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IXPE (Imaging X-ray Polarimetry Explorer)

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NASA has selected a science mission that will allow astronomers to explore, for the first time, the hidden details of some of the most extreme and exotic astronomical objects, such as stellar and supermassive black holes, neutron stars and pulsars. Objects such as black holes can heat surrounding gases to more than a million degrees. The high-energy X-ray radiation from this gas can be polarized – vibrating in a particular direction. The Imaging X-ray Polarimetry Explorer (IXPE) mission will fly three space telescopes with cameras capable of measuring the polarization of these cosmic X-rays, allowing scientists to answer fundamental questions about these turbulent and extreme environments where gravitational, electric and magnetic fields are at their limits. 1) 2) 3)

The mission, slated for launch in 2020, will cost $188 million. This figure includes the cost of the launch vehicle and post-launch operations and data analysis. Principal Investigator Martin Weisskopf of NASA/MSFC (Marshall Space Flight Center) in Huntsville, Alabama, will lead the mission. Ball Aerospace in Broomfield, Colorado, will provide the spacecraft and mission integration. ASI ( Italian Space Agency) will contribute the polarization sensitive X-ray detectors, which were developed in Italy through INFN (National Institute for Nuclear Physics) and the INAF (National Institute of Astrophysics). 4)

The goal of the IXPE mission is to expand understanding of high-energy astrophysical processes and sources, in support of NASA's first science objective in Astrophysics: "Discover how the universe works." Polarization uniquely probes physical anisotropies—ordered magnetic fields, aspheric matter distributions, or general relativistic coupling to black-hole spin—that are not otherwise measurable. It does this by expanding our understanding of high energy astrophysical processes, specifically the polarimetry of cosmic sources with special emphasis on objects such as neutron stars and black holes. By obtaining X-ray polarimetry and polarimetric imaging of cosmic sources, IXPE addresses two specific science objectives: 5)

• Determine the radiation processes and detailed properties of specific cosmic X-ray sources or categories of sources.

• Explore general relativistic and quantum effects in extreme environments.

NASA's Astrophysics Roadmap, "Enduring Quests, Daring Visions", also recommends such measurements. — IXPE uses X-ray polarimetry to expand dramatically X-ray observation space, which historically has been limited to imaging, spectroscopy, and timing. This advance will provide new input to our understanding as to how X-ray emission is produced in astrophysical objects, especially systems under extreme physical conditions — such as neutron stars and black holes. Polarization uniquely probes physical anisotropies — ordered magnetic fields, aspheric matter distributions, or general relativistic coupling to black-hole spin — that are not otherwise easily measurable. Hence, IXPE complements all other investigations in high-energy astrophysics by adding the important and relatively unexplored dimensions of polarization to the parameter space for exploring cosmic X-ray sources and processes, and for using extreme astrophysical environments as laboratories for fundamental physics.

The primary science objectives of IXPE are (Ref. 3):

• Enhance our understanding of the physical processes that produce X-rays from and near compact objects such as neutron stars and black holes.

• Explore the physics of the effects of gravity, energy, electric and magnetic fields at their extreme limits.

IXPE addresses key questions in High Energy Astrophysics:

• What is the spin of a black hole?

• What are the geometry and magnetic-field strength in magnetars?

• Was our Galactic Center an Active Galactic Nucleus in the recent past?

• What is the magnetic field structure in synchrotron X-ray sources?

• What are the geometries and origins of X-rays from pulsars?

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Figure 1: Artist's rendition of the IXPE spacecraft [image credit: HEASARC (High Energy Astrophysics Science Archive Center)] 6)

Hundreds of galactic and extragalactic sources are amenable to meaningful X-ray polarimetry with IXPE. 7) IXPE is 100X more efficient than the polarimeter that first measured the Crab's polarization.

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Figure 2: Time to obtain a specified minimum detectable polarization (MDP) at 99%-confidence versus source flux (10-11 ergs/cm2/s).

Legend to Figure 2: The top axis identifies the all-sky number of extragalactic sources above the limiting flux on the bottom axis. Text near the top of each dashed line also gives the number LMXB (Low Mass X-ray Binary) and HMXB (High Mass X-ray Binary) at that limiting flux. The green line denotes the Crab Nebula, with the green dot marking the time required for the OSO-8 (Orbiting Solar Observatory-8) polarimeter to achieve a 3% detectable polarization at 99%-confidence for the Crab without pulsar contamination.

 

IXPE mission collaborations:

In June 2017, a new partnership between NASA and ASI (Italy's Space Agency) was formed. Robert Lightfoot, NASA's acting administrator, signed an agreement on June 20 with Roberto Battiston, president of ASI, defining the terms of cooperation for the IXPE (Imaging X-ray Polarimetry Explorer) mission during a ceremony at the Paris Air Show in Le Bourget, France. 8)

The IXPE mission will fly three telescope systems capable of measuring the polarization of X-rays emitted by cosmic sources. ASI will contribute IXPE's sophisticated "eyes" — three polarization-sensitive X-ray detectors which were developed in Italy — and the use of its equatorial ground station located at Malindi, Kenya.

NASA will supply the X-ray telescopes and use of its facilities to perform end-to-end X-ray calibration and science operations.

Ball Aerospace in Broomfield, Colorado, will provide the spacecraft and mission integration. Ball Aerospace will also operate the flight system with support from LASP (Laboratory for Atmospheric and Space Physics) at the University of Colorado at Boulder.

Other partners include Stanford University, McGill University and MIT (Massachusetts Institute of Technology).

IXPE is next in the line of NASA SMEX (Small Explorer) program missions. NASA/GSFC (Goddard Space Flight Center) in Greenbelt, Maryland, manages the Explorers Program. NASA's Marshall Space Flight Center in Huntsville, Alabama, leads the mission for the agency's Science Mission Directorate in Washington.

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Figure 3: International relationships, clear institutional roles with well-defined interfaces (Image credit: NASA, Ball)

 


 

Payload Concept

IXPE's payload is a set of three identical, imaging, X-ray polarimetry systems mounted on a common optical bench and co-aligned with the pointing axis of the spacecraft.9) 10) Each system, operating independently, comprises a 4-m-focal length Mirror Module Assembly (grazing incidence x-ray optics) that focuses X-rays onto a polarization-sensitive imaging detector. The focal length is achieved using a deployable boom. Each Detector Unit (DU) contains its own electronics, which communicate with the payload computer that in turn interfaces with the spacecraft. Each DU has a multi-function filter wheel assembly for in-flight calibration checks and source flux attenuation.

Designing an instrument of appropriate sensitivity to accomplish the science objectives summarized above involved a trade of MMA design, detector design, and the number of telescope systems, all versus focal length, and considered boundary conditions of mass and power that are within spacecraft and launch vehicle constraints. These trades were completed and the result is the three telescope system described here which meets science objectives and requirements with margin while placing reasonable and achievable demands on the spacecraft, launch vehicle, and the deployable optical bench. Specifically, three identical systems provide redundancy, a range of detector clocking angles to mitigate against any detector biases, shorter focal length for given mirror graze angles (i.e., given energy response) and thinner/lighter mirrors compared to a single telescope system.

Figure 4 shows the IXPE observatory with key payload elements. The payload uses a deployable x-ray shield to prevent off-axis x-rays from striking the detectors. The deployable boom is covered with a thermal sock (not shown) to maintain a more constant thermal environment. A metrology system consisting of a deployed section-mounted camera which images a metrology target (diode string) on the spacecraft top deck is used to monitor motions between the two ends of the Observatory during science observations.

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Figure 4: IXPE Observatory showing key payload elements (image credit: IXPE Team)

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Figure 5: Mirror module design (image credit: IXPE Team)

 

GPD (Gas Pixel Detector)

The GPD, a contribution of ASI, has the following features/characteristics:

• Detection uses photoelectric effect

• Photoelectron emission aligned with X-ray polarization vector

• Electron multiplier with pixelated detector.

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Figure 6: Measurement concept of the incoming X-ray radiation (image credit: IXPE Team)

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Figure 7: Illustration of the GPD elements (image credit: IXPE Team)

 


 

Spacecraft Concept

The IXPE Observatory is based on the BCP-100 (Ball Commercial Platform) spacecraft architecture. The modular design allows for concurrent payload and spacecraft development with a well-defined, clean interface that reduces technical and schedule risk. The BCP-100 design supports the project goal of incorporating a low-risk spacecraft by using flight-proven components, a simple structural design, and significant design and software reuse from prior missions. The design balances a low-cost and low-risk approach with significant spacecraft capability and flexibility. IXPE is leveraging the BCP-100's flexibility for science payload accommodation. The IXPE payload is mounted on the spacecraft top deck. The IXPE Observatory is designed to launch on a Pegasus XL or larger launch vehicle.

Some background: The BCP-100 design, based on the STP (Space Test Program) Standard Interface Vehicle, supports the project goal of incorporating a low-risk spacecraft by using flight-proven components, a simple structural design, and significant design and software reuse from prior missions. The design balances a low-cost and low-risk approach with significant spacecraft capability and flexibility. The BCP-100 capabilities support a variety of potential small payloads. The standard capability spacecraft can operate over a wide range of low earth orbit altitudes (400 – 850 km) and inclinations (0º to sun-synchronous). The spacecraft design provides the required power over the full range of sun angles. A star tracker is a key element of the attitude determination and control system. It is mounted directly to and aligned with the deployed payload to minimize alignment errors between the spacecraft and payload.

STP Satellite -2 (STPSat-2) was the first use of the STP vehicle and was launched 19 November 2010 on a Minotaur IV from the Kodiak Launch Complex, Alaska. It accommodates 2 separate SERB payloads . STPSat-2 continues extended operations well beyond its 13 month design life, and achieved 6 years on-orbit in December 2016.

 

IXPE spacecraft:

IXPE is the fourth build of the BCP-100 class spacecraft. IXPE is leveraging the flexibility of the BCP-100 architecture to accommodate the IXPE science payload. It is re-configured for launch on a Pegasus XL launch vehicle with the IXPE payload mounted on the spacecraft top deck. The solar array wraps around the spacecraft body and payload.

The Observatory is designed to support IXPE measurement requirements. Key design drivers include pointing stability in the presence of various disturbances, particularly gravity gradient, and minimization of SAA passes which makes the zero degree inclination orbit the best available choice. A nominal IXPE target list is known in advance with targets distributed over the sky. The observatory has observational access to an annulus normal to the Sun line at any given time with a width ±30° from Sun-normal. This orientation allows the payload to collect all necessary science data during the mission while keeping the solar arrays oriented toward the sun and maintaining sufficient power margins. Typically, each science target is visible over an approximate 60 day window and can be observed continuously for a minimum time of 56.7 minutes each orbit. Changes in the IXPE orbit over mission lifetime are sufficiently small, eliminating the need for a propulsion system and its resulting operational complexity.

A view of the deployed IXPE Observatory is shown in Figure 9, while Figure 10 shows the Observatory stowed in a Pegasus XL launch vehicle fairing. When deployed, IXPE is 5.2 m from the bottom of the spacecraft structure to the top of the payload and is 1.1 m in diameter. The solar panels span 2.7 m when deployed. The Observatory launch mass is approximately 300 kg.

The payload is mounted on the +Z face of the spacecraft structure (top deck). This simplifies alignment and integration, and minimizes mass by providing the shortest possible load paths. The star tracker OH (Optical Heads) are mounted on opposite ends of the Observatory anti-boresighted from one another to prevent simultaneous Earth obscuration. One OH is mounted on top of the telescope support structure, co-located and boresighted with the X-ray optics. The second OH is mounted on the bottom of the spacecraft top deck looking out through the PAF ring. Two hemispherical S-band low-gain antennas are mounted on opposite sides of the spacecraft and coupled together to provide omnidirectional communications coverage.

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Figure 8: A set of three MMA (Mirror Module Assemblies) of IXPE focus X-rays onto three corresponding focal plane detector units (image credit: IXPE Team)

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Figure 9: IXPE observatory in its deployed configuration (image credit: IXPE Team)

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Figure 10: IXPE Observatory stowed in a Pegasus XL fairing (image credit: IXPE Team)

IXPE has substantial timeline and technical margins:

• Design Reference Mission (DRM) targets studied in detail during Year 1

• Year 2 is available for follow-up observations, targets of opportunity, survey of additional sources.

Characteristic

Requirement

Capability

Margin

Launch mass

291.7 kg

380 kg

30.3%

Science data storage

4 GB

6 GB

50%

EOL science mode power generation w/30º offset

188 W

257 W

37%

LOS pointing accuracy

53.1 arcsec (3σ)

25.2 arcsec (3σ)

110%

LOS co-alignment accuracy, x-axis

19.8 arcsec (3σ)

9.5 arcsec (3σ)

107%

LOS co-alignment accuracy, y-axis

26.7 arcsec (3σ)

12.8 arcsec (3σ)

109%

LOS pointing knowledge

34.5 arcsec (3σ)

17.3 arcsec (3σ)

100%

Link margins

> 3 dB

> 3.9 dB

> 3 dB

Table 1: Overview of the IXPE mission parameters and margins

Mission status:

• The phase B activities of the IXPE mission started in the spring of 2017.

 

Launch: The IXPE Observatory will launch no earlier than November 20, 2020 on a Pegasus XL launch vehicle of Orbital ATK.

Orbit: Equatorial orbit, altitude = 540 km, inclination =0º.

IXPE mission operations: Upon separation from the LV, the spacecraft autonomously performs solar acquisition, placing itself in a power-positive attitude. Payload checkout begins as soon as the spacecraft has been verified to be active. X-ray targets are known in advance and observed with a single science mode.

IXPE launch and commissioning operations will be conducted from the MOC (Mission Operations Center) at CU/LASP (University of Colorado/Laboratory for Atmospheric and Space Physics) during the first 30 days on-orbit. An expanded ground team will be resident at MOC during this phase. Malindi coverage will be up to 15 passes/day although it is anticipated only half of this number will be used during this phase. During launch and the first week of commissioning the IXPE orbit will be determined by using SN (Space Network) Doppler data. Ball/LASP navigation will perform all ephemeris format conversions as needed for data products. The MOC will monitor the spacecraft using orbit DOWD via SN until the navigation team at Ball/LASP has sufficient data to take over orbit determination duties, which can take up to two weeks after launch.

For the first week of commissioning, the Operations Team will conduct spacecraft subsystem commissioning operation including C&DH, power, telecom, and ADCS calibration. Once the spacecraft is fully operational, the remainder of the commissioning phase (3 weeks) is dedicated to payload turn-on and check out, which includes boom deployment, x-ray shield deployment, DSU checkout and activation and calibration of the detector units. IXPE boom and X-ray shield deployments are not time critical. The boom deployment is treated as a critical event. The time for set up, deployment and confirmation occur over three passes. The commanded deployment events are scheduled to occur over the Malindi ground station.

Science operations: The predictable and repetitive nature of the observations of known targets and high margin for onboard data storage (50%) allow for ease in science planning and operations. Typically, each science target is visible over an approximate 60 day window and can be observed continuously for a minimum time of 56.7 minutes each orbit. Since routine pass operations are handled by ground automation with no spacecraft sequence involvement, changes in the target list may be incorporated until final approval of the sequence. This information is then forwarded on a weekly basis to the MOC by the SOC (Science Operations Center). The target list is encoded in command sequences and uplinked once every 3 days. The overall observing plan will be refined prelaunch, and modified as needed to respond to Observatory anomalies, missed observations and TOOs (Targets of Opportunity). Any missed targets can be generally included in the next week's scheduling queue because the science program is robust to individual missed visits.

Normal Phase E science operations commence with uplink of the first weekly science observation sequence. Malindi coverage transitions to 2-8 passes per day of 10 minutes each. Many of the pre-defined targets can be observed using one observation period with 2 ground contacts per day while other targets are data intensive and require splitting the observations into 2 to 4 observing sequences, filling the recorder (with 50% margin) and downlinking on average 7.5 times per day. Science and calibration data are stored in the C&DH and downlinked daily during the scheduled passes. Downlinks are initiated and monitored by ground automation. The downlink will be through the Malindi station at a rate of 2.0 Mbit/s (Singapore backup). If communications passes are missed, the data are stored in the C&DH memory and downlinked on subsequent passes.

Since science and communications are decoupled due to the omnidirectional passively coupled S-band LGAs (Low Gain Antennas), operations scheduling is straightforward. Science collection and communications can occur simultaneously as long as an LGA is within the required FOV for the 2.0 Mbit/s downlink.

In summary, IXPE brings together an international collaboration for flying an imaging X-ray polarimeter on a NASA Small Explorer. IXPE will conduct X-ray polarimetry for several categories of cosmic X-ray sources from neutron stars and stellar-mass black holes, to supernova remnants, to active galactic nuclei that are likely to be X-ray polarized.

 


 

Ground segment:

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Figure 11: The IXPE mission uses a heritage ground data system (image credit: IXPE Team)

 


1) "NASA Selects Mission to Study Black Holes, Cosmic X-ray Mysteries ," NASA Release 17-002, January 3, 2017, URL: https://www.nasa.gov/press-release/nasa-selects-mission-to-study-black-holes-cosmic-x-ray-mysteries

2) "NASA to Peer Through a New Window at Black Holes and Exotic Astronomical Objects," Satnews Daily, Jan. 8, 2017, URL: http://www.satnews.com/story.php?number=1942514205

3) William Deininger, "IXPE: Imaging X-Ray Polarimetry Explorer Mission," 2017 IEEE Aerospace Conference ,Yellowstone Conference Center, Big Sky, MT, USA, March 4-11, 2017, URL: https://wwwastro.msfc.nasa.gov/ixpe/for_scientists/presentations/20170304_deininger.pdf

4) "IXPE mission: Italy and NASA for new X-ray astronomy," INFN, INAF, January 16, 2017, URL: https://www.researchitaly.it/en/projects/ixpe-mission-italy-and-nasa-for-new-x-ray-astronomy/

5) W. D. Deininger, R. Dissly, J. Domber, J. Bladt, J. Jonaitis, A. Kelley, R Baggett, B. D. Ramsey, S. L. O'Dell, M. C. Weisskopf, P. Soffitta, "Small Satellite Platform Imaging X-Ray Polarimetry Explorer (IXPE) Mission Concept and Implementation," Proceedings of the 31st Annual AIAA/USU Conference on Small Satellites, Logan UT, USA, Aug. 5-10, 2017, paper: SSC17-III-08, URL: http://digitalcommons.usu.edu/smallsat/2017/all2017/85/

6) "IXPE Imaging X-Ray Polarimeter Explorer," NASA, URL: https://wwwastro.msfc.nasa.gov/ixpe/about/index.html

7) M. C. Weisskopf, R. Bellazzini, E. Costa, B. D. Ramsey, S. L. O'Dell, A. F. Tennant, R. F. Elsner), G. Pavlov, G. Matt, H. Marshall, V. Kaspi, R. Romani, P. Soffita, F. Mulieri, "IXPE — the Imaging X-Ray Polarimeter Explorer: Expanding Our View of the Universe," URL: https://files.aas.org/head14/116-15_Martin_C_Weisskopf.pdf

8) "IXPE mission, NASA teams with ASI," ASI, June 20, 2017, URL: http://www.asi.it/en/news/ixpe-mission-nasa-teams-asi

9) Martin C. Weisskopf, Brian Ramsey, Stephen O'Dell, Allyn Tennant, Ronald Elsner, Paolo Soffita, Ronaldo Bellazzini, Enrico Costa, Jeffery Kolodziejczak, Victoria Kaspi, Fabio Muleri, Herman Marshall, Giorgio Matt, Roger Romani, the IXPE Team, "The Imaging X-ray Polarimetry Explorer (IXPE)," Proceedings of. SPIE, Vol. 9905,' Space Telescopes and Instrumentation 2016: Ultraviolet to Gamma Ray, 990517 (11 July 2016),' Elsevier, Results in Physics 6 (2016) 1179–1180, URL: http://dx.doi.org/10.1016/j.rinp.2016.10.021

10) Paolo Soffitta, "XIPE e IXPE," IAPS/INAF, Italy
 


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 (herb.kramer@gmx.net).

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