooling enough to allow large numbers of stars to form (image credit: X-ray: NASA/CXC/MTA-Eötvös University/N. Werner et al.; Illustration: NASA/CXC/M.Weiss)
• September 6, 2017: A new study using data from NASA's Chandra X-ray Observatory and ESA's XMM-Newton suggests X-rays emitted by a planet's host star may provide critical clues to just how hospitable a star system could be. A team of researchers looked at 24 stars similar to the Sun, each at least one billion years old, and how their X-ray brightness changed over time. 49) 50)
Figure 38: This artist's illustration depicts one of these comparatively calm, older Sun-like stars with a planet in orbit around it. The large dark area is a "coronal hole", a phenomenon associated with low levels of magnetic activity. The inset box shows the Chandra data of one of the observed objects, a two billion year old star called GJ 176, located 30 light years from Earth (image credit: X-ray: NASA/CXC/Queens Univ. of Belfast/R.Booth, et al.; Illustration: NASA/CXC/M.Weiss)
- Since stellar X-rays mirror magnetic activity, X-ray observations can tell astronomers about the high-energy environment around the star. In the new study the X-ray data from Chandra and XMM-Newton revealed that stars like the Sun and their less massive cousins calm down surprisingly quickly after a turbulent youth.
- To understand how quickly stellar magnetic activity level changes over time, astronomers need accurate ages for many different stars. This is a difficult task, but new precise age estimates have recently become available from studies of the way that a star pulsates using NASA's Kepler and ESA's CoRoT missions. These new age estimates were used for most of the 24 stars studied here.
- Astronomers have observed that most stars are very magnetically active when they are young, since the stars are rapidly rotating. As the rotating star loses energy over time, the star spins more slowly and the magnetic activity level, along with the associated X-ray emission, drops.
- Although it is not certain why older stars settle down relatively quickly, astronomers have ideas they are exploring. One possibility is that the decrease in rate of spin of the older stars occurs more quickly than it does for the younger stars. Another possibility is that the X-ray brightness declines more quickly with time for older, more slowly rotating stars than it does for younger stars.
• August 16, 2016: A new look at the debris from an exploded star in our galaxy has astronomers re-examining when the supernova actually happened. Recent observations of the supernova remnant called G11.2-0.3 with NASA's Chandra X-ray Observatory have stripped away its connection to an event recorded by the Chinese in 386 CE. 51) 52)
- Historical supernovas and their remnants can be tied to both current astronomical observations as well as historical records of the event. Since it can be difficult to determine from present observations of their remnant exactly when a supernova occurred, historical supernovas provide important information on stellar timelines. Stellar debris can tell us a great deal about the nature of the exploded star, but the interpretation is much more straightforward given a known age.
- New Chandra data on G11.2-0.3 show that dense clouds of gas lie along the line of sight from the supernova remnant to Earth. Infrared observations with the Palomar 5-meter Hale Telescope had previously indicated that parts of the remnant were heavily obscured by dust. This means that the supernova responsible for this object would simply have appeared too faint to be seen with the naked eye in 386 CE. This leaves the nature of the observed 386 CE event a mystery.
- Taking advantage of Chandra's successful operations since its launch into space in 1999, astronomers were able to compare observations of G11.2-0.3 from 2000 to those taken in 2003 and more recently in 2013. This long baseline allowed scientists to measure how fast the remnant is expanding. Using this data to extrapolate backwards, they determined that the star that created G11.2-0.3 exploded between 1,400 and 2,400 years ago as seen from Earth.
- Previous data from other observatories had shown this remnant is the product of a "core-collapse" supernova, one that is created from the collapse and explosion of a massive star. The revised timeframe for the explosion based on the recent Chandra data suggests that G11.2-0.3 is one of the youngest such supernovas in the Milky Way. The youngest, Cassiopeia A, also has an age determined from the expansion of its remnant, and like G11.2-0.3 was not seen at its estimated explosion date of 1680 CE due to dust obscuration. So far, the Crab nebula, the remnant of a supernova seen in 1054 CE, remains the only firmly identified historical remnant of a massive star explosion in our galaxy.
- This latest image of G11.2-0.3 shows low-energy X-rays in red, the medium range in green, and the high-energy X-rays detected by Chandra in blue. The X-ray data have been overlaid on an optical field from the Digitized Sky Survey, showing stars in the foreground.
- Although the Chandra image appears to show the remnant has a very circular, symmetrical shape, the details of the data indicate that the gas that the remnant is expanding into is uneven. Because of this, researchers propose that the exploded star had lost almost all of its outer regions, either in an asymmetric wind of gas blowing away from the star, or in an interaction with a companion star. They think the smaller star left behind would then have blown gas outwards at an even faster rate, sweeping up gas that was previously lost in the wind, forming the dense shell. The star would then have exploded, producing the G11.2-0.3 supernova remnant seen today.
Figure 39: New Chandra data of the supernova remnant G11.2-0.3 raise new questions about the timing of its origin (image credit: X-ray: NASA/CXC/NCSU/K. Borkowski et al; Optical: DSS)
• July 22, 2014: Fifteen years ago, NASA's Chandra X-ray Observatory was launched into space aboard the Space Shuttle Columbia. Since its deployment on July 23, 1999, Chandra has helped revolutionize our understanding of the universe through its unrivaled X-ray vision. 53) 54)
Figure 40: The images of the Tycho and G292.0+1.8 supernova remnants show how Chandra can trace the expanding debris of an exploded star and the associated shock waves that rumble through interstellar space at speeds of millions of miles per hour. The images of the Crab Nebula and 3C 58 show how extremely dense, rapidly rotating neutron stars produced when a massive star explodes can create clouds of high-energy particles light years across that glow brightly in X-rays (image credit: NASA/CXC/SAO)
- Tycho: More than four centuries after Danish astronomer Tycho Brahe first observed the supernova that bears his name, the supernova remnant it created is now a bright source of X-rays. The supersonic expansion of the exploded star produced a shock wave moving outward into the surrounding interstellar gas, and another, reverse shock wave moving back into the expanding stellar debris. This Chandra image of Tycho reveals the dynamics of the explosion in exquisite detail. The outer shock has produced a rapidly moving shell of extremely high-energy electrons (blue), and the reverse shock has heated the expanding debris to millions of degrees (red and green). There is evidence from the Chandra data that these shock waves may be responsible for some of the cosmic rays – ultra-energetic particles – that pervade the Galaxy and constantly bombard the Earth.
- G292.0+1.8: At a distance of about 20,000 light years, G292.0+1.8 is one of only three supernova remnants in the Milky Way known to contain large amounts of oxygen. These oxygen-rich supernovas are of great interest to astronomers because they are one of the primary sources of the heavy elements (that is, everything other than hydrogen and helium) necessary to form planets and people. The X-ray image from Chandra shows a rapidly expanding, intricately structured, debris field that contains, along with oxygen (yellow and orange), other elements such as magnesium (green) and silicon and sulfur (blue) that were forged in the star before it exploded.
- The Crab Nebula: In 1054 AD, Chinese astronomers and others around the world noticed a new bright object in the sky. This “new star” was, in fact, the supernova explosion that created what is now called the Crab Nebula. At the center of the Crab Nebula is an extremely dense, rapidly rotating neutron star left behind by the explosion. The neutron star, also known as a pulsar, is spewing out a blizzard of high-energy particles, producing the expanding X-ray nebula seen by Chandra. In this new image, lower-energy X-rays from Chandra are red, medium energy X-rays are green, and the highest-energy X-rays are blue.
- 3C58: 3C58 is the remnant of a supernova observed in the year 1181 AD by Chinese and Japanese astronomers. This new Chandra image shows the center of 3C58, which contains a rapidly spinning neutron star surrounded by a thick ring, or torus, of X-ray emission. The pulsar also has produced jets of X-rays blasting away from it to both the left and right, and extending trillions of miles. These jets are responsible for creating the elaborate web of loops and swirls revealed in the X-ray data. These features, similar to those found in the Crab, are evidence that 3C58 and others like it are capable of generating both swarms of high-energy particles and powerful magnetic fields. In this image, low, medium, and high-energy X-rays detected by Chandra are red, green, and blue respectively.
• March 5, 2014: Astronomers have used NASA's Chandra X-ray Observatory and the European Space Agency's (ESA's) XMM-Newton to show a supermassive black hole six billion light years from Earth is spinning extremely rapidly. This first direct measurement of the spin of such a distant black hole is an important advance for understanding how black holes grow over time. 55) 56)
- Black holes are defined by just two simple characteristics: mass and spin. While astronomers have long been able to measure black hole masses very effectively, determining their spins has been much more difficult.
- In the past decade, astronomers have devised ways of estimating spins for black holes at distances greater than several billion light-years away, meaning we see the region around black holes as they were billions of years ago. However, determining the spins of these remote black holes involves several steps that rely on one another.
Figure 41: Multiple images of a distant quasar are visible in this combined view from NASA's Chandra X-ray Observatory and the Hubble Space Telescope. The Chandra data, along with data from ESA's XMM-Newton, were used to directly measure the spin of the supermassive black hole powering this quasar. This is the most distant black hole where such a measurement has been made, as reported in our press release (image credit: X-ray: NASA/CXC/Univ of Michigan/R. C. Reis et al; Optical: NASA/STScI)
- "We want to be able to cut out the middle man, so to speak, of determining the spins of black holes across the universe," said Rubens Reis of the University of Michigan in Ann Arbor, who led a paper describing this result that was published online Wednesday in the journal Nature.
- Reis and his colleagues determined the spin of the supermassive black hole that is pulling in surrounding gas, producing an extremely luminous quasar known as RX J1131-1231 (RX J1131 for short). Because of fortuitous alignment, the distortion of space-time by the gravitational field of a giant elliptical galaxy along the line of sight to the quasar acts as a gravitational lens that magnifies the light from the quasar. Gravitational lensing, first predicted by Einstein, offers a rare opportunity to study the innermost region in distant quasars by acting as a natural telescope and magnifying the light from these sources.
- "Because of this gravitational lens, we were able to get very detailed information on the X-ray spectrum – that is, the amount of X-rays seen at different energies – from RX J1131," said co-author Mark Reynolds also of Michigan. "This in turn allowed us to get a very accurate value for how fast the black hole is spinning."
- The X-rays are produced when a swirling accretion disk of gas and dust that surrounds the black hole creates a multimillion-degree cloud, or corona near the black hole. X-rays from this corona reflect off the inner edge of the accretion disk. The strong gravitational forces near the black hole alter the reflected X-ray spectrum. The larger the change in the spectrum, the closer the inner edge of the disk must be to the black hole.
- "We estimate that the X-rays are coming from a region in the disk located only about three times the radius of the event horizon, the point of no return for infalling matter," said Jon M. Miller of Michigan, another author on the paper. "The black hole must be spinning extremely rapidly to allow a disk to survive at such a small radius."
- For example, a spinning black hole drags space around with it and allows matter to orbit closer to the black hole than is possible for a non-spinning black hole.
- By measuring the spin of distant black holes researchers discover important clues about how these objects grow over time. If black holes grow mainly from collisions and mergers between galaxies, they should accumulate material in a stable disk, and the steady supply of new material from the disk should lead to rapidly spinning black holes. In contrast, if black holes grow through many small accretion episodes, they will accumulate material from random directions. Like a merry go round that is pushed both backwards and forwards, this would make the black hole spin more slowly.
- The discovery that the black hole in RX J1131 is spinning at over half the speed of light suggests this black hole, observed at a distance of six billion light years, corresponding to an age about 7.7 billion years after the Big Bang, has grown via mergers, rather than pulling material in from different directions.
- The ability to measure black hole spin over a large range of cosmic time should make it possible to directly study whether the black hole evolves at about the same rate as its host galaxy. The measurement of the spin of the RX J1131-1231 black hole is a major step along that path and demonstrates a technique for assembling a sample of distant supermassive black holes with current X-ray observatories.
• August 15, 2012: Astronomers have found an extraordinary galaxy cluster, one of the largest objects in the universe, that is breaking several important cosmic records. Observations of the Phoenix cluster with NASA's Chandra X-ray Observatory, the National Science Foundation's South Pole Telescope, and eight other world-class observatories may force astronomers to rethink how these colossal structures and the galaxies that inhabit them evolve. 57)
- This galaxy cluster has been dubbed the "Phoenix Cluster" because it is located in the constellation of the Phoenix, and because of its remarkable properties, as explained here and in our press release. Stars are forming in the Phoenix Cluster at the highest rate ever observed for the middle of a galaxy cluster. The object is also the most powerful producer of X-rays of any known cluster, and among the most massive of clusters. The data also suggest that the rate of hot gas cooling in the central regions of the cluster is the largest ever observed.
- Like other galaxy clusters, Phoenix contains a vast reservoir of hot gas — containing more normal matter than all of the galaxies in the cluster combined — that can only be detected with X-ray telescopes like Chandra. This hot gas is giving off copious amounts of X-rays and cooling quickly over time, especially near the center of the cluster, causing gas to flow inwards and form huge numbers of stars. These features are shown in the artist's impression of the central galaxy, with hot gas in red, cooler gas in blue. The gas flows appear as the ribbon-like features and the newly formed stars are blue. An animation portrays the process of cooling and star formation in action. A close-up of the middle of the optical and UV image shows that the central galaxy has much bluer colors than the nearby galaxies in the cluster, revealing the presence of large numbers of hot, massive stars forming.
- These results are striking because most galaxy clusters have formed very few stars over the last few billion years. Astronomers think that the supermassive black hole in the central galaxy of clusters pumps energy into the system. The famous Perseus Cluster is an example of a black hole bellowing out energy and preventing the gas from cooling to form stars at a high rate. Repeated outbursts from the black hole in the center of Perseus in the form of powerful jets, created giant cavities and produced sound waves with an incredibly deep B-flat note 57 octaves below middle C. Shock waves, akin to sonic booms in Earth's atmosphere, and the very deep sound waves release energy into the gas in Perseus, preventing most of it from cooling.
Figure 42: The image on the left shows the newly discovered Phoenix Cluster, located about 5.7 billion light years from Earth. This composite includes an X-ray image from NASA's Chandra X-ray Observatory in purple, an optical image from the 4m Blanco telescope in red, green and blue, and an ultraviolet (UV) image from NASA's Galaxy Evolution Explorer (GALEX) in blue. The Chandra data show hot gas in the cluster and the optical and UV images show galaxies in the cluster and in nearby parts of the sky (image credit: X-ray: NASA/CXC/MIT/M.McDonald; UV: NASA/JPL-Caltech/M.McDonald; Optical: AURA/NOAO/CTIO/MIT/M.McDonald; Illustration: NASA/CXC/M.Weiss)
- In the case of Phoenix, jets from the giant black hole in its central galaxy are not powerful enough to prevent the cluster gas from cooling. Correspondingly, any deep notes produced by the jets must be much weaker than needed to prevent cooling and star formation.
- Based on the Chandra data and also observations at other wavelengths, the supermassive black hole in the central galaxy of Phoenix is growing very quickly, at a rate of about 60 times the mass of the Sun every year. This rate is unsustainable, because the black hole is already very large with a mass of about 20 billion times the mass of the Sun. Therefore, its growth spurt cannot last much longer than about a hundred million years or it would become much bigger than its counterparts in the nearby Universe. A similar argument applies to the growth of the central galaxy. Eventually powerful jets should be produced by the black hole in repeated outbursts, forming the deep notes seen in objects like Perseus and stopping the starburst. 58)
- The Phoenix cluster originally was detected by the National Science Foundation's South Pole Telescope, and later was observed in optical light by the Gemini Observatory, the Blanco 4-meter telescope and Magellan telescope, all in Chile. The hot gas and its rate of cooling were estimated from Chandra data. To measure the star formation rate in the Phoenix cluster, several space-based telescopes were used, including NASA's Wide-field Infrared Survey Explorer and Galaxy Evolution Explorer and ESA's Herschel.
• July 23, 2009: Ten years ago, on July 23, 1999, NASA's Chandra X-ray Observatory was launched aboard the space shuttle Columbia and deployed into orbit. Chandra has doubled its original five-year mission, ushering in an unprecedented decade of discovery for the high-energy universe. With its unrivaled ability to create high-resolution X- ray images, Chandra has enabled astronomers to investigate phenomena as diverse as comets, black holes, dark matter and dark energy. 59)
- "Chandra's discoveries are truly astonishing and have made dramatic changes to our understanding of the universe and its constituents," said Martin Weisskopf, Chandra project scientist at NASA's Marshall Space Flight Center in Huntsville, Ala.
- The science that has been generated by Chandra — both on its own and in conjunction with other telescopes in space and on the ground — has had a widespread, transformative impact on 21st century astrophysics. Chandra has provided the strongest evidence yet that dark matter must exist. It has independently confirmed the existence of dark energy and made spectacular images of titanic explosions produced by matter swirling toward supermassive black holes.
Figure 43: This image of the debris of an exploded star — known as supernova remnant 1E 0102.2-7219, or "E0102" for short — features data from NASA's Chandra X-ray Observatory. E0102 is located about 190,000 light years away in the Small Magellanic Cloud, one of the nearest galaxies to the Milky Way. It was created when a star that was much more massive than the Sun exploded, an event that would have been visible from the Southern Hemisphere of the Earth over 1000 years ago [image credit: X-ray (NASA/CXC/MIT/D. Dewey et al. & NASA/CXC/SAO/J.DePasquale); Optical (NASA/STScI)]
Legend to Figure 43: Chandra first observed E0102 shortly after its launch in 1999. New X-ray data have now been used to create this spectacular image and help celebrate the ten-year anniversary of Chandra's launch on July 23, 1999. In this latest image of E0102, the lowest-energy X-rays are colored orange, the intermediate range of X-rays is cyan, and the highest-energy X-rays Chandra detected are blue. An optical image from the Hubble Space Telescope (in red, green and blue) shows additional structure in the remnant and also reveals foreground stars in the field. — The Chandra image shows the outer blast wave produced by the supernova (blue), and an inner ring of cooler (red-orange) material. This inner ring is probably expanding ejecta from the explosion that is being heated by a shock wave traveling backwards into the ejecta. A massive star (not visible in this image) is illuminating the green cloud of gas and dust to the lower right of the image. This star may have similar properties to the one that exploded to form E0102.
- "The Great Observatories program — of which Chandra is a major part — shows how astronomers need as many tools as possible to tackle the big questions out there," said Ed Weiler, associate administrator of NASA's Science Mission Directorate at NASA Headquarters in Washington. NASA's other "Great Observatories" are the Hubble Space Telescope, Compton Gamma-Ray Observatory and Spitzer Space Telescope.
• October 5, 2005: Scientists have solved the 35-year-old mystery of the origin of powerful, split-second flashes of light known as short gamma-ray bursts. These flashes, brighter than a billion galaxies, yet lasting only a few milliseconds, have been simply too fast to catch - until now. 60) 61)
- Through the unprecedented coordination of observations from several ground-based telescopes and NASA satellites, scientists determined the flashes arise from violent collisions in space. The clashes are either between a black hole and a neutron star or between two neutron stars. In either scenario, the impact creates a new black hole.
- In at least one burst, scientists saw tantalizing, first-time evidence of a black hole eating a neutron star. The neutron star was first stretched into a crescent, then swallowed by the black hole.
- "Gamma-ray bursts in general are notoriously difficult to study, but the shortest ones have been next to impossible to pin down," said Dr. Neil Gehrels, principal investigator for the Swift satellite at NASA's Goddard Space Flight Center, Greenbelt, Md. "All that has changed. We now have the tools in place to study these events," he said.
- Gamma-ray bursts, first detected in the 1960s, are the most powerful explosions known. They are random, fleeting and can occur from any region of the sky. Two years ago, scientists discovered longer bursts, lasting more than two seconds, arise from the explosion of very massive stars. About 30 percent of bursts are short and under two seconds.
- The Swift satellite detected a short burst on May 9, and NASA's High-Energy Transient Explorer (HETE) detected another on July 9. The May 9 event marked the first time scientists identified an afterglow for a short gamma-ray burst, something commonly seen after long bursts.
- "We had a hunch that short gamma-ray bursts came from a neutron star crashing into a black hole or another neutron star, but these new detections leave no doubt," said Dr. Derek Fox, assistant professor of Astronomy & Astrophysics at Penn State University, State College, Pa. Fox is lead author of one Nature report detailing a multi-wavelength observation.
- Fox's team discovered the X-ray afterglow of the July 9 burst with NASA's Chandra X-ray Observatory. A team led by Jens Hjorth, a professor at the University of Copenhagen identified the optical afterglow using the Danish 1.5-meter telescope at the La Silla Observatory in Chile.
Figure 44: An artist's rendering (left) of GRB 050709 depicts a gamma-ray burst that was discovered on 9 July, 2005 by NASA's High-Energy Transient Explorer. The burst radiated an enormous amount of energy in gamma-rays for half a second, then faded away. Three days later, Chandra's detection of the X-ray afterglow (inset) established its position with high accuracy (image credit: X-ray: NASA/CXC/Caltech/D. Fox et al.; Illustration: NASA/D. Berry)
- A Hubble Space Telescope image showed that the burst occurred in the outskirts of a spiral galaxy about 2 billion light years from Earth. This location is outside the star-forming regions of the galaxy and evidence that the burst was not produced by the explosion of an extremely massive star.
- The most likely explanation for GRB 050709 is that it was produced by a collision of two neutron stars, or a neutron star and a black hole. Such a collision would result in the formation of a black hole (or a larger black hole), and could generate a beam of high-energy particles that could account for the powerful gamma-ray pulse as well as observed radio, optical and X-ray afterglows.
- This gamma-ray burst is one of a class of short-duration bursts that now appear to have a different origin from the more powerful, long-duration gamma-ray bursts that last more than two seconds. Long-duration bursts have been connected to black holes formed in the explosion of extremely massive stars, or hypernovas.
• August 26, 2003: NASA has awarded a contract to the SAO (Smithsonian Astrophysical Observatory) in Cambridge, Mass., to provide science and operational support for the Chandra X-ray Observatory, one of the world's most powerful tools to better understand the structure and evolution of the universe. 62)
- The contract will have a period of performance from August 31, 2003, through July 31, 2010, with an estimated value of $373 million. It is a follow-on contract to the existing contract with Smithsonian Astrophysical Observatory that has provided science and operations support to the Observatory since its launch in July 1999. At launch the intended mission life was five years.
- As a result of Chandra's success, NASA extended the mission from five to 10 years. The value of the original contract was $289 million. The follow-on contract with the Smithsonian Astrophysical Observatory will continue through the 10-year mission. The contract type is cost reimbursement with no fee.
- As a result of Chandra's success, NASA extended the mission from five to 10 years. The value of the original contract was $289 million. The follow-on contract with the Smithsonian Astrophysical Observatory will continue through the 10-year mission. The contract type is cost reimbursement with no fee.
- The science data processing tasks include the competitive selection, planning, and coordination of science observations with the general observers and processing and delivery of the resulting scientific data. There are approximately 200 to 250 observing proposals selected annually out of about 800 submitted, with a total amount of observing time of about 20 million seconds.
- Chandra has exceeded expectations of scientists, giving them unique insight into phenomena light years away, such as exotic celestial objects, matter falling into black holes, and stellar explosions.
- X-ray astronomy can only be performed from space because Earth's atmosphere blocks X-rays from reaching the surface. The Chandra Observatory travels one-third of the way to the moon during its orbit around the Earth every 64 hours. At its highest point, Chandra's highly elliptical, or egg- shaped, orbit is 200 times higher than that of its visible- light-gathering sister, the Hubble Space Telescope.
• August 26, 1999: Extraordinary first images from NASA's Chandra X-ray Observatory trace the aftermath of a gigantic stellar explosion in such stunning detail that scientists can see evidence of what may be a neutron star or black hole near the center. Another image shows a powerful X-ray jet blasting 200,000 light years into intergalactic space from a distant quasar. 63) 64) 65)
- Released today, both images confirm that NASA's newest Great Observatory is in excellent health and its instruments and optics are performing up to expectations. Chandra, the world's largest and most sensitive X-ray telescope, is still in its orbital check-out and calibration phase.
- "When I saw the first image, I knew that the dream had been realized," said Dr. Martin Weisskopf, Chandra Project Scientist, NASA's Marshall Space Flight Center, Huntsville, AL. "This observatory is ready to take its place in the history of spectacular scientific achievements."
- "We were astounded by these images," said Harvey Tananbaum, Director of the Smithsonian Astrophysical Observatory's Chandra X- ray Center, Cambridge, MA. "We see the collision of the debris from the exploded star with the matter around it, we see shock waves rushing into interstellar space at millions of miles per hour, and, as a real bonus, we see for the first time a tantalizing bright point near the center of the remnant that could possibly be a collapsed star associated with the outburst."
- After the telescope's sunshade door was opened last week, one of the first images taken was of the 320-year-old supernova remnant Cassiopeia A (Cas A), which astronomers believe was produced by the explosion of a massive star. Material blasted into space from the explosion crashed into surrounding material at 10 million miles per hour. This collision caused violent shock waves, like massive sonic booms, creating a vast 50-million degree bubble of X-ray emitting gas.
- The Cas A Supernova: A supernova occurs when a massive star has used up its nuclear fuel and the pressure drops in the central core of the star. The matter in the core is crushed by gravity to higher and higher densities, and temperatures reach billions of degrees. Under these extreme conditions, nuclear reactions occur violently and catastrophically reversing the collapse. A thermonuclear shock wave races through the now expanding stellar debris, fusing lighter elements into heavier ones and producing a brilliant visual outburst.
- About every fifty years in our galaxy, a massive star explodes. The shell of matter thrown off by the supernova creates a bubble of multi-million degree gas called a supernova remnant. Cas A is a prime example. The hot gas will expand and produce X-rays for thousands of years.
- The nature of the explosion that produced Cas A has been an enigma. Although radio, optical and x-ray observations of the remnant indicate that it was a powerful event, the visual brightness of the outburst was much less than a normal supernova. Apparently Cas A was produced by the explosion of an unusual massive star that had previously ejected most of its outer layers.
- Probing Cas A Mysteries with NASA's Chandra X-ray Observatory: Chandra's spectacularly vivid images of Cas A allow scientists to trace the dynamics of the remnant and its collision with any material ejected by the star before it exploded. Chandra detectors provide scientists with precise x-ray spectra– measurements of the energies of individual x-rays–from the Cas A remnant. These measurements make it possible to identify which heavy elements are present and in what quantities. Chandra's observations should help astronomers to resolve the long-standing mystery as to the nature and origin of Cas A.
- A related mystery is whether the explosion that produced Cas A left behind a neutron star, black hole, or nothing at all. This "First Light" Chandra image of Cas A shows a bright object near the center of the remnant! Longer observations with Chandra can determine if this is the long sought for neutron star or black hole.
Figure 45: Cas A is the remnant of a star that exploded about 300 years ago. The X-ray image shows an expanding shell of hot gas produced by the explosion. This gaseous shell is about 10 light years in diameter, and has a temperature of about 50 million degrees (image credit: NASA/CXC/SAO)
- Importance of Supernovae: The study of remnants of exploded stars, or supernovae, is essential for our understanding of the origin of life on Earth. The cloud of gas and dust that collapsed to form the sun, Earth and other planets was composed mostly of hydrogen and helium, with a small amount of heavier elements such as carbon, nitrogen, oxygen and iron. The only place where these and other heavy elements necessary for life are made, is deep in the interior of a massive star. There they remain until a catastrophic explosion spreads them throughout space.
- Supernovae are the creative flashes that renew the galaxy. They seed the interstellar gas with heavy elements, heat it with the energy of their radiation, stir it up with the force of their blast waves and cause new stars to form.
Figure 46: PKS 0637-752 is so distant that we see it as it was 6 billion years ago. It is a luminous quasar that radiates with the power of 10 trillion suns from a region smaller than our solar system. The source of this prodigious energy is believed to be a supermassive black hole.The X-ray jet observed for the first time by Chandra in PKS 0637-752, is a dramatic example of a cosmic jet. It has blasted outward from the quasar into intergalactic space for a distance of at least 200,000 light years! The jet's presence means that electromagnetic forces are continually accelerating electrons to extremely high energies over enormous distances. Chandra observations, combined with radio observations, should provide insight into this important cosmic energy conversion process (image credit: NASA/CXC/SAO)
Sensor complement (ACIS, HRC, HETG, LETG)
The SIM (Science Instrument Module) consists of the special hardware that provides mechanical and thermal interfaces to the focal-plane scientific instruments (SIs). The most critical functions from an observer's viewpoint are the capability to adjust the telescope focal length and the ability to move the instruments along an axis orthogonal to the optical axis. The SIM houses the two focal instruments - the ACIS and the HRC. Each of these have two principal components — HRC-I and -S and ACIS-I and -S. The focal-plane instrument layout is shown in Figure 48. The SIM moves in both the X-axis (focus) and the Z-axis (instrument and aimpoint selection). Note that the Y-Axis parallels the dispersion direction of the gratings. 66)
The focal-plane instruments are positioned by the SIM Z-axis translation stage with a repeatability to ±0.005 inches over a translation range of 20 inches. The SIM X-axis motion sets the focus to an accuracy of ±0.0005 inches over a range of 0.8 inches. The fine-focus adjustment step is 0.00005 inches.
Figure 47: A schematic of the SIM (Science Instrument Module), image credit: SAO
Figure 48: Arrangement of the ACIS and the HRC in the focal plane. The view is along the axis of the telescope from the direction of the mirrors. For reference, the two back-illuminated ACIS-S chips are shaded. Numbers indicate positions of chips I0-I3 and S0-S5. SIM motion can be used to place the aimpoint at any point on the vertical solid line ( image credit: SAO)
ACIS (Advanced CCD Imaging Spectrometer)
ACIS is an X-ray imager. X-ray photons hitting the camera are detected individually and their position, energy and arrival time recorded. This allows for high resolution (~1 arcsec) imaging, moderate resolution spectroscopy and timing studies. ACIS can also be used with the HETG (High Energy Transmission Grating), and less commonly the LETG (Low Energy Transmission Grating) for high resolution spectroscopy. 67)
ACIS) offers the capability to simultaneously acquire high-resolution images and moderate resolution spectra. The instrument can also be used in conjunction with the HETG or LETG to obtain higher resolution spectra. ACIS contains 10 planar, 1024 x 1024 pixel CCDs (Figure 50); four arranged in a 2×2 array (ACIS-I) used for imaging, and six arranged in a 1×6 array (ACIS-S) used either for imaging or for a grating spectrum read-out. Two CCDs are back-illuminated (BI) and eight are front-illuminated (FI). The response of the BI devices extends to energies below that accessible to the FI chips. The chip-average energy resolution of the BI devices is better than that of the FI devices.
In principle, any combination of up to 6 CCDs can be operated simultaneously. However, because of changes in the thermal environment, the CXC (Chandra X-ray Center) now recommends that fewer CCDs be selected if the science needs can be met with fewer CCDs. Some CCDs can be designated as optional, which means they may be turned off depending on thermal conditions.
The ACIS instrument was built by a team from the Massachusetts Institute of Technology's Center for Space Research and the Pennsylvania State University (PSU) for the Chandra X-ray Observatory.
Figure 49: Top view of the engineering unit of Chandra’s Advanced CCD Imaging Spectrometer (ACIS), showing the 2 x 2 CCD ACIS-I and the 1 x 6 CCD ACIS-S focal planes. In the flight unit, aluminized-polyimide optical blocking filters OBF-I and OBF-S cover the I and the S focal planes, respectively. The OBFs lie within the “snoot” (with door opened since on-orbit check-out), which in turn lies within the ACIS “collimator” that envelopes the ACIS cavity (image credit: NASA & ACIS Team) 68) 69)
Figure 50: A schematic drawing of the ACIS focal plane; insight to the terminology is given in the lower left. Note the aimpoints: on S3 (the `+') and on I3 (the `x'). Note the differences in the orientation of the I and S chips, important when using subarrays. Note also the (Y, Z) coordinate system and the target offset convention as well as the SIM motion (±Z). This view is along the optical axis, from the sky toward the detectors, (-X). The numerous ways to refer to a particular CCD are indicated: chip letter+number, chip serial number, and ACIS chip number (CCD_ID), image credit: SAO
HRC (High Resolution Camera)
The HRC is comprised of two microchannel plate (MCP) imaging detectors: the HRC-I designed for wide-field imaging; and, HRC-S designed to serve as a read-out for the LETG. The HRC-I is placed at right angles to the optical axis, tangent to the focal surface. The HRC-S is made of three flat elements, the outer two of which are tilted to approximate the LETG Rowland circle. The HRC detectors have the highest spatial resolution on Chandra, matching the HRMA point spread function most closely. Under certain circumstances, the HRC-S detector also offers the fastest time resolution (16 µs). 70)
The HRC is a direct descendant of the Einstein and ROSAT mission High Resolution Imagers (HRIs). The ROSAT HRI had the same coating (CsI) as the HRC. The Instrument Principal Investigator is Dr. Ralph Kraft of the SAO (Smithsonian Astrophysical Observatory).
One purpose of the second (output) MCP is to provide additional gain. In addition, reversing the direction of the second MCP's bias angle with respect to the first removes a clear path for positive ions, and hence reduces the possibility of (positive) ion feedback - where an accelerated ion moving in the opposite direction as that of the electrons ends up causing the release of electrons and starts the process all over again.
Table 3: Common HRC Configurations: Summary of the three most common HRC operating modes
Figure 51: Photo of the HRC flight unit of Chandra (image credit: NASA)
Figure 52: A schematic of the HRC focal-plane geometry as viewed along the optical axis from the telescope towards the focal plane (image credit: SAO)
Figure 53 illustrates the features of the HRC MCP s. X-rays enter through a UV/Ion shield , necessary to reduce/avoid signals from UV light, ions, and low energy electrons. Most of these X-rays are then absorbed in the CsI-coated walls of the first (input) of two consecutive MCPs. The axes of the millions of tubes that comprise the input and output MCPs are not parallel to the optical axis but are canted ("biased") at an angle of 6° in opposite directions as shown in Figure 7.3. This bias improves the probability of an interaction. The CsI coating enhances the photoemission over that from a bare MCP. The resulting photoelectrons are then accelerated by an applied electric field. The next interaction with the walls releases several secondary electrons and so on, until a cascade of electrons is produced.
HETG (High Energy Transmission Grating)
HETG is used for high resolution spectroscopy of bright sources in the range 0.4-10 keV (31-1.2 Å). The HETG has been used to measure Doppler velocities of orbiting systems, even as low as 50 km/s, and plasma outflow velocities from a few hundred to 10's of thousands of km/s. Because the HETG can clearly resolve lines from O to Fe-K, detailed line diagnostics can be applied.
The HETG is optimized for high-resolution spectroscopy of bright sources over the energy band 0.4-10 keV. It is most commonly used with ACIS-S. The resolving power (E/ΔE) varies from ~800 at 1.5 keV to ~200 at 6 keV.
In operation with the HRMA (High Resolution Mirror Assembly) and a focal-plane imager, the complete instrument is referred to as the HETGS (High-Energy Transmission Grating Spectrometer). The HETGS provides high resolution spectra (with E/ΔE up to 1000) between 0.4 keV and 10.0 keV for point and slightly extended (few arcsec) sources. Although HETGS operation differs from proportional counter and CCD spectrometers, standard processing of an HETGS observation produces familiar spectrometer data products: PHA, ARF, and RMF files. These files can then be analyzed with standard forward-folding model fitting software, e.g., Sherpa, XSPEC, ISIS , etc. 71)
The HETG itself consists of two sets of gratings, each with different period. One set, the Medium Energy Grating (MEG), intercepts rays from the outer HRMA shells and is optimized for medium energies. The second set, the High Energy Gratings (HEG), intercepts rays from the two inner shells and is optimized for high energies. Both gratings are mounted on a single support structure and therefore are used concurrently. The two sets of gratings are mounted with their rulings at different angles so that the dispersed images from the HEG and MEG will form a shallow X centered at the undispersed (zeroth order) position; one leg of the X is from the HEG, and the other from the MEG. The HETG is designed for use with the spectroscopic array of the Chandra CCD ACIS-S (Advanced Imaging Spectrometer) although other detectors may be used for particular applications.
LETG (Low Energy Transmission Grating)
LETG ) was developed under the direction of Dr. A. C. Brinkman in the Laboratory for Space Research (SRON) in Utrecht, the Netherlands, in collaboration with the Max-Planck-Institut für Extraterrestrische Physik (MPE) in Garching, Germany. The grating was manufactured in collaboration with Heidenhaim GmbH. 72)
The LETG provides the highest spectral resolving power (E/ΔE > 1000) on Chandra at low energies (0.07 - 0.2 keV). The LETG/HRC-S combination is used extensively for high resolution spectroscopy of bright, soft sources such as stellar coronae, white dwarf atmospheres and cataclysmic variables.
LETGS (Low Energy Transmission Grating Spectrometer) comprises the LETG, a focal-plane imaging detector, and the High Resolution Mirror Assembly. The Chandra High Resolution Camera spectroscopic array (HRC-S) is the primary detector designed for use with the LETG. The spectroscopic array of the Chandra CCD Imaging Spectrometer (ACIS-S) can also be used, though with lower quantum efficiency below ~0.6 keV and a smaller detectable wavelength range than with the HRC-S. The High Energy Transmission Grating (HETG) used in combination with ACIS-S offers superior energy resolution and quantum efficiency above 0.78 keV.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).