NuSTAR (Nuclear Spectroscopic Telescope Array)
NuSTAR is a NASA-funded SMEX (Small Explorer) mission that carries the first focusing hard X-ray (5 - 80 keV) telescope to orbit. NuSTAR will offer a factor 50 - 100 sensitivity improvement compared to previous collimated or coded mask imagers that have operated in this energy band. In addition, NuSTAR provides sub-arcminute imaging with good spectral resolution over a 12 arcminute FOV (Field of View).
NuSTAR will carry out a two-year primary science mission that focuses on four key programs: studying the evolution of massive black holes through surveys carried out in fields with excellent multiwavelength coverage, understanding the population of compact objects and the nature of the massive black hole in the center of the Milky Way, constraining explosion dynamics and nucleosynthesis in supernovae, and probing the nature of particle acceleration in relativistic jets in active galactic nuclei. 1) 2) 3) 4)
The NuSTAR program is managed for NASA by the JPL (Jet Propulsion Laboratory) in Pasadena, CA. The PI (Principal Investigatior) of the mission is Fiona A. Harrison of Caltech (California Institute of Technology). The NuSTAR spacecraft and its payload feature three key technologies to accomplish the mission: 5) 6)
• Hard X-ray optics
• Deployable mast (of SRTM heritage)
• CdZnTe (Cadmium Zinc Telluride) detector technology.
The NuSTAR Small Explorer mission will be the first astronomical telescope on-orbit to utilize the new generation of hard X-ray optics and detector technologies to carry out high-sensitivity observations at X-ray energies significantly greater than 10 keV. Figure 35 shows the total effective area for both telescopes as a function of energy, with a comparison to Chandra (NASA) and XMM-Newton (ESA).
In addition to its core science program, NuSTAR will offer opportunities for a broad range of science investigations, ranging from probing cosmic ray origins to studying the extreme physics around collapsed stars to mapping microflares on the surface of the sun.
Background: NuSTAR is a part of NASA's Explorer Program. While the mission was selected for a Phase A study in 2003 and for a further study in 2005, it was ultimately cancelled by NASA in February 2006 due to agency budget limitations. On September 21, 2007 it was announced that the program had been restarted, with an expected launch in August 2011, though this was later delayed to Q1 2012. 7)
Figure 1: Artist's rendition of the NuSTAR spacecraft in orbit (image credit: NASA/JPL)
Legend to Figure 1: The spacecraft features a mast 10 m in length that deploys after launch to separate the optics modules (right) from the detectors in the focal plane (left).
The NuSTAR minisatellite is based on OSC's (Orbital Sciences Corporation) proven LEOStar-2 bus design of Dulles, VA. The spacecraft is three-axis stabilized with a single articulating solar panel and relies predominantly upon a multi-head star camera (µASC) of DTU (Danish Technical University) for attitude sensing. This enables 80% of the sky to be accessed at any given time, which allows ToO (Target of Opportunity) viewing with few restrictions as well as aids in mission planning. 8)
RF communications: There will be daily downlinks via TDRSS, and uplinks will not be routinely required. The pointing strategy will emphasize long observations of survey fields, specific pointed observations and targets of opportunity.
Table 1: Overview of spacecraft parameters
Figure 2: Diagram of the observatory in the stowed (bottom) and deployed (top) configurations (image credit: NuSTAR collaboration)
NuSTAR metrology system:
The NuSTAR X-ray telescope consists of two co-aligned grazing angle incidence x-ray mirrors, coated with depth-graded multilayers, focusing onto two cadmium-zinc-telluride pixel detectors that are separated from the mirrors by 10 m. The two telescopes are operated independently and the sensitivity of the mission is achieved by combining exposures from the two telescopes. The long focal length required by the hard X-ray optics demands the use of a 10 m extendable mast (Figure NO TAG#), manufactured by ATK Space Systems, Goleta, CA. 9)
The observatory must determine the origin (in celestial coordinates) of all detected X-ray photons during post processing, in order to produce sharp images. This task is complicated by distortions due to thermal bending and external forces acting on the mast during orbit.
To track the motion of this mast, the observatory carries two metrology laser subsystems, which are mounted on the bench with the X-ray optics. The metrology lasers are focused on two metrology detectors mounted on the bench with the X-ray detectors. These metrology detectors are based on PSD (Position Sensitive Detectors). To generate a unique aspect solution, the observatory also carries a star tracker camera on the optics (outboard) bench.
Figure 3: Simplified diagram of the metrology system (NuSTAR collaboration)
After spacecraft commissioning, a one-time in-flight calibration of the observatory is performed to establish the launch shifts and the deployed position of the mast. This is done by observing a bright X-ray source (with known celestial coordinates), and simultaneously logging the star tracker data, the laser metrology data and the X-ray detector data. A sketch of the metrology system diagrammed with the mast and a single telescope is shown in Figure 3.
During spacecraft operations, the positions of the laser beams on the PSD detectors are continually recorded at a frequency much higher than the mast oscillations. Also, all star tracker updates are recorded and the positions (and times) on the focal plane where the individual x-ray photons impinge are recorded. All this information is used to generate high-resolution images during on-ground post processing of the data.
The X-ray detectors detect single incoming X-ray photons. That is, the X-ray detectors are not integrating. The described metrology system would not work with an integrating detector (such as a CCD chip) that does not register the arrival time of the individual photons, since the unique aspect solution of the observatory at a certain instant could not be applied to a given detector read-out.
The metrology system is required to measure the translation in 2 axes (those directions transverse to the laser beams) and the clocking angle (rotation around the observatory boresight). Only these 3 DOFs (Degree of Freedom) have the potential to introduce errors large enough that they must be measured. Figure 4 illustrates the DOFs that need to be measured. - Regarding the thermal design, it is required that the metrology system survive a non-operating temperature range of -45ºC to 60ºC.
Figure 4: The DOFs that are required to be measured by the metrology system (image credit: NuSTAR collaboration)
Legend to Figure 4: The sketch is utilizing a coordinate system that is fixed on the optical bench (where the star tracker is mounted).
The science requirement for the NuSTAR observatory that governs the metrology system performance is that the celestial coordinates of a detected bright X-ray source be determined to an accuracy of ~10 arcseconds.
The laser metrology system is implemented as two laser pointers mounted on a bench with the optics. The laser pointers are illuminating 2 PSD detectors mounted ~10 m away on a bench with the X-ray detectors. 10)
Figure 5: Block diagram of the metrology system (image credit: NuSTAR collaboration)
A block diagram of the electronics is shown in Figure 6. The lasers are driven by a processor, through analog laser drivers. The PSD detector is mounted on a PCB (Printed Circuit Board) along with 4 operational amplifiers. The 4 signals for each channel are routed to the main instrument processor where the signal is low pass filtered and multiplexed into a 14 bit A/D converter. The electronics components are space qualified. The electronics power cycle the lasers and make a background measurement 4 times a second to subtract out the background signal produced by dark current, the moon or the Earth in the FOV, sun stray light.
Figure 6: Block diagram of the metrology system (image credit: NuSTAR collaboration)
Figure 7: Schematic view of the metrology laser and its components (image credit: NuSTAR collaboration)
The requirements on the metrology laser call for a beam accuracy of ≤ 130 µm (at ~10 m) over all observatory orientations and orbits following the initial onetime calibration (this implies: "under all sun illumination circumstances"). Thermal stability is the key to achieving this requirement. To minimize the effect of the sun, the structure is primarily made of Invar 36, which has a low CTE (Coefficient of Thermal Expansion). The laser is mounted behind the optics in an invar barrel. This barrel is placed inside another invar barrel to mechanically hold it, to serve as a thermal shield, and to spread the thermal variation from the sun.
Table 2: Metrology system specifications
The NuSTAR metrology system has space qualified two different laser diodes for its environment. Both brands of lasers have demonstrated their capability to be utilized as flight lasers for NuSTAR. It is expected that if one laser diode fails but the other laser diode survives the mission, it will be possible to recover scientific data. Therefore, since, 1) the laser are interchangeable, 2) since no laser diode illustrates any signs of superiority over the other and 3) it is more likely that one laser will survive if 2 different equally reliable lasers are chosen, a project decision was made to fly one laser diode from each vendor on the NuSTAR metrology system (Ref. 10).
Figure 8: Photo of the NuSTAR spacecraft integration at the OSC facility, Dulles, VA (image credit: OSC, NASA/JPL)
Figure 9: Photo of the stowed mast of NuSTAR (image credit: OSC, Caltech)
Launch: The NuSTAR spacecraft was launched on June 13, 2012 on a Pegasus-XL vehicle of OSC (air-launch on the Stargazer L-1011 aircraft). The launch site was the Kwajalein Atoll in the Marshall Islands (Pacific Ocean). 11)
The June launch came almost three months after a planned early March launch date. 12)
Figure 10: Photo of a Pegasus rocket which launches from underneath the L-1011 "Stargazer" aircraft (image credit: NASA, Orbital)
Orbit: Near equatorial circular orbit, altitude ~650 km x 610 km, inclination = 6º.
• October 30, 2017: Black holes are famous for being ravenous eaters, but they do not eat everything that falls toward them. A small portion of material gets shot back out in powerful jets of hot gas, called plasma, that can wreak havoc on their surroundings. Along the way, this plasma somehow gets energized enough to strongly radiate light, forming two bright columns along the black hole's axis of rotation. Scientists have long debated where and how this happens in the jet. 13)
- Astronomers have new clues to this mystery. Using NASA's NuSTAR space telescope and a fast camera called ULTRACAM on the William Herschel Observatory in La Palma, Spain, scientists have been able to measure the distance that particles in jets travel before they "turn on" and become bright sources of light. This distance is called the "acceleration zone." The study is published in the journal Nature Astronomy. 14)
- Scientists looked at two systems in the Milky Way called "X-ray binaries," each consisting of a black hole feeding off of a normal star. They studied these systems at different points during periods of outburst — which is when the accretion disk — a flat structure of material orbiting the black hole — brightens because of material falling in.
- One system, called V404 Cygni, had reached nearly peak brightness when scientists observed it in June 2015. At that time, it experienced the brightest outburst from an X-ray binary seen in the 21st century. The other, called GX 339-4,was less than 1 percent of its maximum expected brightness when it was observed. The star and black hole of GX 339-4 are much closer together than in the V404 Cygni system.
- Despite their differences, the systems showed similar time delays - about one-tenth of a second — between when NuSTAR first detected X-ray light and ULTRACAM detected flares in visible light slightly later. That delay is less than the blink of an eye, but significant for the physics of black hole jets.
- "One possibility is that the physics of the jet is not determined by the size of the disc, but instead by the speed, temperature and other properties of particles at the jet's base," said Poshak Gandhi, lead author of the study and astronomer at the University of Southampton, United Kingdom.
- The best theory scientists have to explain these results is that the X-ray light originates from material very close to the black hole. Strong magnetic fields propel some of this material to high speeds along the jet. This results in particles colliding near light-speed, energizing the plasma until it begins to emit the stream of optical radiation caught by ULTRACAM.
- Where in the jet does this occur? The measured delay between optical and X-ray light explains this. By multiplying this amount of time by the speed of the particles, which is nearly the speed of light, scientists determine the maximum distance traveled.
- This expanse of about 30,000 km represents the inner acceleration zone in the jet, where plasma feels the strongest acceleration and "turns on" by emitting light. That's just under three times the diameter of Earth, but tiny in cosmic terms, especially considering the black hole in V404 Cygni weighs as much as 3 million Earths put together.
- "Astronomers hope to refine models for jet powering mechanisms using the results of this study," said Daniel Stern, study co-author and astronomer based at NASA's Jet Propulsion Laboratory, Pasadena, California.
- Making these measurements wasn't easy. X-ray telescopes in space and optical telescopes on the ground have to look at the X-ray binaries at exactly the same time during outbursts for scientists to calculate the tiny delay between the telescopes' detections. Such coordination requires complex planning between the observatory teams. In fact, coordination between NuSTAR and ULTRACAM was only possible for about an hour during the 2015 outburst, but that was enough to calculate the groundbreaking results about the acceleration zone.
- The results also appear to connect with scientists' understanding of supermassive black holes, much bigger than the ones in this study. In one supermassive system called BL Lacertae, weighing 200 million times the mass of our Sun, scientists have inferred time delays millions of times greater than what this study found. That means the size of the acceleration area of the jets is likely related to the mass of the black hole.
- "We are excited because it looks as though we have found a characteristic yardstick related to the inner workings of jets, not only in stellar-mass black holes like V404 Cygni, but also in monster supermassive ones," Gandhi said.
- The next steps are to confirm this measured delay in observations of other X-ray binaries, and to develop a theory that can tie together jets in black holes of all sizes.
- "Global ground and space telescopes working together were key to this discovery. But this is only a peek, and much remains to be learned. The future is really bright for understanding the extreme physics of black holes," said Fiona Harrison, principal investigator of NuSTAR and professor of astronomy at Caltech in Pasadena.
Figure 11: This artist's concept shows a black hole with an accretion disk — a flat structure of material orbiting the black hole - and a jet of hot gas, called plasma (image credit: NASA/JPL-Caltech)
• On June 13, 2017, the NuSTAR spacecraft was 5 years on Orbit. 15)
• May 9, 2017: Black holes get a bad rap in popular culture for swallowing everything in their environments. In reality, stars, gas and dust can orbit black holes for long periods of time, until a major disruption pushes the material in. -A merger of two galaxies is one such disruption. As the galaxies combine and their central black holes approach each other, gas and dust in the vicinity are pushed onto their respective black holes. An enormous amount of high-energy radiation is released as material spirals rapidly toward the hungry black hole, which becomes what astronomers call an AGN (Active Galactic Nucleus). 16)
- A study using NASA's NuSTAR telescope shows that in the late stages of galaxy mergers, so much gas and dust falls toward a black hole that the extremely bright AGN is enshrouded. The combined effect of the gravity of the two galaxies slows the rotational speeds of gas and dust that would otherwise be orbiting freely. This loss of energy makes the material fall onto the black hole.
- "The further along the merger is, the more enshrouded the AGN will be," said Claudio Ricci, lead author of the study published in the Monthly Notices Royal Astronomical Society. "Galaxies that are far along in the merging process are completely covered in a cocoon of gas and dust." 17)
- Ricci and colleagues observed the penetrating high-energy X-ray emission from 52 galaxies. About half of them were in the later stages of merging. Because NuSTAR is very sensitive to detecting the highest-energy X-rays, it was critical in establishing how much light escapes the sphere of gas and dust covering an AGN.
- The study was published in the Monthly Notices of the Royal Astronomical Society. Researchers compared NuSTAR observations of the galaxies with data from NASA's Swift and Chandra and ESA's XMM-Newton observatories, which look at lower energy components of the X-ray spectrum. If high-energy X-rays are detected from a galaxy, but low-energy X-rays are not, that is a sign that an AGN is heavily obscured.
- The study helps confirm the longstanding idea that an AGN's black hole does most of its eating while enshrouded during the late stages of a merger. "A supermassive black hole grows rapidly during these mergers," Ricci said. "The results further our understanding of the mysterious origins of the relationship between a black hole and its host galaxy."
Figure 12: This illustration compares growing supermassive black holes in two different kinds of galaxies. A growing supermassive black hole in a normal galaxy would have a donut-shaped structure of gas and dust around it (left). In a merging galaxy, a sphere of material obscures the black hole (right), image credit: National Astronomical Observatory of Japan
• March 27, 2017: A supermassive black hole inside a tiny galaxy is challenging scientists' ideas about what happens when two galaxies become one. 18)
- Was 49 is the name of a system consisting of a large disk galaxy, referred to as Was 49a, merging with a much smaller "dwarf" galaxy called Was 49b. The dwarf galaxy rotates within the larger galaxy's disk, about 26,000 light-years from its center. Thanks to NASA's NuSTAR mission, scientists have discovered that the dwarf galaxy is so luminous in high-energy X-rays, it must host a supermassive black hole much larger and more powerful than expected.
- "This is a completely unique system and runs contrary to what we understand of galaxy mergers," said Nathan Secrest, lead author of the study and postdoctoral fellow at the U.S. NRL (Naval Research Laboratory) in Washington.
- Data from NuSTAR and the Sloan Digital Sky Survey suggest that the mass of the dwarf galaxy's black hole is huge, compared to similarly sized galaxies, at more than 2% of the galaxy's own mass. "We didn't think that dwarf galaxies hosted supermassive black holes this big," Secrest said. "This black hole could be hundreds of times more massive than what we would expect for a galaxy of this size, depending on how the galaxy evolved in relation to other galaxies."
- The dwarf galaxy's black hole is the engine of an AGN (Active Galactic Nucleus), a cosmic phenomenon in which extremely high-energy radiation bursts forth as a black hole devours gas and dust. This particular AGN appears to be covered by a donut-shaped structure made of gas and dust. NASA's Chandra and Swift missions were used to further characterize the X-ray emission.
- Normally, when two galaxies start to merge, the larger galaxy's central black hole becomes active, voraciously gobbling gas and dust, and spewing out high-energy X-rays as matter gets converted into energy. That is because, as galaxies approach each other, their gravitational interactions create a torque that funnels gas into the larger galaxy's central black hole. But in this case, the smaller galaxy hosts a more luminous AGN with a more active supermassive black hole, and the larger galaxy's central black hole is relatively quiet.
- An optical image of the Was 49 system, compiled using observations from the Discovery Channel Telescope in Happy Jack, Arizona, uses the same color filters as the Sloan Digital Sky Survey. Since Was 49 is so far away, these colors are optimized to separate highly-ionized gas emission, such as the pink-colored region around the feeding supermassive black hole, from normal starlight, shown in green. This allowed astronomers to more accurately determine the size of the dwarf galaxy that hosts the supermassive black hole.
- The pink-colored emission stands out in a new image because of the intense ionizing radiation emanating from the powerful AGN. Buried within this region of intense ionization is a faint collection of stars, believed to be part of the galaxy surrounding the enormous black hole. These striking features lie on the outskirts of the much larger spiral galaxy Was 49a, which appears greenish in the image due to the distance to the galaxy and the optical filters used.
- Scientists are still trying to figure out why the supermassive black hole of dwarf galaxy Was 49b is so big. It may have already been large before the merger began, or it may have grown during the very early phase of the merger.
- "This study is important because it may give new insight into how supermassive black holes form and grow in such systems," Secrest said. "By examining systems like this, we may find clues as to how our own galaxy's supermassive black hole formed." - In several hundred million years, the black holes of the large and small galaxies will merge into one enormous beast.
Figure 13: This optical image shows the Was 49 system, which consists of a large disk galaxy, Was 49a, merging with a much smaller "dwarf" galaxy Was 49b. The image was compiled using observations from the DCT (Discovery Channel Telescope) in Happy Jack, Arizona, using the same color filters as the Sloan Digital Sky Survey (image credit: DCT/NRL)
Legend to Figure 13: The dark pink-colored emission stands out because of the intense ionizing radiation emanating from the powerful active galactic nucleus of the dwarf galaxy. Buried within this region of intense ionization is a faint collection of stars, shown in light pink, believed to be part of the galaxy surrounding the enormous black hole. These striking features lie on the outskirts of the much larger spiral galaxy Was 49a, which appears greenish in the image due to the distance to the galaxy and the optical filters used. — Since Was 49 is so far away, the colors in this image are optimized to separate highly-ionized gas emission, such as the pink-colored region around the feeding supermassive black hole, from normal starlight, shown in green. This allowed astronomers to more accurately determine the size of the dwarf galaxy that hosts the supermassive black hole.
• March 23, 2017: The Milky Way's close neighbor, Andromeda, features a dominant source of high-energy X-ray emission, but its identity was mysterious until now. As reported in a new study, NASA's NuSTAR (Nuclear Spectroscopic Telescope Array) mission has pinpointed an object responsible for this high-energy radiation. 19)
- The object, called Swift J0042.6+4112, is a possible pulsar, the dense remnant of a dead star that is highly magnetized and spinning, researchers say. This interpretation is based on its emission in high-energy X-rays, which NuSTAR is uniquely capable of measuring. The object's spectrum is very similar to known pulsars in the Milky Way.
- It is likely in a binary system, in which material from a stellar companion gets pulled onto the pulsar, spewing high-energy radiation as the material heats up. "We didn't know what it was until we looked at it with NuSTAR," said Mihoko Yukita, lead author of a study about the object, based at JHU (Johns Hopkins University) in Baltimore. The study is published in The Astrophysical Journal. 20)
- This candidate pulsar is shown as a blue dot in a NuSTAR X-ray image of Andromeda (also called M31), where the color blue is chosen to represent the highest-energy X-rays. It appears brighter in high-energy X-rays than anything else in the galaxy.
- The study brings together many different observations of the object from various spacecraft. In 2013, NASA's Swift satellite reported it as a high-energy source, but its classification was unknown, as there are many objects emitting low energy X-rays in the region. The lower-energy X-ray emission from the object turns out to be a source first identified in the 1970s by NASA's Einstein Observatory. Other spacecraft, such as NASA's Chandra X-ray Observatory and ESA's XMM-Newton had also detected it. However, it wasn't until the new study by NuSTAR, aided by supporting Swift satellite data, that researchers realized it was the same object as this likely pulsar that dominates the high energy X-ray light of Andromeda.
- Traditionally, astronomers have thought that actively feeding black holes, which are more massive than pulsars, usually dominate the high-energy X-ray light in galaxies. As gas spirals closer and closer to the black hole in a structure called an accretion disk, this material gets heated to extremely high temperatures and gives off high-energy radiation. This pulsar, which has a lower mass than any of Andromeda's black holes, is brighter at high energies than the galaxy's entire black hole population.
- Even the supermassive black hole in the center of Andromeda does not have significant high-energy X-ray emission associated with it. It is unexpected that a single pulsar would instead be dominating the galaxy in high-energy X-ray light.
- "NuSTAR has made us realize the general importance of pulsar systems as X-ray-emitting components of galaxies, and the possibility that the high energy X-ray light of Andromeda is dominated by a single pulsar system only adds to this emerging picture," said Ann Hornschemeier, co-author of the study and based at NASA's Goddard Space Flight Center, Greenbelt, Maryland.
- Andromeda is a spiral galaxy slightly larger than the Milky Way. It resides 2.5 million light-years from our own galaxy, which is considered very close, given the broader scale of the universe. Stargazers can see Andromeda without a telescope on dark, clear nights.
Figure 14: NuSTAR has identified a candidate pulsar in Andromeda — the nearest large galaxy to the Milky Way. This likely pulsar is brighter at high energies than the Andromeda galaxy's entire black hole population (image credit: NASA/JPL-Caltech/GSFC/JHU)
• February 28, 2017: There's a new record holder for brightest pulsar ever found — and astronomers are still trying to figure out how it can shine so brightly. It's now part of a small group of mysterious bright pulsars that are challenging astronomers to rethink how pulsars accumulate, or accrete, material. 21)
- A pulsar is a spinning, magnetized neutron star that sweeps regular pulses of radiation in two symmetrical beams across the cosmos. If aligned well enough with Earth, these beams act like a lighthouse beacon — appearing to flash on and off as the pulsar rotates. Pulsars were previously massive stars that exploded in powerful supernovae, leaving behind these small, dense stellar corpses.
- The brightest pulsar, as reported in the journal Science, is called NGC 5907 ULX. In one second, it emits the same amount of energy as our sun does in 3.5 years. The ESA XMM-Newton satellite found the pulsar and, independently, NASA's NuSTAR (Nuclear Spectroscopic Telescope Array) mission also detected the signal. This pulsar is 50 million light years away, which means its light dates back to a time before humans roamed Earth. It is also the farthest known neutron star. 22)
- "This object is really challenging our current understanding of the accretion process for high-luminosity pulsars," said Gian Luca Israel, from INAF-Osservatorio Astronomica di Roma, Italy, lead author of the Science paper. "It is 1,000 times more luminous than the maximum thought possible for an accreting neutron star, so something else is needed in our models in order to account for the enormous amount of energy released by the object."
- The previous record holder for brightest pulsar was reported in October 2014. NuSTAR had identified M82 X-2, located about 12 million light-years away in the "Cigar Galaxy" galaxy Messier 82 (M82), as a pulsar rather than a black hole. The pulsar reported in Science, NGC 5907 ULX, is 10 times brighter.
Figure 15: NGC 5907 ULX is the brightest pulsar ever observed. This image comprises X-ray emission data (blue/white) from ESA's XMM-Newton space telescope and NASA's Chandra X-ray Observatory, and optical data from the Sloan Digital Sky Survey (galaxy and foreground stars). The inset shows the X-ray pulsation of the spinning neutron star (image credit: ESA/XMM-Newton, NASA/Chandra and SDSS)
• January 31, 2017: Scientists observing a neutron star in the "Rapid Burster" system may have solved a 40-year-old mystery surrounding its puzzling X-ray bursts. 23)
- Discovered in the 1970s, the Rapid Burster is a binary system comprising a low-mass star in its prime and a neutron star — the compact remnant of a massive star's demise. The gravitational pull of the neutron star strips its companion of some of its gas, which then forms an accretion disk and spirals toward the neutron star.
- Most neutron star binary systems continuously release large amounts of X-rays, punctuated by additional X-ray flashes every few hours or days. But scientists have wondered for decades about what accounts for the Rapid Burster's sudden, erratic and extremely intense X-ray emissions — a phenomenon seen only in one other binary system.
- In the new study, researchers discovered that the neutron star's magnetic field creates a gap between the star and the disk around it, largely preventing it from feeding on matter from its stellar companion. Gas builds up until, under certain conditions, it hits the neutron star all at once, producing intense flashes of X-rays.
- The new results provide the first evidence for what causes these so-called "type-II" bursts. The discovery was made with the European Space Agency's XMM-Newton mission and NASA's NuSTAR (Nuclear Spectroscopic Telescope Array) and Swift missions. 24) 25)
- In such a binary system, the gravitational pull of the dense neutron star is stripping gas away from its stellar companion (a low-mass star, not shown in the images of Figure 16). The gas forms an accretion disk and spirals towards the neutron star.
- Before the burst, the fast-spinning magnetic field of the neutron star keeps the gas flowing from the companion star at bay, preventing it from reaching closer to the neutron star and effectively creating an inner edge at the center of the disk (top left of Figure 16). During this phase, only small amounts of gas leak towards the neutron star.
- However, as the gas continues to flow and accumulate near this edge, it spins faster and faster (top right), and eventually catches up with the spinning velocity of the magnetic field (bottom left and right of Figure 16).
Figure 16: These four images show an artist's impression of gas accreting onto the neutron star in the binary system MXB 1730-335, also known as the "Rapid Burster" (image credit: NASA/JPL, Caltech)
• Sept. 8, 2016: The central energy source of enigmatic pulsating ULX (Ultra Luminous X-ray ) sources could be a neutron star according to numerical simulations performed by a research group led by Tomohisa Kawashima at NAOJ (National Astronomical Observatory of Japan) . 26) 27)
- ULXs, which are remarkably bright X-ray sources, were thought to be powered by black holes. But in October 2014, the X-ray space telescope "NuSTAR" detected unexpected periodic pulsed emissions in a ULX named M82 X-2. The discovery of this object named "ULX-pulsar" has puzzled astrophysicists. Black holes can be massive enough to provide the energy needed to create ULXs, but black holes shouldn't be able to produce pulsed emissions. In contrast, "pulsars," a kind of neutron star, are named for the pulsed emissions they produce, but they are much fainter than ULXs. A new theory is needed to explain "ULX-pulsar."
- ULXs are thought to be caused by an object with strong gravity accreting gas from a companion star. As the gas falls towards the object, it collides with other gas. These collisions heat the gas until it gets hot enough to start glowing. The photons (in this case X-rays) emitted by this luminous gas are what astronomers actually observe. But as the photons travel away from the center, they push against the incoming gas, slowing the flow of gas towards the center. This force is called the radiation pressure force. As more gas falls onto the object, it becomes hotter and brighter, but if it becomes too bright the radiation pressure slows the infalling gas so much that it creates a "traffic jam." This traffic jam limits the rate at which new gas can add additional energy to the system and prevents it from getting any brighter. This luminosity upper limit, at which the radiation pressure balances the gravitational force, is called the Eddington luminosity.
- The Eddington luminosity is determined by the mass of the object. Because pulsars have masses hundreds of thousands of times less than the black holes thought to be powering ULXs, their Eddington luminosities are much lower than what would be needed to account for bright ULXs. But Kawashima and his team started to wonder if there might be a way for pulsars to avoid the traffic jam caused by the Eddington luminosity. "The astrophysicists have been so puzzled," he explains, "It may be difficult to sustain super-critical accretion onto neutron stars because neutron stars have solid surfaces, unlike black holes. It was a grand challenge to elucidate how to realize super-critical accretion onto neutron stars exhibiting pulsed emissions."
- For normal pulsars, researchers use an "accretion columns" model where the infalling gas is guided by the pulsar's strong magnetic field so that it lands on the magnetic poles. If the magnetic pole is misaligned with the neutron star's rotation axis (much like how 'magnetic north' is different from 'true north' on Earth), then the location of the magnetic pole will revolve around the rotation axis as the neutron star spins. If the magnetic pole points towards Earth, it appears bright to us, but when it rotates away, the emissions seem to disappear. This is similar to how a lighthouse seems to blink as the direction of its beam rotates.
- In order to address the mystery of ULX-pulsar, Kawashima and his team performed simulations to see if there is some way the accretion columns of gas could flow smoothly without a traffic jam and become hundreds of times brighter than the Eddington luminosity. "No one knew if super-critical column accretion could actually be realized on a neutron star," explains Shin Mineshige at Kyoto University, "It was a tough problem because we needed to simultaneously solve the equations of hydrodynamics and radiative transfer, which required advanced numerical techniques and computational power." In the 1970's, a few astrophysicists briefly addressed the calculation of moderately (not extremely) super-critical accretion columns, however they had to make many assumptions to make the calculations workable. "But thanks to recent developments in techniques and computer resources," says Ken Ohsuga at NAOJ, "we are now at the dawn of the radiation-hydrodynamic simulations era." The codes are already used for studies focused on black hole simulations. Thus, prompted by the discovery of ULX-pulsar, this team applied their radiation-hydrodynamic code to simulate super-critical accretion columns onto neutron stars, and carried out the simulations on the NAOJ supercomputer "ATERUI."
- The team found that it actually is possible for the infalling gas to avoid an Eddington luminosity induced traffic jam in super-critical column accretion. In their simulations, the accreting gas forms a shock front near the neutron star. Here, a huge amount of the kinetic energy of the infalling gas is converted to thermal energy. The gas just below the shock surface is rapidly heated by this energy and emits a huge number of photons. But rather than pushing back against the infalling gas as the previous models suggested, the photons are directed out the sides of the column. This means without a traffic jam, more gas can fall in rapidly, be heated by the shock front and produce more photons, so that the process isn't forced to slow down.
- The NAOJ team's model can account for the observed characteristics of ULX-pulsar: a high luminosity and directed beams of photons which will appear to blink as the neutron star rotates. Surprisingly, the direction of the photon beams is at a right angle to the polar beams expected in a standard pulsar model. This is the first simulation to support the idea that the central engine of the ULX-pulsar is a neutron star. This team is planning to further develop their work by using this new lighthouse model to study the detailed observational features of the ULX-pulsar M82 X-2, and to explore other ULX-pulsar candidates.
Figure 17: The new lighthouse model (a snapshot from Movie 1) and simulation results from the present research (inset on the right.) In the simulation results, the red indicates stronger radiation, and the arrows show the directions of photon flow. In this figure, many photons are produced near the surface of the neutron star and escape from the side of the accretion column. (image credit: NAOJ)
• July 28, 2016: Supermassive black holes in the universe are like a raucous choir singing in the language of X-rays. When black holes pull in surrounding matter, they let out powerful X-ray bursts. This song of X-rays, coming from a chorus of millions of black holes, fills the entire sky — a phenomenon astronomers call the cosmic X-ray background. 28)
- NASA's Chandra mission has managed to pinpoint many of the so-called active black holes contributing to this X-ray background, but the ones that let out high-energy X-rays — those with the highest-pitched "voices" — have remained elusive.
- New data from NASA's NuSTAR have for the first time, begun to pinpoint large numbers of the black holes belting out the high-energy X-rays. Or, in astronomer-speak, NuSTAR has made significant progress in resolving the high-energy X-ray background (Figure 18).
- "We've gone from resolving just two percent of the high-energy X-ray background to 35 percent," said Fiona Harrison, the principal investigator of NuSTAR at Caltech in Pasadena and lead author of a new study describing the findings in an upcoming issue of The Astrophysical Journal. "We can see the most obscured black holes, hidden in thick gas and dust."
- The results will ultimately help astronomers understand how the feeding patterns of supermassive black holes change over time. This is a key factor in the growth of not only black holes, but also the galaxies that host them. The supermassive black hole at the center of our Milky Way galaxy is dormant now, but at some point in the past, it too would have siphoned gas and bulked up in size.
- As black holes grow, their intense gravity pulls matter toward them. The matter heats up to scorching temperatures, and particles get boosted to close to the speed of light. Together, these processes make the black hole surroundings glow with X-rays. A supermassive black hole with a copious supply of fuel, or gas, will give off more high-energy X-rays. — NuSTAR is the first telescope capable of focusing these high-energy X-rays into sharp pictures.
- "Before NuSTAR, the X-ray background in high energies was just one blur with no resolved sources," said Harrison. "To untangle what's going on, you have to pinpoint and count up the individual sources of the X-rays."
- "We knew this cosmic choir had a strong high-pitched component, but we still don't know if it comes from a lot of smaller, quiet singers, or a few with loud voices," said co-author Daniel Stern, the project scientist for NuSTAR at NASA's Jet Propulsion Laboratory in Pasadena, California. "Now, thanks to NuSTAR, we're gaining a better understanding of the black holes and starting to address these questions."
- High-energy X-rays can reveal what lies around the most buried supermassive black holes, which are otherwise hard to see. In the same way that medical X-rays can travel through your skin to reveal pictures of bones, NuSTAR can see through the gas and dust around black holes, to get a deeper view of what's going on inside.
Figure 18: The blue dots in this field of galaxies, known as the COSMOS field, show galaxies that contain supermassive black holes emitting high-energy X-rays. The black holes were detected by NASA's NuSTAR, which spotted 32 such black holes in this field and has observed hundreds across the whole sky so far.
Legend to Figure 18: The other colored dots are galaxies that host black holes emitting lower-energy X-rays, and were spotted by NASA's Chandra X-ray Observatory. Chandra data show X-rays with energies between 0.5 to 7 keV, while NuSTAR data show X-rays between 8 to 24 keV.
• Jan. 5, 2016: NuSTAR has captured the best high-energy X-ray view yet of a portion of our nearest large, neighboring galaxy, Andromeda. The space mission has observed 40 "X-ray binaries" — intense sources of X-rays comprised of a black hole or neutron star that feeds off a stellar companion. The results will ultimately help researchers better understand the role of X-ray binaries in the evolution of our universe. According to astronomers, these energetic objects may play a critical role in heating the intergalactic bath of gas in which the very first galaxies formed. 29)
Figure 19: NuSTAR has imaged a swath of the Andromeda galaxy — the nearest large galaxy to our own Milky Way galaxy (image credit: NASA/JPL, Caltech, GSFC)
- Andromeda, also known as M31, can be thought of as the big sister to our own Milky Way galaxy. Both galaxies are spiral in shape, but Andromeda is slightly larger than the Milky Way in size. Lying 2.5 million light-years away, Andromeda is relatively nearby in cosmic terms. It can even be seen by the naked eye in dark, clear skies.
- Other space missions, such as NASA's Chandra X-ray Observatory, have obtained crisper images of Andromeda at lower X-ray energies than the high-energy X-rays detected by NuSTAR. The combination of Chandra and NuSTAR provides astronomers with a powerful tool for narrowing in on the nature of the X-ray binaries in spiral galaxies.
- In X-ray binaries, one member is always a dead star or remnant formed from the explosion of what was once a star much more massive than the sun. Depending on the mass and other properties of the original giant star, the explosion may produce either a black hole or neutron star. Under the right circumstances, material from the companion star can "spill over" its outermost edges and then be caught by the gravity of the black hole or neutron star. As the material falls in, it is heated to blazingly high temperatures, releasing a huge amount of X-rays.
• Dec. 17, 2015: NuSTAR finds a cosmic clumpy doughnut around a black hole. The most massive black holes in the universe are often encircled by thick, doughnut-shaped disks of gas and dust. This deep-space doughnut material ultimately feeds and nourishes the growing black holes tucked inside (Figure 20). 30)
- Until recently, telescopes weren't able to penetrate some of these doughnuts, also known as tori. "Originally, we thought that some black holes were hidden behind walls or screens of material that could not be seen through," said Andrea Marinucci of the Roma Tre University in Italy, lead author of a new Monthly Notices of the Royal Astronomical Society study describing results from NASA's NuSTAR (Nuclear Spectroscopic Telescope Array) and the European Space Agency's XMM-Newton space observatory.
- With its X-ray vision, NuSTAR recently peered inside one of the densest of these doughnuts known to surround a supermassive black hole. This black hole lies at the center of a well-studied spiral galaxy called NGC 1068, located 47 million light years away in the Cetus constellation.
- The observations revealed a clumpy, cosmic doughnut. "The rotating material is not a simple, rounded doughnut as originally thought, but clumpy," said Marinucci.
- Doughnut-shaped disks of gas and dust around supermassive black holes were first proposed in the mid-1980s to explain why some black holes are hidden behind gas and dust, while others are not. The idea is that the orientation of the doughnut relative to Earth affects the way we perceive a black hole and its intense radiation. If the doughnut is viewed edge-on, the black hole is blocked. If the doughnut is viewed face-on, the black hole and its surrounding, blazing materials can be detected. This idea is referred to as the unified model because it neatly joins together the different black hole types, based solely upon orientation.
- In the past decade, astronomers have been finding hints that these doughnuts aren't as smoothly shaped as once thought. They are more like defective, lumpy doughnuts that a doughnut shop might throw away.
- The new discovery is the first time this clumpiness has been observed in an ultra-thick doughnut, and supports the idea that this phenomenon may be common. The research is important for understanding the growth and evolution of massive black holes and their host galaxies.
Both NuSTAR and XMM-Newton observed the supermassive black hole in NGC 1068 simultaneously on two occasions between 2014 to 2015. On one of those occasions, in August 2014, NuSTAR observed a spike in brightness. NuSTAR observes X-rays in a higher-energy range than XMM-Newton, and those high-energy X-rays can uniquely pierce thick clouds around the black hole. The scientists say the spike in high-energy X-rays was due to a clearing in the thickness of the material entombing the supermassive black hole.
- NGC 1068 is well known to astronomers as the first black hole to give birth to the unification idea. "But it is only with NuSTAR that we now have a direct glimpse of its black hole through such clouds, albeit fleeting, allowing a better test of the unification concept," said Marinucci.
- The team says that future research will address the question of what causes the unevenness in doughnuts. The answer could come in many flavors. It's possible that a black hole generates turbulence as it chomps on nearby material. Or, the energy given off by young stars could stir up turbulence, which would then percolate outward through the doughnut. Another possibility is that the clumps may come from material falling onto the doughnut. As galaxies form, material migrates toward the center, where the density and gravity is greatest. The material tends to fall in clumps, almost like a falling stream of water condensing into droplets as it hits the ground.
Figure 20: Galaxy NGC 1068 can be seen in close-up in this view from NASA's Hubble Space Telescope. NuSTAR's high-energy X-rays eyes were able to obtain the best view yet into the hidden lair of the galaxy's central, supermassive black hole (image credit: NASA/JPL, CalTech)
Legend to Figure 20: This active black hole — shown as an illustration in the zoomed-in inset — is one of the most obscured known, meaning that it is surrounded by extremely thick clouds of gas and dust. The NuSTAR data revealed that the torus of gas and dust surrounding the black hole, also referred to as a doughnut, is more clumpy than previously thought. doughnuts around active, supermassive black holes were originally proposed in the mid-1980s to be smooth entities. More recently, researchers have been finding that doughnuts are not so smooth but have lumps. NuSTAR's latest finding shows that this is true for even the thickest of doughnuts.
• On July 13, 2015, the NuSTAR mission was 3 years on orbit.
• July 8, 2015: X-rays light up the surface of our sun in a bouquet of colors in this new image (Figure 21) containing data from NASA's NuSTAR (Nuclear Spectroscopic Telescope Array). The high-energy X-rays seen by NuSTAR are shown in blue, while green represents lower-energy X-rays from the X-ray Telescope instrument on the Hinode spacecraft, named after the Japanese word for sunrise. The yellow and red colors show ultraviolet light from NASA's SDO (Solar Dynamics Observatory). 31)
- NuSTAR usually spends its time investigating the mysteries of black holes, supernovae, and other high-energy objects in space. But it can also look closer to home to study our sun. The active areas of the sun are filled with flares, which are giant eruptions on the surface of the sun that spew out charged particles and high-energy radiation. They occur when magnetic field lines become tangled and broken, and then reconnect. Due to its extreme sensitivity, NuSTAR's telescope cannot view the larger flares. But it can help measure the energy of smaller microflares, which produce only one-millionth the energy of the larger flares.
- NuSTAR may also be able to directly detect hypothesized nanoflares, which would be only one-billionth the energy of flares. Nanoflares — which may help explain why the sun's atmosphere, or corona, is much hotter than expected — would be hard to spot due to their small size. However, nanoflares may emit high-energy X-rays that NuSTAR has the sensitivity to detect. Astronomers suspect that these tiny flares, like their larger brethren, can send electrons flying at tremendous velocities. As the electrons zip around, they give off high-energy X-rays.
- Astronomers are also excited to use NuSTAR's images of the sun to pinpoint where energy from flares is released. While it is known that the energy is generally liberated in the upper solar atmosphere, the locations and detailed mechanisms are not precisely known.
- Cosmologists are looking forward to using NuSTAR's solar observations, too. There is a slim chance the telescope could detect a hypothesized dark matter particle called the axion. Dark matter is a mysterious substance in our universe that is about five times more abundant than the regular matter that makes up everyday objects and anything that gives off light. NuSTAR might be able to address this and other mysteries of the sun.
Figure 21: Flaring, active regions of our sun are highlighted in this new image combining observations from several telescopes. High-energy X-rays from NASA's NuSTAR (Nuclear Spectroscopic Telescope Array) are shown in blue; low-energy X-rays from Japan's Hinode spacecraft (Solar-B) are green; and extreme ultraviolet light from NASA's SDO (Solar Dynamics Observatory) is yellow and red (image credit: NASA/JPL-Caltech, GSFC, JAXA)
• April 29, 2015: Peering into the heart of the Milky Way galaxy, NASA's NuSTAR has spotted a mysterious glow of high-energy X-rays that, according to scientists, could be the "howls" of dead stars as they feed on stellar companions. The project scientists see a completely new component of the center of our galaxy with NuSTAR's images. The center of our Milky Way galaxy is bustling with young and old stars, smaller black holes and other varieties of stellar corpses – all swarming around a supermassive black hole called Sagittarius A. 32)
- Astronomers have four potential theories to explain the baffling X-ray glow, three of which involve different classes of stellar corpses. When stars die, they don't always go quietly into the night. Unlike stars like our sun, collapsed dead stars that belong to stellar pairs, or binaries, can siphon matter from their companions. This zombie-like "feeding" process differs depending on the nature of the normal star, but the result may be an eruption of X-rays.
- According to one theory, a type of stellar zombie called a pulsar could be at work. Pulsars are the collapsed remains of stars that exploded in supernova blasts. They can spin extremely fast and send out intense beams of radiation. As the pulsars spin, the beams sweep across the sky, sometimes intercepting the Earth, like lighthouse beacons.
- Other possible culprits include heavy-set stellar corpses called white dwarfs, which are the collapsed, burned-out remains of stars not massive enough to explode in supernovae. Our sun is such a star, and is destined to become a white dwarf in about five billion years. Because these white dwarfs are much denser than they were in their youth, they have stronger gravity and can produce higher-energy X-rays than normal. Another theory points to small black holes that slowly feed off their companion stars, radiating X-rays as material plummets down into their bottomless pits.
- Alternatively, the source of the high-energy X-rays might not be stellar corpses at all, astronomers say, but rather a diffuse haze of charged particles, called cosmic rays. The cosmic rays might originate from the supermassive black hole at the center of the galaxy as it devours material. When the cosmic rays interact with surrounding, dense gas, they emit X-rays.
- However, none of these theories match what is known from previous research, leaving the astronomers largely stumped.
Figure 22: NuSTAR has captured a new high-energy X-ray view (magenta) of the bustling center of our Milky Way galaxy. The smaller circle shows the area where the NuSTAR image was taken -- the very center of our galaxy, where a giant black hole resides. That region is enlarged to the right, in the larger circle, to show the NuSTAR data (image credit: NASA/JPL, Caltech)
Legend to Figure 22: The NuSTAR picture is one of the most detailed ever taken of the center of our galaxy in high-energy X-rays. The X-ray light, normally invisible to our eyes, has been assigned the color magenta. The brightest point of light near the center of the X-ray picture is coming from a spinning dead star, known as a pulsar, which is near the giant black hole. While the pulsar's X-ray emissions were known before, scientists were surprised to find more high-energy X-rays than predicted in the surrounding regions, seen here as the elliptical haze. 33)
Astronomers aren't sure what the sources of the extra X-rays are, but one possibility is a population of dead stars. The background picture was captured in infrared light by NASA's Spitzer Space Telescope. The NuSTAR image has an X-ray energy range of 20 to 40 keV.
• Feb. 19, 2015: Astronomers have discovered that the NuSTAR and XMM-Newton missions of NASA and ESA, respectively, are showing that fierce winds from a supermassive black hole blow outward in all directions — a phenomenon that had been suspected, but difficult to prove until now. This discovery has given astronomers their first opportunity to measure the strength of these ultra-fast winds and prove they are powerful enough to inhibit the host galaxy's ability to make new stars. 34) 35)
Supermassive black holes blast matter into their host galaxies, with X-ray emitting winds traveling at up to one-third the speed of light. In the new study, astronomers determined PDS 456, an extremely bright black hole known as a quasar more than 2 billion light-years away, sustains winds that carry more energy every second than is emitted by more than a trillion suns.
NuSTAR and XMM-Newton simultaneously observed PDS 456 on five separate occasions in 2013 and 2014. The space telescopes complement each other by observing different parts of the X-ray light spectrum: XMM-Newton views low-energy and NuSTAR views high-energy spectra.
Previous XMM-Newton observations had identified black hole winds blowing toward us, but could not determine whether the winds also blew in all directions. XMM-Newton had detected iron atoms, which are carried by the winds along with other matter, only directly in front of the black hole, where they block X-rays. Combining higher-energy X-ray data from NuSTAR with observations from XMM-Newton, scientists were able to find signatures of iron scattered from the sides, proving the winds emanate from the black hole not in a beam, but in a nearly spherical fashion.
Figure 23: Artist's rendition of suppermassive black holes at the cores of galaxies blast out radiation and ultra-fast winds as observed by NASA's NuSTAR and ESA's XMM-Newton missions (image credit: NASA/JPL-Caltech, ESA)
The data plot of Figure 24 from NuSTAR and XMM-Newton determines for the first time the shape of ultra-fast winds from supermassive black holes, or quasars. The winds blow in every direction, in a nearly spherical fashion, coming from both sides of a galaxy (only one side is shown in the artist's impression here). 36)
The plot shows the brightness of X-ray light from an extremely luminous quasar called PDS 456, with the highest-energy rays on the right. XMM-Newton sees the lower-energy X-rays, and NuSTAR the higher ones. XMM Newton had previously observed PDS 456 in 2001. At that time, it had measured the X-rays up to an energy level of 11 keV. From those data, researchers detected a dip in the X-ray light, called an absorption feature (see dip in plot). The dip is caused by iron atoms – which are carried by the winds along with other matter – absorbing the X-ray light of a particular energy. What's more, the absorption feature is 'blue-shifted," meaning that the winds are speeding toward us.
These data told researchers that at least some of the winds were blowing toward us – but they didn't reveal whether those winds were confined to a narrow beam along our line of sight, or were blowing in all directions. That's because XMM-Newton had only detected absorption features, which by definition occur in front of a light source, in this case, the quasar. To probe what was happening at the other sides of the quasar, the astronomers needed to find an emission feature, which would indicate that the iron was scattering X-ray light at a particular energy in all directions, not only toward the observer.
NuSTAR and XMM-Newton teamed up to observe PDS 456 simultaneously in 2013 and 2014, and the results of that campaign are shown in this plot. NuSTAR data are represented as orange circles and XMM-Newton as blue squares. The NuSTAR data reveal the baseline of the "continuum" quasar light (see gray line) – or what the quasar would look like without any winds. What stands out is the bump to the left of the dips. That is an iron emission signature, the telltale sign that the black-hole winds blow to the sides and in all directions.
Figure 24: XMM-Newton and NuSTAR spectrum of the quasar PDS 456 (image credit: NASA/JPL-Caltech, Keele University, UK)
• Dec. 22, 2014: For the first time, a mission designed to set its eyes on black holes and other objects far from our solar system has turned its gaze back closer to home, capturing images of our sun. NASA's NuSTAR has taken its first picture of the sun, producing the most sensitive solar portrait ever taken in high-energy X-rays. 37) 38) 39)
- While the sun is too bright for other telescopes such as NASA's Chandra X-ray Observatory, NuSTAR can safely look at it without the risk of damaging its detectors. This first solar image from NuSTAR demonstrates that the telescope can in fact gather data about sun. And it gives insight into questions about the remarkably high temperatures that are found above sunspots — cool, dark patches on the sun. Future images will provide even better data as the sun winds down in its solar cycle.
- With NuSTAR's high-energy views, it has the potential to capture hypothesized nanoflares — smaller versions of the sun's giant flares that erupt with charged particles and high-energy radiation. Nanoflares, should they exist, may explain why the sun's outer atmosphere, called the corona, is sizzling hot, a mystery called the "coronal heating problem." The corona is, on average 1 million degrees Celsius, while the surface of the sun is relatively cooler at 6,000 degrees Celsius. It is like a flame coming out of an ice cube. Nanoflares, in combination with flares, may be sources of the intense heat.
Figure 25: X-rays stream off the sun in this image showing observations from by NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, overlaid on a picture taken by NASA's SDO (Solar Dynamics Observatory), image credit: NASA/JPL-Caltech/GSFC
Legend to Figure 25: This is the first picture of the sun taken by NuSTAR. The field of view covers the west limb of the sun. The NuSTAR data, seen in green and blue, reveal solar high-energy emission (green shows energies between 2 and 3 keV volts, and blue shows energies between 3 and 5 keV). The high-energy X-rays come from gas heated to above 3 million degrees. The red channel represents ultraviolet light captured by SDO at wavelengths of 171 Å, and shows the presence of lower-temperature material in the solar atmosphere at 1 million degrees. - This image shows that some of the hotter emission tracked by NuSTAR is coming from different locations in the active regions and the coronal loops than the cooler emission shown in the SDO image.
• October 8, 2014: A research team led by Caltech astronomers of Pasadena California have found a pulsating, dead star beaming with the energy of about 10 million suns. This is the brightest pulsar – a dense stellar remnant left over from a supernova explosion – ever recorded. The discovery was made with NASA's NuSTAR (Nuclear Spectroscopic Telescope Array). 40) 41) 42) 43)
- The surprising find is helping astronomers better understand mysterious sources of blinding X-rays, called ULXs (Ultraluminous X-ray sources). Until now, all ULXs were thought to be black holes. The new data from NuSTAR show at least one ULX, about 12 million light years away in the galaxy Messier 82 (M82), is actually a pulsar.
ULXs are generally thought to be black holes feeding off companion stars — a process called accretion. They also are suspected to be the long-sought after "medium-size" black holes – missing links between smaller, stellar-size black holes and the gargantuan ones that dominate the hearts of most galaxies. But research into the true nature of ULXs continues toward more definitive answers.
NuSTAR did not initially set out to study the two ULXs in M82. Astronomers had been observing a recent supernova in the galaxy when they serendipitously noticed pulses of bright X-rays coming from the ULX known as M82 X-2. Black holes do not pulse, but pulsars do.
- The key to NuSTAR's discovery was its sensitivity to high-energy X-rays, as well as its ability to precisely measure the timing of the signals, which allowed astronomers to measure a pulse rate of 1.37 seconds. They also measured its energy output at the equivalent of 10 million suns, or 10 times more than that observed from other X-ray pulsars. This is a big punch for something about the mass of our sun and the size of Pasadena.
Astronomers are planning follow-up observations with NASA's NuSTAR, Swift and Chandra spacecraft to find an explanation for the pulsar's bizarre behavior. The NuSTAR team also will look at more ULXs, meaning they could turn up more pulsars. At this point, it is not clear whether M82 X-2 is an oddball or whether more ULXs beat with the pulse of dead stars. NuSTAR, a relatively small telescope, has thrown a big loop into the mystery of black holes.
Figure 26: A rare and mighty pulsar (pink) can be seen at the center of the galaxy Messier 82 in this new multi-wavelength portrait. NASA's NuSTAR mission discovered the "pulse" of the pulsar — a type of dead star — using is high-energy X-ray vision (image credit: NASA/JPL, Caltech)
• August 12, 2014: NuSTAR has captured an extreme and rare event in the regions immediately surrounding a supermassive black hole. A compact source of X-rays that sits near the black hole, called the corona, has moved closer to the black hole over a period of just days. The corona recently collapsed in toward the black hole, with the result that the black hole's intense gravity pulled all the light down onto its surrounding disk, where material is spiraling inward. 44)
- As the corona shifted closer to the black hole, the gravity of the black hole exerted a stronger tug on the X-rays emitted by it. The result was an extreme blurring and stretching of the X-ray light. Such events had been observed previously, but never to this degree and in such detail. Supermassive black holes are thought to reside in the centers of all galaxies. Some are more massive and rotate faster than others. The black hole in this new study, referred to as Markarian 335, or Mrk 335, is about 324 million light-years from Earth in the direction of the Pegasus constellation. It is one of the most extreme of the systems for which the mass and spin rate have ever been measured. The black hole squeezes about 10 million times the mass of our sun into a region only 30 times the diameter of the sun, and it spins so rapidly that space and time are dragged around with it.
- The data plot of Figure 27, captured by NuSTAR, shows X-ray light streaming from regions near a supermassive black hole known as Markarian 335. The light is coming from two areas: a superheated disk of material feeding the black hole, called the accretion disk; and a cloud of particles traveling near the speed of light, called the corona. The exact shape and nature of coronas are not clear, but researchers know that X-ray light from the corona is reflected off the accretion disk.
That reflected light, and the corona's direct light, are mapped in this plot over a range of X-ray energies. (This is the highest range of X-rays, which NuSTAR was specially designed to see.) The yellow line is a model that shows what the data are predicted to look like if X-ray light has been stretched, or blurred. The blue line shows what the plot would look like without the blurring effects. The white dots show the actual NuSTAR data, indicating the light is extremely blurred.
What's blurring the light? It's a combination of factors. First, there is a Doppler shift happening due to the spinning disk. As one side of the disk moves toward us and the other side away, the light is squeezed or stretched. A second effect has to do with the enormous speeds of the spinning black hole, which approach the speed of light. A final effect is from the enormous gravity of the black hole, which pulls on the light, making it harder to escape its grasp. The light loses energy in this process. All of these factors contribute to the smeared look of the data as seen in the plot.
These data were taken after a dramatic dip in brightness had first been observed by NASA's Swift satellite. NuSTAR's high-energy X-ray data pointed to the cause for the observed change: Markarian 335's corona had shifted closer to the black hole itself — and this closer proximity meant that the black hole's gravity could yank harder on the corona's light, stretching it to lower energies than observed before. The astronomers say that the corona moved over a period of days, and is still in the close configuration. They don't know if and when it would move back to where it was previously, or why the corona moved. NuSTAR and other high-energy telescopes are busy trying to crack these mysteries (Ref. 44).
Figure 27: This plot of data, captured by NuSTAR, shows X-ray light streaming from regions near a supermassive black hole known as Markarian 335 (image credit: NASA/JPL-Caltech/Institute for Astronomy, Cambridge)
• February 19, 2014: NuSTAR has created the first map of radioactive material in a supernova remnant. The results, from a remnant named Cassiopeia A (Cas A), reveal how shock waves likely rip apart massive dying stars. 45) 46) 47)
Figure 28: This is the first map (false color image) of radioactivity in a supernova remnant, the blown-out bits and pieces of a massive star that exploded. The blue color shows radioactive material mapped in high-energy X-rays using NuSTAR (image credit: NASA/JPL-Caltech, CXC, SAO)
Legend to Figure 28: The mystery of how Cassiopeia A exploded is unraveling thanks to new data from NASA's NuSTAR. In this image, NuSTAR data, which show high-energy X-rays from radioactive material, are colored blue. Lower-energy X-rays from non-radioactive material, imaged previously with NASA's Chandra X-ray Observatory, are shown in red, yellow and green. The new view shows a more complete picture of Cassiopeia A, the remains of a star that blew up in a supernova event whose light reached Earth about 350 years ago, when it could have appeared to observers as a star that suddenly brightened. The remnant is located 11,000 light-years away from Earth.
NuSTAR is the first telescope capable of taking detailed pictures of the radioactive material in the Cassiopeia A supernova remnant. While other telescopes have detected radioactivity in these objects before, NuSTAR is the first capable of pinpointing the location of the radioactivity, creating maps. When massive star explode, they create many elements: non-radioactive ones like iron and calcium found in your blood and bones; and radioactive elements like titanium-44, the decay of which sends out high-energy X-ray light that NuSTAR can see.
By mapping titanium-44 in Cassiopeia A, astronomers get a direct look at what happened in the core of the star when it was blasted to smithereens. These NuSTAR data complement previous observations made by Chandra, which show elements, such as iron, that were heated by shock waves farther out from the remnant's center.
In this image, the red, yellow and green data were collected by Chandra at energies ranging from 1 to 7 keV. The red color shows heated iron, and green represents heated silicon and magnesium. The yellow is what astronomers call continuum emission, and represents a range of X-ray energies. The titanium-44, shown in blue, was detected by NuSTAR at energies ranging between 68 and 78 keV (Ref. 45).
• January 2014: NuSTAR is now executing its primary science mission, and with an expected orbit lifetime of 10 years, the project anticipates proposing a guest investigator program, to begin in late 2014 (Ref. 67).
• January 2014: NuSTAR's unique viewpoint, in seeing the highest-energy X-rays, is showing the project well-studied objects and regions in a whole new light. Figure 29of NuSTAR shows the energized remains of a dead star, a structure nicknamed the "Hand of God" after its resemblance to a hand. 48) 49)
- The new "Hand of God" image shows a nebula 17,000 light years away, powered by a dead, spinning star called PSR B1509-58, or B1509 for short. The dead star, called a pulsar, is the leftover core of a star that exploded in a supernova. The pulsar is only about 19 km in diameter but packs a big punch: it is spinning around nearly seven times every second, spewing particles into material that was upheaved during the star's violent death. These particles are interacting with magnetic fields around the ejected material, causing it to glow with X-rays. The result is a cloud that, in previous images, looked like an open hand. - One of the big mysteries of this object, called a pulsar wind nebula, is whether the pulsar's particles are interacting with the material in a specific way to make it appear as a hand, or if the material is in fact shaped like a hand.
- With approximately 10 times greater spatial resolution and more than 100 times greater sensitivity than previous missions in this energy band, NuSTAR has opened the high-energy sky to sensitive study. Over the first year of the mission, NuSTAR has undertaken a range of studies, from observations of energetic events towards the center of the Milky Way galaxy to detailed studies of distant supermassive black holes. 50)
Figure 29: High-energy X-ray view of the 'Hand of God' (image credit: NASA/JPL, Caltech, McGill University)
Legend to Figure 29: NuSTAR has imaged the structure in high-energy X-rays for the first time, shown in blue. Lower-energy X-ray light, previously detected by NASA's Chandra X-ray Observatory, is shown in green and red. NuSTAR's view is providing new clues to the puzzle. The hand actually shrinks in the NuSTAR image, looking more like a fist, as indicated by the blue color. The northern region, where the fingers are located, shrinks more than the southern part, where a jet lies, implying the two areas are physically different. The red cloud at the end of the finger region is a different structure, called RCW 89. Astronomers think the pulsar's wind is heating the cloud, causing it to glow with lower-energy X-ray light.
In this image, X-ray light seen by Chandra with energy ranges of 0.5 to 2 keV and 2 to 4 keV is shown in red and green, respectively, while X-ray light detected by NuSTAR in the higher-energy range of 7 to 25 keV is blue.
• August 29, 2013: NuSTAR is giving the wider astronomical community a first look at its unique X-ray images of the cosmos. The first batch of data from the black-hole hunting telescope is publicly available today, Aug. 29, via NASA's HEASARC (High Energy Astrophysics Science Archive Research Center). 51)
- The images, taken from July to August 2012, shortly after the spacecraft launched, comprise an assortment of extreme objects, including black holes near and far. The more distant black holes are some of the most luminous objects in the universe, radiating X-rays as they ferociously consume surrounding gas. One type of black hole in the new batch of data is a blazar, which is an active, supermassive black hole pointing a jet toward Earth. Systems known as X-ray binaries, in which a compact object such as a neutron star or black hole feeds off a stellar companion, are also in the mix, along with the remnants of stellar blasts called supernovas.
• Feb. 27, 2013: Two X-ray space observatories, NASA's NuSTAR and the ESA's XMM-Newton missions, have teamed up to measure definitively, for the first time, the spin rate of a black hole with a mass 2 million times that of our sun. 52)
The supermassive black hole lies at the dust- and gas-filled heart of a galaxy called NGC 1365, and it is spinning almost as fast as Einstein's theory of gravity will allow. The observations also are a powerful test of Einstein's theory of general relativity, which says gravity can bend space-time, the fabric that shapes our universe, and the light that travels through it.
NuSTAR is designed to detect the highest-energy X-ray light in great detail. It complements telescopes that observe lower-energy X-ray light, such as XMM-Newton and NASA's Chandra X-ray Observatory. Scientists use these and other telescopes to estimate the rates at which black holes spin (Figure 31).
Figure 30: The artist's concept illustrates a supermassive black hole with millions to billions times the mass of our sun. Supermassive black holes are enormously dense objects buried at the hearts of galaxies (image credit: NASA/JPL)
Until now, these measurements were not certain because clouds of gas could have been obscuring the black holes and confusing the results. With help from XMM-Newton, NuSTAR was able to see a broader range of X-ray energies and penetrate deeper into the region around the black hole. The new data demonstrate that X-rays are not being warped by the clouds, but by the tremendous gravity of the black hole. This proves that spin rates of supermassive black holes can be determined conclusively. Measuring the spin of a supermassive black hole is fundamental to understanding its past history and that of its host galaxy.
Supermassive black holes are surrounded by pancake-like accretion disks, formed as their gravity pulls matter inward. Einstein's theory predicts the faster a black hole spins, the closer the accretion disk lies to the black hole. The closer the accretion disk is, the more gravity from the black hole will warp X-ray light streaming off the disk (Ref. 52).
Figure 31: Artist's view of two models of black hole spin (image credit: NASA/JPL, Caltech)
Legend to Figure 31: Scientists measure the spin rates of supermassive black holes by spreading the X-ray light into different colors. The light comes from accretion disks that swirl around black holes, as shown in both of the artist's concepts. They use X-ray space telescopes to study these colors, and, in particular, look for a "fingerprint" of iron — the peak shown in both graphs, or spectra — to see how sharp it is. Prior to observations with NASA's NuSTAR (Spectroscopic Telescope Array), and the European Space Agency's XMM-Newton telescope, there were two competing models to explain why this peak might not appear to be sharp.
The "rotation" model shown at top of Figure 31 held that the iron feature was being spread out by distorting effects caused by the immense gravity of the black hole. If this model were correct, then the amount of distortion seen in the iron feature should reveal the spin rate of the black hole.
The alternate model held (bottom of Figure 31) that obscuring clouds lying near the black hole were making the iron line appear artificially distorted. If this model were correct, the data could not be used to measure black hole spin.
NuSTAR helped to solve the case, ruling out the alternate "obscuring cloud" model. Its high-energy X-ray data — shown at top as green bump to the right of the peak — revealed that features in the X-ray spectrum are in fact coming from the accretion disk and not from the obscuring clouds. Together with XMM-Newton, the space observatories were able to make the first conclusive measurement of a black hole's spin rate, and more generally, confirm that the "gravitational distortion" model is accurate (Ref. 52). 53)
The solid lines of Figure 32 show two theoretical models that explain low-energy X-ray emission seen previously from the spiral galaxy NGC 1365 by XMM-Newton. The red line explains the emission using a model where clouds of dust and gas partially block the X-ray light, and the green line represents a model in which the emission is reflected off the inner edge of the accretion disk, very close to the black hole. 54)
The blue circles show the latest measurements from XMM-Newton, and the yellow circles show the data from NuSTAR. While both models fit the XMM-Newton data equally well, only the disk reflection model fits the NuSTAR data.
The results show that the iron feature, the sharp peak at left, is being affected black hole's immense gravity and not intervening clouds. The degree to which the iron feature is spread out reveals the spin rate of the black hole.
Figure 32: Two X-ray observatories are better than one (image credit: NASA/JPL, Caltech)
• Feb. 21, 2013: NuSTAR has been in orbit around Earth for more than eight months since its launch in June 2012, studying black holes and probing the nature of the high-energy X-ray universe. Mission and science operations have settled down into a mostly predictable daily routine, and the science team is making good progress toward achieving the primary, or "Level 1," science goals. Examples of some of the key NuSTAR observations performed to date include mapping of the central regions of our Milky Way galaxy, studying the remnants of exploded stars in our galaxy, and surveys of several well-studied extragalactic fields. 55)
• January 2013: Since launch, the NuSTAR team has been fine-tuning the telescope. The mission has looked at a range of extreme, high-energy objects already, including black holes near and far, and the incredibly dense cores of dead stars. In addition, NuSTAR has begun black-hole searches in the inner region of the Milky Way galaxy and in distant galaxies in the universe. 56) 57)
Figure 33: This new view of the spiral galaxy IC 342, seven million light years away, includes data from NuSTAR (image credit: NASA/JPL,Caltech/DSS)
Legend to Figure 33: This new view of spiral galaxy IC 342, also known as Caldwell 5, includes data from NASA's NuSTAR spacecraft. High-energy X-ray data from NuSTAR have been translated to the color magenta, and superimposed on a visible-light view highlighting the galaxy and its star-studded arms. NuSTAR is the first orbiting telescope to take focused pictures of the cosmos in high-energy X-ray light; previous observations of this same galaxy taken at similar wavelengths blurred the entire object into one pixel.
The two magenta spots are blazing black holes first detected at lower-energy X-ray wavelengths by NASA's Chandra X-ray Observatory. With NuSTAR's complementary data, astronomers can start to home in on the black holes' mysterious properties. The black holes appear much brighter than typical stellar-mass black holes, such as those that pepper our own galaxy, yet they cannot be supermassive black holes or they would have sunk to the galaxy's center. Instead, they may be intermediate in mass, or there may be something else going on to explain their extremely energetic state. NuSTAR will help solve this puzzle.
The image (Figure 33) shows NuSTAR X-ray data taken at 10 to 35 keV. The visible-light image is from the Digitized Sky Survey.
• November 2012: The first three months of science, or "Phase E," operations have been a busy time for NASA's NuSTAR. Since the science operations began on August 1, 2012, the NuSTAR team has wrestled with learning to point the telescope's flexible system of optics, mast, spacecraft bus and solar array. Characterization of the behavior of NuSTAR in space took an additional six weeks, but has now been completed. Several adjustments to attitude control parameters — factors in pointing the telescope — were necessary to bring the spacecraft into the required specifications, and the satellite's slew rate, or the speed at which it points to new targets, has recently been enhanced by a factor of two. 58)
Calibrating the telescope is now a primary focus for the science team. NuSTAR has made a number of coordinated observations of famous, bright X-ray sources like the Crab nebula, Hercules X-1, 3C 273 and IC 4329A. The first two are neutron stars in our Milky Way galaxy, and the latter two are accreting black holes in the centers of nearby galaxies. These coordinated calibration campaigns have been done with many of the low- and high-energy X-ray astronomical satellites currently in orbit: NASA's Chandra X-ray and Swift observatories, ESA's (European Space Agency's) XMM-Newton and INTEGRAL telescopes, and JAXA's (Japan Aerospace Exploration Agency's) Suzaku.
• On June 28, 2012, NuSTAR has taken its first snapshots of the highest energy X-rays in the cosmos. The first images from the observatory show Cygnus X-1, a black hole in our galaxy that is siphoning gas off a giant-star companion. This particular black hole was chosen as a first target because it is extremely bright in X-rays, allowing the NuSTAR team to easily see where the telescope's focused X-rays are falling on the detectors. 59)
• On June 21, 2012 (9 days after launch), NuSTAR successfully deployed its lengthy mast, giving it the ability to see the highest energy X-rays in the universe. 60)
The NuSTAR team will now begin to verify the pointing and motion capabilities of the satellite, and fine-tune the alignment of the mast. In about five days, the team will instruct NuSTAR to take its "first light" pictures, which are used to calibrate the telescope.
• Confirmation of the successful deployment of NuSTAR's solar arrays has been received. All systems are nominal. 61)
Sensor complement: (NuSTAR)
The instrument has the same name as the spacecraft. NuSTAR is based in large part on the technologies developed for the HEFT (High-Energy Focusing Telescope) balloon experiment. The NuSTAR instrument team is from the following institutions: Caltech, Columbia University, DTU (Danish Technical University), LLNL (Lawrence Livermore National Laboratory),NASA/GSFC (Goddard Space Flight Center), and UCB (University of California, Berkeley). 62) 63) 64) 65) 66)
NuSTAR operates in the band from 3 to 79 keV, extending the sensitivity of focusing far beyond the ~10 keV high-energy cutoff achieved by all previous X-ray satellites. The inherently low background associated with concentrating the X-ray light enables NuSTAR to probe the hard X-ray sky with a more than 100-fold improvement in sensitivity over the collimated or coded mask instruments that have operated in this bandpass. Using its unprecedented combination of sensitivity and spatial and spectral resolution, NuSTAR will pursue five primary scientific objectives: 67)
1) probe the obscured AGN (Active Galactic Ncleus) activity out to the peak epoch of galaxy assembly in the universe (at z <~ 2) by surveying selected regions of the sky
2) study the population of hard X-ray-emitting compact objects in the Galaxy by mapping the central regions of the Milky Way
3) study the non-thermal radiation in young supernova remnants, both the hard X-ray continuum and the emission from the radioactive element 44 Ti
4) observe blazars contemporaneously with ground-based radio, optical, and TeV telescopes, as well as with Fermi and Swift, to constrain the structure of AGN jets
5) observe line and continuum emission from core-collapse supernovae in the Local Group, and from nearby Type Ia events, to constrain explosion models. During its baseline two-year mission, NuSTAR will also undertake a broad program of targeted observations.
The NuSTAR instrument
Instrument: The NuSTAR instrument is the first astronomical telescope in orbit to utilize the new generation of hard X-ray optics and solid-state detector technologies to carry out high-sensitivity observations at X-ray energies significantly greater than 10 keV. It consists of two co-aligned depth-graded multilayer coated grazing incidence optics focused onto solid state CdZnTe pixel detectors with a focal length of 10.15 m. Figure 35 shows the total effective area for both telescopes as a function of energy, with a comparison to Chandra and XMM. The energy band extends from about 5 - 79 keV, being limited at the low-energy end by the optics thermal cover and shield entrance window, and at the high energy end by the K-edge (at 78.4 keV) in the Platinum mirror coatings.
For (focusing) X-ray telescopes, the standard metric for specifying angular resolution is the HPD (Half-Power Diameter), which is also called the HEW (Half-Energy Width). This is the angular diameter of the image of a point source, which contains half the flux (at a given energy) focused by the telescope. From the standpoint of detecting and measuring sources with an X-ray telescope, the HPD proves more useful than other imaging metrics—e.g., full width at FWHM (Full Width Half maximum) and RMS (Root-Mean-of Squares) image blur. However, the RMS is useful in formulating imaging error budgets for the geometric-optics terms that govern the PSF (Point Spread Function) core. 68)
Table 3: Key instrument performance parameters (Ref. 1)
Figure 34: Two X-ray telescopes and the electromagnetic spectrum in energy and wavelength presentation (image credit: NASA/JPL, Caltech)
The instrument FOV is energy-dependent due to changes in multilayer reflectance as a function of energy and optics shell radius, which results in overall loss of reflectance and more vignetting at high energy (Figure 36). The spectral resolution is 500 eV at energies below ~30 keV, and increases to 1.2 keV at the upper end of the energy range. The 2 µs temporal resolution, determined by the bit rate allocated in the telemetry stream for time tags, is more than adequate to meet the scientific requirements. The intrinsic temporal resolution of the detector is better than 1 µs. The target of opportunity (ToO) response time is required to be less than 24 hours; however, on average the turnaround will be faster, with targets typically acquired within 6 hours.
Figure 35: Effective area for two telescopes as a function of energy compared with the Chandra and XMM focusing telescopes (image credit: NuSTAR collaboration)
Table 4: Baseline mission science plan
Figure 36: Effective area as a function of off-axis angle, as a fraction of on-axis area, for several energies (image credit: NuSTAR collaboration)
The NuSTAR instrument is launched in a compact, stowed configuration, and after launch a 10 m mast is deployed to achieve a focal length of 10.15 m. Since the absolute deployment location of the mast is difficult to measure on the ground, due to complications associated with complete gravity off-loading, an adjustment mechanism is built into the last section of the mast to enable a one-time alignment to optimize the location of the optical axes on the focal plane. This mechanism provides two angular adjustments as well as rotation. The mast is not perfectly rigid, it is being subjected to thermal distortions particularly when going in and out of Earth's shadow which translate into changes in telescope alignment of 1 -2 arcmin. These mast alignment changes are measured by the combination of an optics bench-mounted star tracker and a laser metrology system.
The same combination of sensors also provides the absolute instrument aspect. In order to limit the FOV open to the detectors, and therefore the diffuse cosmic background, an aperture stop consisting of three rings deploys with the mast. The aperture stop is shown in the deployed configuration in Figure 37. In the stowed configuration, the top will be 0.83 m above the focal plane surface.
Figure 37: Diagram of the NuSTAR instrument showing the principal elements (image credit: NuSTAR collaboration)
A blowup of an individual optics module is also shown in Figure 39. Each layer of the optic has an upper and lower conic shell (equivalent to the parabola-hyperbola sections of a Wolter-I optic). Each shell is composed of multiple glass segments formed by thermal slumping. Each piece of glass is coated with a depth-graded multilayer to enhance reflectivity. The enhanced reflectivity provided by the multilayers, along with the shallow graze angles afforded by the long focal length of the optics (10.15 m) provide high effective area over the NuSTAR energy band of 6-79 keV, and a FOV of 12 arcminutes by 12 arcminutes. There are 133 concentric layers which together form each optic. 69) 70) 71)
The glass layers (a Titanium-glass-epoxy-graphite composite structure) are built up on a Titanium mandrel. Titanium support spiders located on the top and bottom of each optic connect it to the optical bench. The compliant, radially-symmetric spiders accommodate thermal expansion effects. Thermal covers (5 µm polyimide) on the entrance and exit apertures of the optic reduce thermal gradients by blocking direct view of the sun and deep space. Three flight modules (called FM0, FM1 and FM2 respectively) are being fabricated. The first two modules are the flight units, and the third module is to provide for more extensive X-ray characterization than is permitted for either of the flight modules, given the compressed delivery schedule of the optics..
Figure 38: Schematic view of a focusing near-grazing X-ray telescope (image credit: NuSTAR collaboration)
Figure 39: NuSTAR instrument showing a blowup of an optics module (image credit: NuSTAR collaboration)
The layers of glass are physically separated from and attached to each other by means of precisely-machined graphite spacers which run lengthwise down the glass segments, and which constrain the glass to the proper conical shape of the Wolter 1 geometry. The spacers are 1.2 mm wide (except in the inner 5 layers where they are 1.6 mm wide to provide more bonding area in this high stress region) in order to minimize X-ray shadowing. The spacers are bonded to the Titanium mandrel and to the glass layers by means of a low outgassing epoxy (Henkel F131). The inner 65 layers of glass form integral shells by means of ~60º sectors, and the outer 65 layers by means of ~30º sectors (dodecants), see Figure 40 (left). The number of graphite spacers is fixed at 5 per glass segment for fabrication simplicity. The transition from sextants to dodecants provides for a reduction in the spacer span between glass segments as the radius of the layers grows, thus maintaining good control of the glass figure. Figure 40 (right) shows the details of the 3 sextant layers of glass that serve as an "intermediate mandrel" for transitioning from sextant to dodecant layers.
Figure 40: Optic drawing showing glass segmentation, support spiders and polyimide thermal shield (left); on the right is the end view of optics showing intermediate mandrel (IM) details; IM glass (blue); IM spacers (light blue); spacers (gray); sextants and dodecants above/below the IM (image credit: NuSTAR collaboration)
The right side image of Figure 40 shows the details of the 3 sextant layers of glass that serve as an "intermediate mandrel" for transitioning from sextant to dodecant layers. The mandrel, sextant and dodecant glass layers and spacers are visible. Double width spacers azimuthally tie the dodecants to the sextant glass at the intermediate mandrel, increasing torsional stiffness and providing a path for distribution of launch loads. The glass segments are coated with depth-graded multilayers. All three flight modules use W/Si for the outer layers. FM0 uses Pt/SiC for its inner layers, while FM1 and FM2 use Pt/C for the inner layers. In order to optimize multilayer reflectivity, the optic's layers are divided into 10 groups, with a different multilayer recipe for each group.
Table 5: Summary of the optics module parameters
Figure 41: One of NuSTAR's two mirrors, or optics, assembled in a clean room at Columbia University's Nevis Laboratory (image credit: NuSTAR collaboration)
Figure 42: NuSTAR FM0 (Flight Module 0) with 106 layers on assembly machine of Columbia University's Nevis Laboratory (image credit: NuSTAR collaboration)
Focal plane modules: Each focal plane consists of four CdZnTe pixel sensors coupled to a custom low-noise ASIC (Application Specific Integrated Circuit). Each hybrid contains a 32 x 32 array of 600 µm pixels with a resulting plate scale of 12.300/pixel, so that the mirror point spread function is over-sampled. The sensors are placed in a 2 x 2 array with a minimal (~ 500 µm) gap between them to fill a total subtended FOV of 13 arcmin on a side (Figure 43). Table 6 summarizes the primary characteristics of the focal plane.
Figure 43: The NuSTAR focal plane configuration (left) and photo of an engineering test module (right), image credit: NuSTAR collaboration)
Table 6: Summary of the focal plane configuration
To achieve a low energy threshold and good spectral performance, the detector readout is designed for very low noise. The electronic noise contribution (including detector leakage current) to the energy resolution is 400 eV, and the low-energy threshold is 2.5 keV for an event registering in a single pixel. Over most of the energy range the detector spectral resolution is limited by charge collection uniformity in the CdZnTe crystal. At low energies, between 5 and 30 keV, the average spectral resolution for a typical flight detector is 500 eV FWHM (Full Width Half Maximum), while at 60 keV it is 1.0 keV, and at 86 keV it is 1.2 keV.
The focal plane will be passively cooled in flight to between 0-5ºC. The passive cooling is enabled by the low-power dissipation of the detector readout chip (50 µW/pixel). At in-flight operating temperatures, the detector leakage current is a negligible contributor to the resolution. In addition to measuring the deposited energy and arrival time for each event, the readout architecture enables a depth of interaction measurement which can be used both to maximize photo peak efficiency at high energy, where charge trapping effects can lead to a low-energy `tail' on the energy resolution, and in addition reject background from the back portion of the detector.
The readout of each focal plane module is controlled by an FPGA-embedded microprocessor. Because each pixel triggers independently, and the electronics shaping time is short, there are no pile-up issues equivalent to the CCD focal planes on XMM and Chandra. The maximum rate that events can be processed is 400 cps (cycles per second) in each telescope; however, pulse pileup does not occur until substantially higher rates (~ 105 cps). The readout system is designed so that source fluxes can be measured up to count rates of 104 cps. At the nominal faint-source count rates, the readout dead time is < 2%.
The focal plane is surrounded by an active 2 cm thick CsI(Na) shield and incorporates a deployable aperture stop. The CsI shield extends 20 cm above the detector, and has an opening angle of 16º, while the passive aperture stop defines a much narrower opening of 4º diameter. Figure 44 shows the expected background counts per unit detector area. At low energies the background is dominated by diffuse leakage through the portion of the aperture stop FOV not blocked by the optics bench. The spectral features between 25 and 35 keV are fluorescence from the CsI shield. The background level shown in Figure 44 assumes the use of the depth-of-interaction measurement to reject interactions in the back of the detector, which results in about a factor two background reduction at 60 keV (Ref. 69).
Figure 44: Predicted detector background count rate per unit area as a function of energy (image credit: NuSTAR collaboration)
Figure 45: Photo of one of two NuSTAR focal plane modules (image credit: NuSTAR collaboration) 72)
Data will be publicly available at HEASARC (High Energy Astrophysics Science Archive Research Center) of NASA/GSFC following validation at the science operations center located at Caltech.
The MOC (Mission Operations Center) is located at UCB/SSL. Command uplinks and data downlinks will be through a ground station, operated by ASI (Agenzia Spaziale Italiana), located in Malindi, Kenya. Most science targets will be viewed for a week or more, so that after a 30-day in-orbit checkout and commissioning period, commanding will be rare. The turnaround time for ToO (Target of Opportunity) observations depends largely on timing relative to the ground station passes. The Malindi station is visible once per 90 minute orbit, but commands can take up to 12 hours to prepare given that the MOC is not staffed 24 hours/day.
The science data will be transferred from the MOC to the SOC (Science Operations Center). The SOC is in charge to process and validate the data, and distribute products to the science team. All science data will be converted to FITS (Flexible Image Transport System) format conforming to OGIP (Office of Guest Investigator Programs) standards, and analysis software will adopt the FTOOLS approach and environment. The NuSTAR science data has no proprietary period, and after a six-month interval during which the instrument calibration will be understood and the performance verified, data will enter the public science archive, located at the HEASARC.
NuSTAR Mission System: 73)
• Core components:
- MOC (Mission Operations Center)
- FDC (Flight Dynamics Center)
- SOC (Science Operations Center)
- Remote ground stations
- Secure network links to remote ground stations
- Acquisition data upload via secure circuits
- Post-pass telemetry file delivery via Open Internet
- Standardized data flows between MOC, FDC, SOC
Figure 46: Core components and interfaces of the NuSTAR mission at USB/SSL (image credit: USB/SSL)
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20) M. Yukita, A. Ptak, A. E. Hornschemeier, D. Wik, T. J. Maccarone, K. Pottschmidt, A. Zezas, V. Antoniou, R. Ballhausen, B. D. Lehmer, A. Lien, B. Williams, F. Baganoff, P. T. Boyd, T. Enoto, J. Kennea, K. L. Page, Y. Choi, "Identification of the Hard X-ray Source Dominating the E > 25 keV Emission of the Nearby Galaxy M31," The Astrophysical Journal, Draft version March 22, 2017, URL: https://arxiv.org/pdf/1703.07318.pdf
21) Elizabeth Landau, Markus Bauer, "NuSTAR Helps Find Universe's Brightest Pulsars," NASA/JPL, Feb. 28, 2017, URL: http://www.jpl.nasa.gov/news/news.php?feature=6760&utm_
22) Gian Luca Israel, Andrea Belfiore, Luigi Stella, Paolo Esposito, Piergiorgio Casella, Andrea De Luca, Martino Marelli, Alessandro Papitto, Matteo Perri, Simonetta Puccetti, Guillermo A. Rodríguez Castillo, David Salvetti, Andrea Tiengo, Luca Zampieri, Daniele D'Agostino, Jochen Greiner, Frank Haber, Giovanni Novara, Ruben Salvaterra, Roberto Turolla, Mike Watson, Joern Wilms, Anna Wolter, "An accreting pulsar with extreme properties drives an ultraluminous x-ray source in NGC 5907," Science, 20 Feb 2017, DOI: 10.1126/science.aai8635
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28) Elizabeth Landau, "Chorus of Black Holes Sings in X-Rays," NASA, July 28, 2016, URL: http://www.nasa.gov/feature/jpl/chorus-of-black-holes-sings-in-x-rays
29) Whitney Clavin, "Andromeda Galaxy Scanned with High-Energy X-ray Vision," NASA, Jan. 5, 2016, URL: http://www.nasa.gov/feature/jpl/Andromeda
30) "NuSTAR Finds Cosmic Clumpy Doughnut Around Black Hole," NASA/JPL, Dec. 17, 2015, URL: http://www.jpl.nasa.gov/news/news.php?release=2015-379
31) "Searing Sun Seen in X-rays," NASA, July 8, 2015, URL: http://www.nasa.gov/jpl/nustar/searing-sun-seen-in-x-rays
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36) "XMM-Newton and NuSTAR spectrum of the quasar PDS 456," ESA, Feb. 19, 2015, URL: http://sci.esa.int/xmm-newton/55480-xmm-newton-
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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).