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Newsletter No. 66 May 1995


  1. Notes from the Editor - K. Hurley
  2. News from NASA Headquarters - A. Bunner, L. Kaluzienski, G. Riegler, and D. Weedman
  3. The Forthcoming SAX Mission - A. Parmar and L. Piro
  4. HIREGS Long Duration Balloon Flight - S. Boggs
  5. Next Generation X-Ray Observatory - W. Cash
  6. The Hard X-ray Telescope (HXT) - P. Gorenstein
  7. Energetic X-ray Imaging Survey Telescope (EXIST) - J. Grindlay and T. Prince
  8. A Next Generation Orbiting High-Energy Gamma-Ray Telescope (GLAST) - P. Michelson
  9. The Energetic Transient Array - G. Ricker
  10. Large Area X-ray Spectroscopy Mission - H. Tananbaum
  11. Burst Arc Second Imaging and Spectroscopy (BASIS) - B. Teegarden
  12. X-ray Spectroscopy with NGXO - N. E. White and R. Petre
  13. Meetings
  14. NSF Program Dates - J. Wright
  15. News Notes

Notes from the Editor



September 1994, NASA issued a Research Announcement for "New Mission Concepts in Astrophysics" to define future space missions for the years beyond 2000. Eight of the accepted proposals fall into the High Energy Astrophysics area, and descriptions of them can be found in this newsletter.

News from NASA Headquarters


Daniel W. Weedman

This summer's budget season in Congress is a crucial and challenging period whose outcome will impact the goals of the NASA Astrophysics Division in a major way. For most of 1995, we struggled within NASA to accommodate anticipated cuts that would progressively reduce the annual agency budget, reaching a 10% reduction from current levels by the year 2000. The objective was to accommodate these cuts by internal reorganization, streamlining, and privatizing of activities so that on-going science programs were not affected and new starts could be included. It appeared, as of mid-May, that such accommodation had been reached.

The recommendations will require meaningful changes in many NASA activities. For Astrophysics, the biggest impacts will be at Ames Research Center and Marshall Space Flight Center, where it is recommended that the space science efforts be transitioned into independent research institutes, to be established in geographic proximity to the centers. The institute for Ames scientists would be interdisciplinary, with focus on astrobiology. At MSFC, the primary Astrophysics activities would relate to the GRO and AXAF missions. Also, MSFC will be the NASA center for technical development of large astronomical optics for future missions, from high energy through visible/infrared. Flight operations of NASA research aircraft will be transferred from Ames to Dryden Research Center, which requires major changes in operations plans for the airborne astronomy program. Astrophysics-related activities at Goddard Space Flight Center and JPL will not be changed drastically; GSFC will have primary implementation responsibility for Earth-orbital space science missions and JPL for deep space missions. NASA Headquarters will be dramatically reorganized, by adopting a corporate structure with a further 40% reduction in personnel. Space science activities will be overseen by the Space Science Enterprise, one of five Enterprises in the agency (Space Science, Mission to Planet Earth, Human Exploration and Development of Space, Aeronautics, and Space Technology).

An outcome of NASA's restructuring was the determination that the Agency's investment in science would not decrease, providing optimism that all proposed new starts could be maintained. Simultaneously with the conclusion of the internal NASA studies accommodating the reductions described above, new budget targets for NASA were included in House and Senate Budget Committee resolutions, requiring in the House version an additional 13% annual reduction by 2000. In this version, a $14.46 billion NASA budget in 1995 would become a $11.51 billion budget in 2000. This would be about one-half of the agency budget anticipated three years ago as being in place by the end of the decade. Reductions would begin immediately, removing about $750M from the FY 1996 budget already pending in Congress. As a result, there is now great uncertainty about the ability to afford new science missions. It is this uncertainty that poses the greatest concern for SOFIA and SIRTF, the two new Astrophysics missions included in the 1996 budget submission.

The evolution of NASA structure and budgeting is part of continuing discussions with members of the community who participate in various committees and working groups that report to the Astrophysics Division. Names of these members and updates on other information from Headquarters are included in our Home Page at

High Energy Astrophysics Supporting Research & Technology (SR&T) Program Solicitation

In consultation with our advisory groups, the High Energy Astrophysics branch has decided to merge together our previously separate solicitations for basic research in X- and gamma-ray astrophysics into a single solicitation. The first announcement for the integrated program is presently planned for release in late 1995/early 1996 and will solicit proposals for basic research in areas relevant to NASA's flight program in high energy astrophysics (~0.1 keV-100 GeV). Nominal awards will be made for a period of three years and the total funding available for the program is expected to be ~ $7.5M per year. The solicitation/review process will be conducted on a three-year cycle. Questions concerning this may be directed to Dr. L. Kaluzienski at (202) 358-0370 or

New Chair for HEAMOWG:

Bruce Margon has accepted the position of chair of the AURA Board of Directors, the institution which (among other things) oversees the Space Telescope Science Institute. Bruce has decided to divest himself of other demands on his time and possible conflicts of interest. Unfortunately, this means that Bruce is stepping down as HEAMOWG Chair. Effective now, Steve Kahn (UC-Berkeley) has agreed to take on the position of Chair of the HEAMOWG. Steve is planning to move from Berkeley to Columbia University this July.

Delay of SMEX AO:

The decision has been made to delay the planned next SMEX Announcement of Opportunity (AO) by one year, from this summer until the summer of 1996, because of the current Pegasus launch system problems and the resulting delays in the launches of FAST and/or SWAS.

XMM Update:

A few developments in ESA's XMM cornerstone mission program have occurred recently. The XMM spacecraft contractor, Dornier, was selected last October and Dornier has been under Phase B contract since January 1995. XMM will be launched in summer 1998 into a 48 hour orbit. The latest XMM X-ray mirror performance test results have been very positive. Two Mirror Demonstration Models (one 2-shell mirror and one 3-shell flight-like model) have produced a half energy width at 1.5 keV in the range 6 to 18 arc seconds, better than XMM specifications.

Restructuring of Data Archives at GSFC:

Guenter Riegler, Astrophysics Division, NASA HQ

As a response to the President's January 1995 budget requests for 1996-2000, NASA conducted a thorough review ("zero-based review", ZBR) of NASA's infrastructure. A detailed description is available at As part of the ZBR, NASA scrutinized the National Space Science Data Center (NSSDC) which is a part of the Space Science Data Operations Office (SSDOO) at the Goddard Space Flight Center. NSSDC, known for the long-term archive of space science data, actually consists of several interrelated functions: (a) data management, (b) long-term archives (level zero mission data sets, often on magnetic tapes, film, CD-ROMs, etc.), (c) on-line or "near-line" data sets (8 Terabytes of space science data for ROSAT, ASCA, etc., growing at the rate of 3 TB/year), and (d) value-added tasks (data format standards, interoperable catalogs, discipline-specific data manipulation tools, documentation, etc.). The SSDOO is also responsible for data processing and mission operations management for ROSAT, ASCA, GRO, XTE, and other Astrophysics and Space Physics missions.

The guidelines from the ZBR state that a portion of the NSSDC should be privatized beginning in fiscal year 1996; the transition is to be completed during FY97. To fulfill this charter, a joint Headquarters-GSFC team will be formed to establish user requirements, minimize impact to operating missions, develop implementation options, and to select and implement the privatization.

Because of this emphasis on privatizing data archives, I have been asked about the possible impact on the "HEASARC", the US archive for High Energy Astrophysics Data at GSFC. The HEASARC is, of course, linked to the NSSDC because much of the High Energy Astrophysics data physically reside on the NSSDC's data archive and distribution system ("NDADS"). It is my personal view that the HEASARC is not at all involved in the data archive privatization, since most of the HEASARC effort involves science value-added work on data analysis software, calibration and documentation, format definition, and the user interface to the archive.

The Forthcoming SAX Mission

by Arvind Parmar (SSD/ESA) and Luigi Piro (IAS/Frascati)

SAX is an Italian/Dutch spacecraft that will be launched into low inclination orbit at a height of 600 km by an Atlas G-Centaur in March 1996. The ground station is in Malindi, Kenya and scientific operations will be conducted from Rome. The spacecraft is three axis stabilized and will provide a pointing accuracy of 1' during its 2 to 4 year lifetime. The average telemetry rate is 70 kbps. The main scientific goal of the mission is to conduct broad-band spectroscopy over a wide energy range of 0.1-200 keV

The mission is complementary to XTE, having similar high-energy sensitivity, but with an emphasis on spectroscopy (with imaging capabilities below 10 keV), as opposed to timing. The broad energy coverage cannot be achieved with a single instrument, so the payload is necessarily complex. This consists of a Phoswich detector system (PDS; 15-300 keV) of comparable sensitivity to HEXTE on XTE, a high pressure gas scintillation proportional counter (HPGSPC) that offers a factor ~5 better energy resolution than the PDS over the energy range 3-120 keV with an effective area of 350 cm^2, and a set of four X-ray imaging mirrors each of 120 cm^2 geometric area. An imaging GSPC is located at the focal plane of each mirror system. In three systems, these are sensitive in the energy range 1.0-10.0 keV providing somewhat better position resolution than the ASCA GIS, but with lower effective area. In one case, the detector utilizes a thin window and a driftless configuration to provide a response between 0.1-10.0 keV. Below about 0.5 keV, the energy resolution of this instrument is better than that of current CCDs. All these instruments are coaligned, have FOVs of about 1 degree, and provide microsecond timing capabilities.

In addition, there are two Wide Field Cameras (WFC; 2-30 keV) that point in opposite directions along an axis perpendicular to the other instruments. These provide ~2 mCrab sensitivity in an uncrowded region of the sky in 10^4 sec and sources can be positioned to 2' accuracy within the 20 x 20 degree FWHM FOVs. It is expected that the WFCs will detect many transient events, to which the main complement of instruments can be pointed within 8 hours.

Gamma ray burst detection capability in the range 100-600 keV will be also provided by the active shields of the PDS.

Following a Performance Verification phase after launch, 80% of the observing time will be devoted to Core Program observations (open to the Italian and Dutch scientific communities, and scientists from SSD and MPE). This figure decreases to 60% in the second year and 50% in subsequent years. The rest of the observing time will be open to scientists worldwide. An Announcement of Opportunity to propose for SAX will be released to the World wide community in July of this year. Selection of the initial observing program by the Time Allocation Committee will be made later in the year. Data analysis software is being prepared by the instrument teams and will be publically available. In the case of the imaging instruments, this is based around the FTOOLS package. Further information on SAX and the AO can be obtained from the Assistant Project Scientist, Luigi Piro ( Anyone interested in being included in the SAX mailing list can send their address to

HIREGS Long Duration Balloon Flight

HIREGS January 1995 Long Duration Balloon Flight from Antarctica

S. E. Boggs

Space Sciences Laboratory

University of California, Berkeley

The High Resolution Gamma-Ray and Hard X-Ray Spectrometer (HIREGS) successfully completed a long duration balloon flight from Antarctica in January 1995, collecting ~8 days of spectra from 20 keV to 20 MeV to study the Galactic center region and several other interesting sources. Analysis is still in the preliminary stages, but HIREGS possibly recorded the best gamma-ray spectroscopic measurements to date, which will allow us to study the shapes of the 0.511 MeV electron-positron annihilation line, the 1.809 MeV 26Al nucleosynthesis line, and the cyclotron absorption features of 2 accreting X-ray pulsars. For this flight, we also added hard x-ray (<200 keV), 3-FWHM fine collimators over half of the detectors which allowed us to measure the diffuse background continuum at the Galactic Center, as well as the spectra of up to 3 separate point sources simultaneously. Several transient sources were on during the flight and in the HIREGS field of view, including GX1+4 and Nova Scorpii with the fine collimators we should be able to separate their spectra from the diffuse background.

HIREGS consists of an array of twelve 6.7 cm diameter by 6.1 cm high n-type germanium (Ge) coaxial detectors, cooled by liquid nitrogen and enclosed on the sides and bottom by a 5 cm thick bismuth germanate (BGO) shield. A 10 cm thick drilled cesium iodide (CsI) collimator in front provides a ~24 FWHM field of view. Additional hard X-ray (<200kev) passive collimators inserted in the CsI collimator cover half of the detectors, limiting these fields of view to 3.5 x 24 FWHM. These passive collimators are pointed in three different directions, effectively scanning in the hard X-ray range any region of the sky we are observing.

The payload was launched from Williams Field, McMurdo Station on 9 January 1995 on a 29 million cubic foot balloon. The payload was cutdown on 1 February 1995, having circumnavigated the antarctic continent in a little under 23 days at an average altitude of 39 km. Data was collected for 8 of these days before a system failure occurred. Half of this data was collected from the Galactic Center, and the other half was shared between the Galactic Plane (l=335), Vela X-1, GX301-2, and the region around Nova Scorpii.

Ballooning in Antarctica has many scientific as well as political benefits. The long duration of the flights is the largest benefit, with circumnavigation of the South Pole taking 10-20 days (as opposed to 10 hours to 2 days for most continental US flights). The 24 hr/day sunlight provides constant solar power, as well as limiting the diurnal altitude excursions. For sources in the southern hemisphere, 24 hr/day continuous observations are possible. The zonal winds keep the balloon at a nearly constant latitude, allowing launch and recovery operations to run from the same base. Large payloads (1-2 tons) can be flown for a fraction of the cost of satellite missions. There are no international boundaries to cross and no risk of accidental landing in populated areas.

While working in Antarctica remains challenging, the potential benefits of long duration balloon flights more than compensate for the geographic inconvenience, and yearly advances in the working conditions at McMurdo Station are making research in general much easier. Having heard the stories of my predecessors, I felt very fortunate to have telephone and Internet access in the laboratory, as well as a fairly comfortable dormitory room and decent food.

More information and pictures can be found on the HIREGS Internet home page at

HIREGS is a collaborative effort involving the University of California, Berkeley; the University of California, San Diego; Lawrence Berkeley Laboratory; and the Centre d'Etude Spatiale des Rayonnements, Toulouse, France. We are grateful for the work of all the participants from these institutions, as well as the participation of the ballooning teams from the National Scientific Balloon Foundation and the Physical Sciences Laboratory. We would also like to thank the National Science Foundation and the United States Navy for their support in Antarctica. This research was supported in part by NASA grant NAGW-3816.

Next Generation X-ray Observatory

Webster Cash

University of Colorado

The Next Generation X-ray Observatory Concept Study has the goal of pushing x-ray imaging to unprecedented levels of resolution. Our goal is to develop the concept for an x-ray observatory that will feature angular resolution at least ten times finer than AXAF.

The future of x-ray instrumentation lies in the direction of increased collecting area and higher spatial resolution. If our telescopes can improve in these areas, then all of x-ray astronomy, including both spectroscopy and polarimetry, will benefit. While larger collecting area to probe ever fainter objects is usually the first priority of astronomers, increased collecting areas are of limited value if clarity of imaging does not also improve.

Many of our most pressing scientific questions can be better addressed with high resolution x-ray telescopes. For example, we could probe deeper into the structure of cooling flows, and map the activity. We could resolve the jets of AGN's closer to the central engine. We could resolve the fine scale structure and dynamics of supernova remnants. We could split the coronae of close binary stars and search for their interactions. Indeed, most areas of endeavor would be well served.

To this end we have developed a new class of x-ray optic based on the use of spherical surface optical elements. The new approach greatly simplifies the technical path to subarcsecond imaging in the x-ray. The University of Colorado and Ball Aerospace will be studying the best means of building an observatory with resolution better than 0.1 arcseconds, and effective collecting area in excess of 100 square centimeters. This would provide throughput comparable to the IPC of Einstein, and resolution comparable to the Hubble Space Telescope.

The Hard X-ray Telescope

Paul Gorenstein, SAO

The Hard X-Ray Telescope or HXT mission concept is based upon a system of focusing telescopes that collectively, observe simultaneously over a very broad band of energy, from the ultraviolet to 100 keV and in several narrow bands extending to 1 MeV. In long, pointed observations HXT is expected to have an order of magnitude more sensitivity and finer angular resolution in the 10 to 100 keV band than all current and future missions. It will also have considerably more sensitivity for detecting narrow lines in the 100 keV to 1 MeV regime. The focusing telescopes permit the use of small, cooled detector arrays of relatively low mass. Hence, the energy resolution is very high. The primary objective of HXT is the study of non-thermal emission processes which dominate the regime above 10 keV In this area it will have considerably more sensitivity than all missions currently in development. HXT can be launched into low Earth orbit by an enhanced Delta rocket.

HXT contains two types of hard X-ray telescopes. One type, of which there are 15 units, is based upon an optical design that has large projected area at small graze angles and utilizes a novel type of multilayer coating to extend the cutoff of a grazing incidence telescope from 10 to 100 keV. This system is called the modular multilayer telescope or MMT. The substrates are double conical reflectors that are produced by replication from masters, similar to the XMM process. We expect the angular resolution to be about 30" HPD and the field of view to be a few arcminutes. There is a two stage imaging detector at each focus, a CCD which intercepts X-rays below 10 keV followed immediately down stream by either a germanium strip array or cadmium zinc telluride array which detects the 10 to 100 keV photons. Defining the hard x-ray detector type will be an objective of the study. The MMT telescopes account for most of the payload mass.

The other type of telescope is an adjustable array of several hundred crystals that focus by Laue scattering, and is called the Laue Crystal Telescope or LCT. Individual picomotors adjust the angle of each crystal so that many diffract photons of a fixed energy to the same point along the optic axis where they converge upon a single array of cooled germanium detectors that can move within a range between 3m to 10m from the crystal array. When all crystals are adjusted to diffract the same nominal energy, the LCT actually images several bands, including those of higher and lower order, each a few per cent wide, between 100 keV and 1 MeV. The bandwidths and field of view increase with the mosaicity of the crystals which can be controlled to an extent that is not completely known. Greater mosaicity is beneficial only up to a point because reflection efficiency and angular resolution falls. Determining the optimum and practical values of mosaicity is another objective of the study. The LCT will have high sensitivity for detecting narrow X-ray lines of known energy such as those expected from Type 1 supernova and possible 511 keV lines from compact binaries and AGNs.

The UV monitor is a three telescope system that provides coverage in the ultraviolet band to supplement the soft and hard X-ray measurements. It is especially useful in the study of time correlated changes across the broad electromagnetic spectrum of an AGN.

The front aperture of the payload is shown in the sketch and the effective area at several energies is listed in the table.

To view this table, click here.

The institutions which proposed the study are: the Smithsonian Astrophysical Observatory, the Naval Research Laboratory, the Marshall Space Flight Center, the Goddard Space Flight Center, the Argonne National Laboratory, the Danish Space Research Institute, the Brera Astronomical Observatory, and the Centre d'Etude Spatiale des Rayonnements (Toulouse). We welcome contributions from others to the mission concept study.

Energetic X-ray Imaging Survey Telescope (EXIST)

J. Grindlay (CfA) and T. Prince (CIT)


N. Gehrels (GSFC), C. Hailey (LLNL), B. Ramsey (MSFC), G. Skinner (SRG), J. Tueller (GSFC), P. Ubertini (IAS), and M. Weisskopf (MSFC)

Need for Hard X-ray Imaging Survey

A critically important region of the astrophysical spectrum is the hard x-ray band, from ~ 5-600 keV. In this band, an unusually rich range of astrophysical processes occur in both compact and diffuse sources. The band includes the transition from primarily thermal objects -- either optically thin, like supernova remnants or galaxy clusters, or optically thick, like the blackbody emission components of high luminosity x-ray binaries (all of which are generally still detectable at 10 keV) -- to objects which are primarily non-thermal, or at least display significantly Comptonized spectra. Examples of the very hottest thermal plasmas directly measurable in astronomical objects, the ~ 10^8-10^9 K coronae around or above accretion disks in compact binaries and active galactic nuclei, are best studied in this hard x-ray band. In particular, the rapid pace of discovery of hard x-ray emission from black hole (BH) binaries (the so-called x-ray novae) as well as relatively low luminosity neutron star (NS) binaries (the bursters) have shown that hard x-ray spectra offer particularly direct clues to the study of black holes and accretion flows onto BHs vs. NSs.

Surprisingly, for such a pivotal region of the astronomical spectrum, the hard x-ray universe is still relatively unexplored. Only one survey (HEAO-A4, in 1978-80) has been carried out in this energy range but at relatively low sensitivity and without the benefit of imaging. Yet the scientific impact of survey missions on astronomy is amply demonstrated at X-ray wavelengths by ROSAT and at IR wavelengths by IRAS. In the hard X-ray and low-energy gamma-ray bands (5 - 600 keV), a sensitive imaging sky survey is yet to be carried out. The HEAO-A2 and A4 instruments carried out the first all-sky observations above 10 keV, achieving a sensitivity of several mCrab at 2-10 keV energies (A2) and 20-100 mCrab at energies 13-180 keV (cf. Figure 1a). The French coded aperture telescope SIGMA on the Russian GRANAT satellite demonstrated the power of wide-field imaging in the ~ 35 - 300 keV band but it was not extremely sensitive and observed only a small fraction of the sky (primarily the Galactic Center). The OSSE instrument on GRO has significantly better sensitivity than either HEAO-A4 or SIGMA above 100 keV, but does not cover the range below 100 keV well, and its relatively narrow field of view and GRO pointing constraints has precluded large-area surveys. XTE, with its sensitive broad-band coverage (2-200 keV), also lacks the survey capability as well as 511 keV coverage.

Figure 1a. EXIST Survey continuum sensitivity

Figure 1b. EXIST Survey line sensitivity

Finally, a survey mission which incorporates very wide field of view detectors and telescopes can also address the general objectives mentioned above for broad temporal coverage: from short bursts to transient sources. The BATSE detectors on CGRO have shown the power of monitoring for both transients and of course the detailed study of gamma ray bursts. A wide-field survey mission can contribute greatly to both of these major topics in gamma-ray astronomy.

EXIST Mission Concept Overview

We have begun to study a mission which would conduct the first high sensitivity all-sky imaging survey (5-600 keV) with a wide-field coded aperture telescope. Grazing incidence telescopes, with response limited to <~80 keV and very small fields of view, cannot do this. The Energetic X-ray Imaging Survey Telescope (EXIST) mission characteristics and baseline instrument, as proposed and accepted for study in the recent NASA solicitation for Mission Concepts, are described in detail by Grindlay et al (1995).

EXIST would be an all-sky survey mission lasting > ~1 year (for > ~2 passes through the entire sky). The sensitivities for continuum and line detection are given in Figures 1a and 1b and are quite spectacular: EXIST would be > ~ 100 times as sensitive as HEAO-A4 over the whole sky and ~ 10 times as sensitive as typical XTE/HEXTE pointings (in 1 degree fields) which will cover only ~ 2% of the sky. Below 200 keV, EXIST could achieve line sensitivities comparable to deep pointings with INTEGRAL in its limited sky coverage. The nominal imaging resolution would be 10 arcmin, with <~1 arcmin source locations for bright sources and < ~10 arcsec positions for bright gamma-ray bursts.

After an extensive Survey, the EXIST mission could continue as a pointed Observatory of unprecedented sensitivity and resolution or it could continue as a powerful survey instrument, with occasional deep pointings. The Observatory sensitivities are typically a factor of 2-3 more sensitive than the Survey sensitivities (Figure 1), depending on exposure time, and are even more sensitive at lower energies (<~ 30-50 keV) because of the larger on-axis effective area in the co-aligned telescope field of view.

The EXIST mission would include a large array of new-technology Cadmium-Zinc-Telluride (CZT) solid state detectors. These are pixellated to give the high spatial resolution (as well as good energy resolution) needed for a compact coded aperture imaging telescope design. The mission could be launched on the newly defined NASA ``Med-Lite'' launch vehicle.

The basic mission consists of a survey mode with a ROSAT-like scanning strategy, i.e. a rotation scan motion perpendicular to the ecliptic with one complete 360-degree scan every orbit. The baseline instrument originally proposed consists of 4 coded aperture telescope modules, each of approximately 2500 cm^2 detector area. A possible telescope layout on the spacecraft is shown in the introductory figure above. However, other telescope and detector combinations are possible and are being considered as part of the Concept Study. A key feature of the instrument is an energy dependent field-of-view (FOV): 6 degrees by 20 degrees at energies below approximately 40 keV, and 20 degrees by 20 degrees at energies above approximately 80 keV. The energy-dependent FOV allows optimization of the survey observations in the two background regimes: diffuse-dominated and internal-dominated. In survey mode, it is desirable that the scan swath width be maximally wide to optimize chances for detection of transient behavior. Our design goal is half-sky coverage over one orbit. Use of offset telescopes allows a wide scan-path while maintaining acceptable signal-to-background ratios. The nominal offset (scan mode) total field of view would be 80 degrees by 20 degrees (FWHM). In the pointed mode, the telescopes could be co-aligned to achieve the optimal on-axis point source sensitivity.


Grindlay, J.E. et al 1995, Proc. SPIE, in preparation.

GLAST: A Next Generation Orbiting High-Energy Gamma-Ray Telescope

The Gamma-ray Large Area Space Telescope (GLAST), a proposed next-generation high-energy gamma-ray telescope for studying emission from astrophysical sources in the 10 MeV to 300 GeV energy range, has been selected for study by NASA as a new astrophysics mission concept. The primary scientific targets of GLAST include active galactic nuclei, gamma-ray bursts, neutron stars, and diffuse galactic and extragalactic high-energy radiation.

Like previous high-energy telescopes, GLAST relies on the unambigious identification of incident gamma-rays by detection of the electron and positron that result from pair creation in a thin converter material. Measurement of the energy and direction of the electron-positron shower provides information about the energy and direction of the incident gamma-ray. In contrast to earlier orbiting telescopes, the GLAST design utilizes modern solid-state particle detector technology and recently developed advanced space-qualified computers. In particular, GLAST uses position-sensitive silicon strip detectors, interleaved between thin converters, to track particles rather than using a gaseous particle tracker. Because of this technical approach, the telescope design can be easily optimized to a range of sizes. For example, accomodation of GLAST within a Delta II size launch system results in an instrument with capabilities well beyond those of EGRET currently operating on the Compton Observatory; namely, a broader energy range, larger effective area, wider field of view, and single- photon angular resolution 2 to 5 times more precise than EGRET's resolution. GLAST will have a maximum effective area of 8000 sq cm above 300 MeV, a field of view of 2.6 sr, and a single photon angular resolution (rms projected) of 0.3 deg at 1 GeV, approaching 0.03 deg above 20 GeV. With these capabilities, GLAST will have a point source flux detection threshold nearly 2 orders of magnitude below that of EGRET and is expected to detect several thousand sources from an all-sky survey.

A series of scientific and technical workshops to develop the GLAST mission concept will be held during the next 18 months. The first scientific workshop will be on July 27-28, 1995 at Stanford University. For further information on GLAST, please contact Peter Michelson by e-mail at or Bill Atwood at

The Energetic Transient Array (ETA)

a Gamma-ray Burst Astrometry Mission

US Investigator Team for the ETA:

George R. Ricker (Principal Investigator), John P. Doty, Peter G. Ford, Alan M. Levine, Manuel Martinez-Sanchez, Roland K. Vanderspek
MIT Center for Space Research, 77 Massachusetts Avenue, Cambridge MA 02139

Kevin C. Hurley, J. Garrett Jernigan
University of California, Space Sciences Lab., Berkeley CA 94720

Stanford E. Woosley
University of California, Santa Cruz, Board of Study in Astronomy & Astrophysics, Santa Cruz, CA 95064

Donald Q. Lamb
University of Chicago, Dept. of Astronomy & Astrophysics, 5640 S. Ellis, Chicago IL 60637

Dieter H.B. Hartmann
Dept. of Physics & Astronomy, Clemson University, Clemson SC 29634-1911

Thomas L. Cline, Jay P. Norris
NASA/Goddard Space Flight Center, Lab. for High Energy Astrophysics Code 668.1 Greenbelt MD 20771

Gerald J. Fishman, John M. Horack
NASA/Marshall Space Flight Center, ES-62, Huntsville AL 35812

Participating Institutions with hardware or testing roles in the ETA program include:

  • Massachusetts Institute of Technology, Center for Space Research 77 Massachusetts Avenue, Cambridge, MA 02139
  • University of California at Berkeley, Space Sciences Laboratory Grizzly Peak & Centennial Drive, Berkeley, CA 94720
  • NASA/Marshall Space Flight Center, Code ES 62 Huntsville, AL 35812
  • NASA/Lewis Research Center 21000 Brookpark Road, Mail Stop SPDT-1, Cleveland, OH 44135
  • Phillips Laboratory, Electric Propulsion Laboratory OL-AC PL/RKAS, 4 Draco Drive, Edwards Air Force Base, CA 93524-7190


The Energetic Transient Array (ETA) will measure ~10^3 gamma-ray burst (GRB) locations per year, with positions as accurate as ~1 arcsec, resulting in error boxes ~10 -100 times finer than previously possible, and definitely answering the question as to whether GRBs repeat. ETA positions will enable optical/IR/radio counterpart searches to fainter than 26 mag. Six (6) dedicated, 45 kg, ETA microsats will be deployed in ~1AU radius solar orbits using an ion engine-driven carrier. The ETA is a low cost mission, well-suited to flight as a MIDEX.

Introduction and Overview of the Energetic Transient Array (ETA)

Gamma-ray bursts (GRBs) are a deep and abiding mystery, whose origin continues to baffle an ever-growing community of intensely-interested astronomers and physicists. The goal of our proposed study is to develop an astrophysics mission concept, the Energetic Transient Array (ETA), which will measure positions of ~10^2 GRBs to an unprecedented accuracy of ~1 arcsecond. An additional ~10^3 bursts will be localized to better than ~1 arcminute. Thus, the fundamental scientific question of whether GRBs repeat (Section 2, below) will definitely be answered by the ETA. Furthermore, the ETA positions will be sufficiently accurate that the most powerful optical, radio, and IR facilities on and above the planet can finally be brought to bear on the GRB mystery (Section 2). No other mission concept of which we are aware has the capacity of providing the arcsecond positions that are required for spectroscopic study and deep imaging of GRB locations.

The ETA relies on an idea first proposed ~5 years ago (Ricker 1990). Effectively, the ETA is a "space-borne very long baseline (VLB) network" for gamma-ray astrophysics, and requires at least 4 widely-spaced GRB "receivers," just as does a VLB network in the radio band. (In fact, we have chosen 6 such GRB detectors.) The ETA draws upon the previously exploited concept of the "interplanetary network" (IPN) of small GRB detectors on separated (~1 AU) spacecraft, but the ETA differs from the IPN in two important ways. First, the ETA detectors are identical from one dedicated ETA platform to the next, so that the problems of mismatched detector sensitivities and inadequate resources (quantity of telemetry, "on-time" , etc.) which have plagued the usual "piggy-back" IPN systems will not effect the ETA. Second, the ETA will be "designed," in terms of appropriately chosen platform spacings within the solar system, and not just "happen," as has often been the case for the IPN. The fact that the ETA is "designed" also means that system-level hardware- and software-induced propagation time uncertainties will not plague it, as has been the case in a number of piggy-back systems, for which IPN users had to struggle (sometimes unsuccessfully) to obtain pre- and post-launch spacecraft end-to-end timing measurements and post hoc geometry corrections of sufficient accuracy. Thus ETA will be able to fully utilize template matching of GRBs measured with well-calibrated, well-understood detector systems. To assure ample telemetry to and from the msats, we plan to use the superb, 18.3 meter X-band radio antenna at MIT's Westford Site.

For the ETA, we propose an inter-platform spacing of ~0.1 - 2 AU range for a network of 6 identical microsats (each ~45 kg in mass) with sensitive, large area (1300 cm^2 per microsat), BATSE-type GRB detectors covering the ~50 - 500 keV energy band. The aggregate detector area-solid angle product of the constellation of ETA msats, coupled with the low and stable background in interplanetary space, will assure that the GRB detector rate for ETA (~1000 yr-1) significantly exceeds that of BATSE (~300 year-1). We plan to emulate the successful approach we previously took with the High Energy Transient Experiment (HETE, scheduled for a late 1995 launch; Ricker et al. 1992), in which we developed a small, inexpensive msat for a pre-launch mission cost of <$15m, or about 30%-50% of the cost of a SMEX. The HETE effort has been very much in the style of a "cheaper, better, faster" mission, offering a maximum science-per-dollar return.

For the ETA, we will extend the HETE paradigm by implementing two important new technical innovations in a low-cost, science mission. The first innovation is the production and launch of a sufficient number of microsats to achieve "production run" economies. For a production run of 6 microsats, we estimate that the highly-capable ETA microsats can be fabricated at a unit cost of ~$1M. The second innovation for the ETA is its planned first use of an important new technology in an interplanetary setting: solar electric propulsion. The ETA carrier will rely upon a robust, Xe-gas propelled, stationary plasma thruster (SPT) unit which has been extensively flight-tested in Russian earth-orbiting missions. The SPT engines have recently completed exhaustive, burn time testing extending over 3000-5000 hours in the US ( at the Jet Propulsion Laboratory, and at NASA Lewis Research Center-LeRC). The ETA carrier is an ideal test bed to utilize the SPT-70, the thruster we have selected, since the mission can be optimized for a microsat deployment period of 3-4 months. Furthermore, our mode of operating the ETA carrier will make no stringent demands on thrust level stability. Also, thrusting occurs well away from planetary neighborhoods, yet the carrier remains near ~1 AU from the Sun, assuring adequate solar energy flux for the photovoltaic generators the SPT-70 requires. We have arranged a Space Act Agreement with the Electric Propulsion Group at LeRC, and look forward to their participation and assistance in the proposed study on issues related to carrier propulsion and mission design.

Our team also includes the USAF Philips Laboratory Electric Propulsion Group, which will be responsible for coordinating the efforts within industry to optimize an SPT engine system for the ETA. Finally, we have examined another means of making the ETA even more cost-effective and scientifically productive: international cooperation with CNES, the French national space agency.

It is with great eagerness and anticipation that we look forward to the scientific and technical possibilities offered by the ETA effort we are proposing. At the end of the year's effort for the which the ETA Team is receiving funding as a small mission concept, we should be well-poised to move toward a flight program. We envision this program leading to a mid-1999 MEDLITE launch, followed by a highly productive 2-3 year ETA Science Mission, resulting in exciting new scientific insights into the GRB mystery which the ETA is almost guaranteed to produce.

Scientific Background and Objectives for the ETA

Despite twenty years of intense study of cosmic gamma-ray bursts by observers and theorists, no one knows for sure what they are, where they come from, or even whether they are a single phenomenon. The primary impact of the Compton Gamma-Ray Observatory (CGRO) has been to intensify the debate about whether GRBs are Galactic or cosmological in origin. Either way, they are telling us something new and unexpected about the nature of the universe. As a result, GRBs have attracted increasing interest from astronomers and physicists.

Accurate localization of GRBs provides a powerful means of attacking the question of whether or not GRBs repeat and the identification of GRB counterparts at other wavelengths. Confirmation that GRBs repeat would provide a severe constraint on possible models of GRB sources. Discovery of counterparts at other wavelengths would connect the study of GRBs with the rest of astronomy, bringing to bear on the GRB mystery all the power of ground-based and space-based radio, infrared, optical, UV, and X-ray telescopes. It would likely provide immediate and powerful information about the distances to GRB sources. The timely dissemination of many accurate GRB positions is crucial to the potential success of such efforts.

The Burst and Transient Source Experiment (BATSE) on board CGRO has obtained ~ 10 degree positions for more than 1000 GRBs, and has recently begun to disseminate the positions of new bursts within ~ 10-20 seconds of the BATSE trigger time, via the BACODINE network. The Interplanetary Network (IPN) has provided E1' positions for a handful of bursts by cross-correlating the time histories of bright bursts observed by GRB instruments aboard three or more interplanetary spacecraft. Earlier, determination and dissemination of IPN GRB positions took weeks to months; recently, this delay has been reduced to hours. However, with the death of the Mars Observer, there is now only one deep space GRB detector (on Ulysses) so that sources can be located only within arc-shaped error boxes which are an arcminute or so wide and ~ 10 degrees long. There will not be another deep space GRB detector until late 1996 at the earliest, and most likely not until 1998. In the meantime, HETE will provide 10"-10' positions from a single spacecraft for ~ 30 GRBs per year in real time (~ 3-5 seconds after they trigger HETE).

In contrast, the ETA mission would provide <1" positions for ~10^2 GRBs per year, and <1' positions for ~10^3 GRBs per year. Below we describe the major advances that ETA would make possible regarding the detection of repeating and the identification of GRB counterparts at other wavelengths.

Repetition of Gamma-Ray Bursts

Recently, by studying clustering in the angular distribution of the GRBs in the BATSE 1B catalogue, Quashnock and Lamb (1993, 1994) found evidence that GRBs repeat. Wang and Lingenfelter (1993) have also presented evidence that five particular bursts arise from a single repeating source. If "classical'' GRBs do repeat, it significantly constrains the range of allowed burst models. Many Galactic models (such as episodic accretion, starquakes or thermonuclear flashes involving neutron stars) predict repeated bursts, whereas most cosmological models invoke a singular, cataclysmic event (such as coalescence of a neutron star-neutron star or neutron star-black hole binary, or a "failed" supernovae) in order to generate the tremendous amount of energy that these models require. Thus confirmation that GRBs repeat would favor a Galactic origin for the bursts. It would also suggest a close relationship between "classical'' GRBs and soft gamma-ray repeaters (SGRs), which are thought to be Galactic neutron stars.

Whether one uses the two-point angular correlation function or nearest- and farthest-neighbor distributions, the ability to detect repeating depends upon i) the total number N of observed GRBs, ii) the fraction f of GRBs that come from repeating sources, iii) the number of repetitions nrep from each repeating source, and iv) (most importantly) the error sigma in the GRB positions. The nearest- neighbor analysis of the BATSE 1B catalogue performed by Quashnock and Lamb (1993) indicates that f ~ 20%, and that the number of repetitions from each repeating source is ~ 3-4. More recent two-point angular correlation function and nearest- neighbor analyses of the 2B catalogue performed by Meegan et al. (1994), Hartmann et al. (1994), and Lamb and Quashnock (1994) indicate that f ~ 20%.

A repeating fraction f ~ 20% may be detectable using the 260 bursts in the BATSE 1B catalogue, given the typical total (statistical plus systematic) BATSE positional error sigma ~ 7 degrees (1 standard deviation). BATSE has now detected well over 1000 GRBs. Unfortunately, this does not translate straightforwardly into a better ability to detect repeating, because of source confusion. The average angular separation between 1000 bursts is ~ 7 degrees, the same size as the typical BATSE positional error; thus for 1000 or more bursts, one can never know which burst comes from which source. The problem of source confusion will be mitigated only slightly if it proves possible to reduce the size of the BATSE systematic error (currently 4 degrees). This is because the statistical error dominates the total positional error for the vast majority of GRBs.

In contrast, ETA would detect ~ 1000 GRBs per year with a typical total positional errors sigma ~ 1" - 1' (2 standard deviations). With such positional accuracies, more than 108 GRBs could be observed before source confusion becomes a problem! Clearly, ETA would have the ability to detect any repeating, if it occurs!

In addition, we remark that, whether one uses the two-point angular correlation function or the nearest- and farthest-neighbor distributions, the ability to detect repeating depends crucially on the nonlinear nature of the clustering. The efficiency with which BATSE observes any point on the sky is about 34% due to Earth blocking, the South Atlantic Anomaly, and the dead time while the data for each triggered event is transmitted to the ground. In contrast, ETA would have an exposure near 100%. Thus sources that produced 2-5 bursts in one year of BATSE observations would produce 6-15 bursts in one year of ETA observations, dramatically increasing the chances of detecting repeating. ETA's advantage cannot be overcome by longer BATSE observations unless the repeating sources remain burst-active throughout the observing period.

Counterparts of Gamma-Ray Bursts

The "holy grail'' of GRB studies remains the identification of a quiescent counterpart, some object which can be studied with the full force of large and sophisticated astronomical instruments. Such observations provide the best chance of determining the distance scale to GRBs, as they did earlier in the case of radio sources and X-ray sources, and have done very recently in the case of SGRs.

If GRBs come from the coalescence of neutron star-neutron star binaries (Narayan, Paczynski, and Piran 1992), "failed'' supernovae, or other baryonic constituents of the universe, one expects GRB sources to be distributed like luminous matter. Bright GRBs would then be associated with bright galaxies or clusters of galaxies.

However, some sixteen ~ 1' error boxes have now been searched with the VLA, IRAS, in the near IR, optical, and in the extreme UV with the ROSAT Wide Field Camera, and none of these searches has revealed any galaxy brighter than, typically, mR = 19.5. As Fenimore et al. (1993) and Schaefer (1994) have pointed out, the negative results obtained so far severely constrain cosmological models of GRBs. The lack of host galaxies in these small error boxes rules out an association between GRBs and spiral galaxies or typical AGN, and suggests that if GRBs are cosmological, they may be associated with subluminous, dwarf irregular, or faint blue galaxies.

Tyson (1988) gives the integral number-magnitude counts distribution from deep optical CCD images of the sky as log N (mR We note that GRB instruments which achieve only ~ 10"x10" error boxes will already be highly source-confused (~1 optical source per beam) at R = + 25.5, a limiting magnitude commonly reached by 3-4 meter class telescopes with state-of-the-art CCD cameras. Thus, such GRB error boxes would be of little value for optical or radio source identification based on positional coincidence. They are also much too coarse for sensitive spectroscopy.


  • Fenimore, E. E. 1993, Nature, 366, 40.
  • Hartmann, D. H., et al. 1994, ApJ, in press.
  • Lamb, D. Q., and Quashnock, J. M. 1994, MNRAS, submitted.
  • Meegan, C., et al. 1994, ApJ, in press.
  • Narayan, R., Paczynski, B., and Piran, T. 1992, ApJ, 395, L83.
  • Quashnock, J. M., and Lamb, D. Q. 1993, MNRAS, 265, L59.
  • Ricker, G. R., et al, 1990,
  • Ricker, G. R., et al, 1992, Proceedings, Los Alamos Workshop on Gamma-Ray Bursts, Taos, NM, C. Ho, R. Epstein, & E. Fenimore, eds. (Cambridge University Press, UK) p. 288.
  • Schaefer, B. 1994, talk at Aspen Center for Physics Workshop on "Gamma-Ray Bursts."
  • Tyson, T. 1988, A.J., 96, 1.
  • Wang, V. C., and Lingenfelter, R. E. 1993, ApJ, 416, L13.

Large Area X-ray Spectroscopy Mission

Principal Investigator: Dr. Harvey Tananbaum, SAO


  • Dr. Leon VanSpeybroeck, Smithsonian Astrophysical Observatory
  • Dr. Martin Weisskopf, Marshall Space Flight Center
  • Prof. Claude Canizares, Massachusetts Institute of Technology
  • Dr. George Ricker, Massachusetts Institute of Technology
  • Dr. Thomas Markert, Massachusetts Institute of Technology
  • Prof. Steven Kahn, Univ. of California, Berkeley
  • Dr. Eric Silver, Lawrence Livermore National Laboratory
  • Prof. Bruce Margon, Univ. of Washington
  • Prof. Oberto Citterio, Osserv. Astron. di Brera
  • Dr. Stephen S. Murray, Smithsonian Astrophysical Observatory
  • Mr. James Bilbro, Marshall Space Flight Center
  • Dr. Marshall Joy, Marshall Space Flight Center
  • Dr. Ronald Elsner, Marshall Space Flight Center
  • Dr. Stephen O'Dell, Marshall Space Flight Center
  • Dr. Frederik Paerels, Univ. of California, Berkeley
  • Dr. Scott Anderson, Univ. of Washington
  • Dr. Mauro Ghigo, Osserv. Astron. di Brera


Spectroscopy provides essential data for detailed physical studies in all branches of astronomy. X-ray astronomy is no exception. Current missions have initiated such studies, and the data already show multiple spectral components and a richness and variety of absorption and emission features for essentially all classes of X-ray emitters. Missions currently in development such as Spectrum-XG, SAX, AXAF-I, XMM, and ASTRO-E will extend these spectroscopic capabilities. However, much larger area and higher throughput, accompanied by relatively high spectral resolution, are required for programs such as time resolved spectroscopy of stellar flares, spectra of X-ray binaries and supernova remnants in M31, and studies of the evolution of clusters of galaxies and quasars.

We propose to study a Large Area X-ray Spectroscopy Mission conceived and sized for programs such as these in order to address a range of fundamental astrophysical questions such as:

  • the role of flares and microflares in heating stellar coronae
  • the impact of metallicity on the Eddington limit in accreting binaries
  • the enrichment of the interstellar and intracluster media
  • the formation of galaxies from cooling gas and
  • the nature of the environment around quasars and lower level active galactic nuclei.

The essence of our Large Area X-ray Spectroscopy concept is to build six identical modest satellites, each carrying a highly packed assembly of replicated mirror shells and X-ray spectrometers, launched into a solar, drift-away orbit (similar to that studied for the SIRTF program). We envision a large area (~ 3.3 sq. m around 1 keV), and a relatively high spectral resolution (E/Delta E = 800 at 20 Angstrom) with the ability to study extended sources (up to 1' or 2') as well as point-like objects. Consonant with the changing environment at NASA and in the world at-large, we propose to achieve these increased capabilities at substantially lower cost than for previous missions.

A proposal to do substantially more science for significantly lower cost must have some specificity to be credible. Therefore, we have organized a team with extensive relevant experience and have conceptually designed a strawman payload. The strawman draws heavily on existing capabilities and includes recent technological developments in replication optics and detectors. We fit the payload into an existing launch vehicle. The strawman provides a basis for computing specific examples of the science which can be done and for estimating the cost of the program. Still, the strawman is not meant to be definitive - we will draw on future technological advances and will involve many other members of the science community in our study through an open process. In this way, we expect to iterate and improve on the science objectives and on our approach to the hardware.

Mission cost is constrained by baselining existing instruments and spacecraft upgraded by new technology as appropriate, by restricting payload weight, by choosing an orbit with a benign environment and permitting simple operations, and by assuming a modest amount of risk for individual units given that the program comprises 6 identical payloads. Our estimated cost is $50 - $55M per satellite or $300 - $330M for 6 units. Launch costs would be additional ($45M per launch on a Delta II, perhaps $25-30M per launch for a new vehicle such as the Lockheed LLV-3).

The proposed approach resonates extraordinarily well with NASA's strategy for the future. The program provides a robust approach, distributing risk over several launches using a number of small, lightweight, inexpensive satellites to achieve the required large area and scientific sensitivity. The development time is relatively short - 3 years from program start to first launch, with subsequent launches every 4-6 months. The solar orbit supports very simple operational scenarios and safe modes, and is conducive to long life (> 5 years). There are specific opportunities for student involvement and incorporation and diffusion of new technology. The program proposes to do ``Observatory'' class science for ``Discovery'' class costs, as a facility open for all scientists to use. International cooperation is already a part of the program starting with this proposal.

Burst Arc Second Imaging and Spectroscopy (BASIS)

Bonnard Teegarden The BASIS mission concept has two primary objectives 1) determination of gamma-ray burst positions to ~ 5 arcsec accuracy for ~ 100 bursts/yr and 2) high sensitivity burst spectroscopy in the 0.2 - 200 keV range. Burst localizations at the arc second level offer the exciting prospect for counterpart identification. With a sample size of ~ 200 detected bursts it should be possible to fully exploit the most sensitive ground-based telescopes in searching for burst counterparts. It will also be possible to unambiguously determine whether or not bursts repeat. The energy spectra of gamma-ray bursts are only poorly studied below ~ 20 keV. BASIS will provide unprecedented sensitivity and sky coverage on this energy range. It will have the capability of measuring absorption columns of > 3 x 10^20 cm^-2 for bright bursts and > 3 x 10^21 cm^-2 for dim bursts. Galactic absorption, if present, should be easily detectable. Bright extragalactic bursts should also have detectable absorption. Correlation with the measured burst absorption with the known galactic absorption column will very likely allow one to distinguish between galactic disk and halo models. Bursts originating in distant galaxies should also show measurable absorption and brightness-absorption correlations would be expected.

Precise burst localization is accomplished using an array of three wide-field (1.5 sr ea.) compact coded aperture telescopes. The technical innovation that makes this possible is the use CdZnTe strip detectors for fine position determination. These are hi-Z room temperature semiconductor detectors ideally suited for operation in the hard X-ray range where GRB fluxes peak. A working prototype detector with a spatial resolution of 100 microns is now operating at the Goddard Space Flight Center. An array of these devices with a total sensitive area of ~ 200 cm^2 is envisioned for each telescope. This large collecting area combined with the wide field-of-view (~ 4 sr) will insure the detection and localization of ~ 100 bursts/yr. Imaging is accomplished using a two-scale coded aperture mask (100 micron and 2 cm). The coarse scale is used for the prompt on-board determination of 0.1 - 1=B degree localizations. These will be made available within seconds to ground-based observatories using the INMARSAT satellites and a BACODINE-style network. Precise burst positions (~ 5 arcsec) will be produced through post-processing on the ground using the 100 micron fine mask scale.

Soft X-ray spectroscopy (0.2 - 20 keV) is accomplished with an array of 3 conventional thin-window xenon gas proportional counters. In the X-ray region the background is dominated by the diffuse cosmic X-ray background. A rapidly orienting 3=B degree venetian-blind collimator is used to suppress this background while maintaining wide-field coverage. The on-board prompt coarse burst location from the coded mask telescope mentioned above is used to orient the collimator. These detectors are designed to be complementary to the coded mask telescopes and will produce high-quality spectra for ~ 100 bursts/yr.

In addition to the 200 cm^2 fine positioning detectors, the coded-mask telescopes incorporate arrays of CdZnTe detectors that are optimized for spectroscopy. Each telescope will have ~ 1000 cm^2 of these devices. With this capability we will be able to perform high-resolution burst spectroscopy (dE/E ~ 3% at 100 keV) with unprecedented sensitivity. This should provide a definitive answer to the question of the existence of cyclotron lines in gamma-ray bursts.

X-ray Spectroscopy with NGXO

An approved proposal in response to NRA 94-OSS-15

Nick White, LHEA and Goddard Space Flight Center.

Robert Petre, LHEA and Goddard Space Flight Center.


We are studying a Next Generation X-ray Observatory, NGXO, that will provide a major increase in spectral capability, at a relatively low cost. The NGXO mission is a natural follow-on mission to the AXAF, XMM and Astro-E observatories, and will be well placed to capitalize on the expected discoveries from these great observatories. The mission concept consists of two co-aligned telescope systems that provide coverage from 0.3-60 keV. One is optimized to cover the 0.3-12 keV band with high spectral resolution using a non-dispersive spectrometer (an array of quantum calorimeters with 2eV resolution) to yield close to unit quantum efficiency. The second will be the first focusing telescope to operate in the 10-60 keV energy range, and will be capable of superior imaging and spectroscopic observations in that band. The sensitivities of the two telescopes are matched to make possible many thousands of high quality X-ray spectral observations, from an available population of ~10^6 galactic and extragalactic X-ray sources.

NGXO is capable of addressing many new and exciting astrophysical problems including: determining the mass of black holes in X-ray binaries; measuring the 0.3-60 keV X-ray spectrum from AGN and determining their contribution to the X-ray background; measuring Compton reflection spectra from cold material in accretion driven systems; determining the Hubble constant using resonant line absorption of QSO spectra by rich clusters; searching for a hot 10^7K intergalactic medium; mapping the dynamics of the intracluster medium; mapping the ionization state, abundance and emission from supernova remnants on a 15 arc second angular scale; and measuring mass motion in stellar flares and the dynamics of accretion flows.

The baseline NGXO mission concept is sized to fit a Delta launch vehicle. The total end-to-end mission cost is estimated to be $399M, (which includes $75M MO&DA for an assumed 5 yr mission lifetime and a 20% contingency).

The NGXO Spectrometer

The core instruments on the AXAF, XMM and Astro-E observatories are CCD cameras and these will provide a spectro-imaging capability with a moderate energy resolution of ~100 eV over the 0.4-10 keV band. These missions also carry higher spectral resolution devices (gratings on XMM and AXAF and a micro-calorimeter array on Astro-E) and these will gather more detailed spectra of the brighter sources. The collecting areas of the grating spectrometers are factors of 5-10 less than the CCD cameras on their respective missions and will require long exposures of 10^5-10^6 s to obtain high quality spectra. Also the gratings cannot be used to study extended sources because the spatial extent of the X-ray emission will hamper their sensitivity to line emission by creating confusion between spatial and spectral features (supernova remnants and clusters of galaxies are amongst the most line-rich X-ray sources). The Astro-E XRS will be able to observe extended sources, but the ~10 eV resolution is insufficient at 1 keV to fully resolve many of key line complexes required for plasma diagnostics.

Figure 1a. The resolution as a function of energy for the spectrometers to be flown on AXAF (LETG, METG, HETG), XMM (RGS1 and RGS2, 1st and 2nd orders), Astro-E and that proposed for NGXO.

Figure 1b. The effective area of the spectrometers as a function of energy.

The baseline NGXO high throughput X-ray spectrometer, HTXS, is an advanced micro-calorimeter array capable of 2 eV resolution from 0.3-12 keV. This resolution is well within the theoretical capabilities of an X-ray calorimeter and provides at 1 keV an energy resolution comparable to the gratings on XMM. At 6 keV 2 eV resolution is comparable to that obtained with the crystal spectrometers used to study the solar X-ray spectrum. The major advantage of using a non-dispersive spectrometer is the increased quantum efficiency compared with the current generation of grating spectrometers. The calorimeter array is at the focus of a large area (~2,000 cm^2), lightweight, foil grazing-incidence mirror with an angular resolution of 15 arc second half power radius. In Figure 1 we compare the spectral resolution (R=E/DE) and the effective area as a function of energy of the current generation of higher resolution spectrometers with that expected from the NGXO HTXS. The NGXO 0.3-12 keV spectral resolving power (E/DE) of 150-6000, in combination with the high throughput, gives a resolving power a factor of 5 better than the calorimeters to be flown on Astro-E, with a factor of 5 increase in effective area and two times improved angular resolution. At 1 keV the NGXO HTXS 2 eV resolution delivers comparable spectral resolution to the XMM RGS, but with a factor of 10 increase in effective area (compared to two XMM RGS modules).

The NGXO spectrometer will gather high quality spectra of sources routinely observed with moderate spectral resolution by the CCD detectors on AXAF, XMM and Astro-E, but well beyond the grasp of the high resolution spectrometers on the currently planned missions. In addition, observations of extended sources can be made that are not possible with the dispersive spectrometers on AXAF and XMM. To illustrate this increase in capability Figure 2 shows simulated spectra centered on the Fe K He-like complex at 6.7 keV and Fe L complex at 1 keV for the nearby bright stellar coronal source AR Lac. With 2 eV resolution the NGXO HTXS spectrum fully resolves the density sensitive He-like complex of of Fe K satellite lines centered on 6.65 keV. The classical forbidden, resonance and intercombination line diagnostics of the He-like transitions gives a direct density diagnostic. Around 1 keV all the important Fe L shell blends from Fe XVII through FeXXIV are resolved. These Fe L lines are often the strongest seen in celestial plasmas from stars, clusters and galaxies. There are other K line complexes available in the 0.3-10 keV band including N, O, Ne, Mg, Si, S, Ca, Ar, and Ni, and these will also be available at far greater sensitivity for both point and extended sources.

The planned resolution gives a maximum sensitivity to bulk velocities >100 km s^-1 at the iron K line and will probe the dynamics of a wide range of dynamic phenomena with temperatures of 10^6 to 10^8K, ranging from stellar flares to relativistic accretion disks. The absolute Fe K line energies will be determined to an accuracy of ~10 km s^-1, allowing for the first time a determination of binary parameters, redshift and relative velocities approaching those possible from optical spectroscopy. The sensitivity of NGXO to radial velocity variations in X-ray line emission from the vicinity of the X-ray source has the potential to deliver accurate mass measurements of black holes, neutron stars and white dwarfs in binary systems with orbital periods of hours.

Figure 2a. Spectral comparison for the Astro-E XRS and XMM RGS with the NGXO HTXS. This uses simulations based on an 80,000 s observation of AR Lac. This is a 2 temperature spectrum with 7 and 24 million degrees, one third solar abundance model derived from spectral fits obtained by ASCA. A single XMM RGS 1st order spectrum is shown.

Figure 2

Figure 2b. This is the same simulation as Figure 2a, but for the iron K He-like blend at 6.7 keV. In this case the Astro E XRS is compared with the NGXO HTXS.

The NGXO Hard X-ray Telescope

The current generation of imaging X-ray have an upper energy threshold of 10 keV, above which the effective areas drop precipitously. The first results from ASCA are showing that sensitive observations of the >10 keV band are necessary to constrain the continuum spectra of many classes of X-ray source. This is crucial in highly absorbed sources and sources where the Compton reflection of X-rays from cold material is present. Extended energy coverage is also necessary to differentiate between thermal and non-thermal emission. The second X-ray telescope on NGXO makes use of multilayer coatings to enhance the high energy performance of a grazing-incidence mirror to give significant throughput up to 60 keV. A CdZnTe (or similar) imaging detector will be used as a high quantum efficiency spectrometer.

The hard X-ray capability of the NGXO hard X-ray telescope, HXT, in combination with the high resolution spectrometer, will make it possible to detect the interaction of X-rays with cold material and non-thermal phenomena. It is in this hard X-ray band that Compton reflection from cool material in AGN, X-ray binaries, and cataclysmic variables is detectable. Simultaneous measurements of the iron K lines and the reflection spectrum will be used to constrain the overall geometry, ionization state and abundances in these systems. Hard X-ray observations are also essential to constrain the continuum spectrum for heavily absorbed X-ray sources (e.g. Seyfert II galaxies and magnetic CVs) measured by the spectrometer. This mission will measure the spectra of the sources that contribute to the X-ray background over the 0.3-60 keV band, where the background is dominant. The sensitivities of the two co-aligned telescopes are matched to obtain high quality spectra over the entire 0.3-60 keV spectrum of AGN at a flux level of >5*10^-13 erg cm^-2 s^-1 (in the Einstein IPC band) in 10^5 s. The NGXO will permit the deepest search yet for the hard X-ray signature of a dormant/hidden AGN at the center of our own and nearby galaxies. We have simulated a spectrum using NGXO for a 10^5 s observation of an AGN at a flux of 5.0*10^-13 erg cm^-2 s^-1 (over the 0.1-4.0 keV band). The simulation, shown in Figure 3, included a Compton reflection spectrum for an inclination of 10 degrees. The fit is to a simple power law model to illustrate the significance of the reflection spectrum, which is clearly detected above 20 keV. It will be possible to map the reverberations in the strength of the Compton reflection spectrum, in response to changes in the central luminosity of the X-ray source.

Figure 3.

Figure 3: A simulation of an AGN spectrum, including the effects of Compton reflection, for a 100,000s NGXO exposure of a source at a flux of 5.0*10^-13 erg cm^-2 s^-1 (over the 0.1-4.0 keV band). This was for a cold disk, with an inclination of 10 degrees. The simulation included the reflection spectrum; the model fit shown is for a simple power law, and the residuals in the lower panel demonstrate the significance of the reflection bump at higher energies.

The Study Phase Objectives

The goal of our proposed study is to develop a conceptual design for an NGXO mission (instrumentation, spacecraft and operations) that achieves our stated scientific objectives at a minimum of cost and technical risk. Many of the required components utilize technology currently under development at GSFC as part of the NASA SR&T program. This study will serve as a catalyst to demonstrate the viability of the proposed technology.

A large part of the new science possible with NGXO rests on the expectation that the spectral resolution of a microcalorimeter can be improved by a factor of five. Based on the expertise gained by the LHEA in the development of the first X-ray micro-calorimeter, this seems an achievable step. At the end of the study phase it will be demonstrated that 2 eV resolution is feasible. The field of view of the spectrometer is limited by the maximum number of pixels that can be reliably fabricated and contacted electrically. The trade-offs between the array size, the science goals and the spacecraft pointing requirements will be investigated to determine the optimum array size.

For both telescope systems it is desirable to maximize the focal length, to achieve the highest possible throughput at high energies. The choice between a two- and a four-reflection system depends primarily on the focal length allowed by an extendable optical bench (EOB) and the launch vehicle. Based on studying the cost and technical trade-offs between the launcher capabilities, the EOB, and the focal length of the mirror system, a recommendation will be made regarding the optimum telescope configuration. For focal lengths up to 10 m, a four-reflection mirror in effect doubles the focal length by providing shallower grazing angles. But this is at the expense of efficiency and possibly spatial resolution from the additional reflections. For focal lengths >10 m a two-reflection mirror provides the required collecting area without the additional cost of fabricating and aligning two more reflection stages.

To achieve the required reflection efficiency, the Hard X-ray Imaging Telescope requires the application of a multilayer coating on its conical mirror surfaces. This approach offers great promise as a means of producing a high throughput, low weight, low cost system that can deliver both arc minute spatial resolution and considerable effective area up to 60 keV. While multilayer technology has been successfully applied to enhance the performance of mirrors in narrow bands, it has not yet been fully developed for a hard X-ray telescope. During the study phase a program to study the feasibility and cost of applying multilayers to foil optics will be pursued. Other approaches to high energy imaging optics will be investigated, including the use of micro-channel plates as hard X-ray optics.

The CdZnTe detectors currently represent the most promising technology for the hard X-ray imaging spectrometer. As part of a NASA-funded Gamma-ray SR&T program, LHEA is already carrying out a development program for these detectors, and by the end of the study phase we will be able to demonstrate the feasibility of building a flight system. In parallel, we will investigate the use of alternate semiconductor materials, both previously used for hard X-ray spectroscopy (such as Si and Ge) and as yet unexploited (such as InSb).

The Study Team

  • Principal Investigator: Nicholas White
  • Lead Co-Investigator: Robert Petre
  • The Calorimeter: Richard Kelley, Caroline Stahle and Andrew Szymkowiak.
  • The Mirrors: Peter Serlemitsos, Robert Petre and Yang Soong.
  • The Mission Parameters: Frank Marshall.
  • The Hard X-ray Detector: Niel Gehrels and A. Parsons.
  • Key Participants: Elihu Boldt, Steve Holt, K. Arnaud, Richard Mushotzky, Keith Jahoda, Jean Swank, Tim Kallman, Koji Mukai, Steve Drake, and Ian George.
  • Consultants: Herb Schnopper, Finn Christensen, Bob Warwick, George Fraser, and Paolo Giommi.

The figures can also be found here


3rd Compton Symposium, Munich, Germany, 12-14 June 1995. Contact:

Astrophysical Sources of Gravitational Waves, 7-10 July 1995, State College, PA. Contact:

Aspen Center for Physics. Programs on Physics of Dense Stellar Systems, Big Bang Nucleosynthesis, Inflation: from Theory to Observation and Back, Elementary Processes in Astrophysical Dense Matter, and others. Contact:, or Sally Mencimer, Aspen Center for Physics, 600 W. Gillespie, Aspen, CO 81611, (303) 925-2585

High Energy Solar Physics Workshop, August 16-18, Goddard Space Flight Center, contact

Interpretation of Gamma-Ray Sources and Related High-Energy Sources, Institute for Theoretical Physics, University of California, Santa Barbara, mid-August - December 1995; the workshop will start with a conference on Nonthermal Gamma-Ray Sources, August 21-25. Contact:

24th International Cosmic Ray Conference, Rome, Italy, August 28 - September 8, 1995. Contact: or VAXROM::ICRC95

Cosmic Ray, Particle and Astroparticle Physics, Florence, Italy, 11-13 September. Contact:

TeV Gamma-Ray Astrophysics, Padova, Italy, 11-13 September. Contact:

Rontgenstrahlung from the Universe, 25-29 September, Wurzburg, Germany. Contact:

187th AAS, San Antonio, TX, January 1996. Contact:

Annual HEAD meeting, around Easter break, 1996, at UC San Diego. Announcement will be sent out to the membership

31st COSPAR Scientific Assembly, 14-21 July 1996, Birmingham, UK. Contact:

NSF Program Dates

James P.Wright, NSF

Effective in calendar year 1995, the NSF Division of Astronomical Sciences has changed the deadline for the Stellar Astronomy and Astrophysics (SAA) Program from May to July 1 and has instituted TARGET DATES for SAA AND SEVERAL OTHER Programs:

  • Stellar Astronomy and Astrophysics (SAA):
    Target Date - 1 July 1995
  • Extragalactic Astronomy and Cosmology (EXC):
    Target Date - 1 August 1995
  • Galactic Astronomy (GAL):
    Target Date - 1 September 1995
  • Planetary Astronomy (PLA):
    Target Date - 1 September 1995

Target Dates will remain in effect for these Programs in the same months in future years until further notice.

News Notes

Friends of Good News! On November 16, in Rome, the Balzan Prize was awarded to Sir Fred Hoyle and Martin Schwarzschild by the President of Italy. In the 1940s Fred went to Princeton and constructed with Schwarzschild the theory of red giants and the numerical techniques of stellar evolution. That theory was cited for this marvelous prize to two of the greatest astronomers that ever looked up!

A tote of rum all 'round!

Don Clayton


HEADNEWS, the electronic newsletter of the High Energy Astrophysics Division of the American Astronomical Society, is issued by the Secretary-Treasurer, at the University of California Space Sciences Laboratory, Berkeley, CA 94720-7450. The HEAD Executive Committee Members are:

  • Martin Elvis, Chair (
  • Neil Gehrels, Vice-Chair (
  • Kevin Hurley, Secretary-Treasurer (
  • Lynn Cominsky (
  • Chuck Dermer (
  • Chryssa Kouveliotou (
  • Rick Rothschild, Member (
  • Mel Ulmer, Member (
  • Diana Worrall, Member (
  • Virginia Trimble, Member and Past Chair (

Please send newsletter correspondence to

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