Newsletter No. 67 November 1995

IN THIS ISSUE:

1. Notes from the Editor - K. Hurley
2. News from NASA Headquarters - Alan Bunner
3. SAX Announcement of Opportunity - Alan Bunner and Guenther Riegler
4. High Energy Astrophysics Science Archive Research Center - Jesse Allen
5. Burst Locations with an ArcSecond Telescope (BLAST) - Neil Johnson et al
6. Hot Interstellar Medium Spectrometer (HIMS) - Wilt Sanders
7. Upcoming HEAD Meetings - M. Elvis, N. Gehrels, and K. Hurley
8. Future Meetings

Notes from the Editor

Kevin Hurley, Secretary-Treasurer (khurley@sunspot.ssl.berkeley.edu)

In May of this year, NASA issued an Announcement of Opportunity for

Medium-class

Explorer Missions (MIDEX). Three high energy astrophysics proposals were accepted in the first round of selections, and two of them (BLAST and HIMS) are described in detail in this issue. As always, this newsletter can be accessed through the AAS Web site, but note that theiraddress and homepage layout have changed. To find the HEAD newsletter, go to http://www.aas.org, then click on "Divisions", and then on "High Energy Astrophysics Division".

News from NASA Headquarters
Alan Bunner, NASA Headquarters

As of about the end of this year, the Office of Space Science at NASA Headquarters will be reorganized such that your representatives, advocates, and discipline contacts will have changed. The primary interfaces at NASA HQ with the scientific community will now be through a Board of Directors headed by Dr. Wes Huntress. The "Science Program Board of Directors" has primary responsibility for planning and strategy, advocacy, budget priorities, and interface with the community. The Science Program Directors are (1) Alan Bunner, science director for "Physics of the Cosmos:" high energy astrophysics, EUV astronomy, submillimeter and radio astronomy, gravity physics and general relativity. (2) Ed Weiler, science director for "Astronomical Search for Origins and Planetary Systems:" UV, visible, and infrared astronomy, including HST, SOFIA, SIRTF, and the interdisciplinary field in which celestial astronomy and planetary astronomy overlap, including the search for new planetary systems. (3) Jurgen Rahe, director for "Solar System Exploration," the exploration of our solar system, and (4) George Withbroe, director for "the Sun-Earth Connection," including solar physics, space physics, solar wind and cosmic rays, magnetospheric and ionospheric physics. I expect that four advisory committees of outside scientists will soon be formed that will report to each of the four science directors above. In addition to these interfaces with the scientific community, there will continue to be several other avenues for community scientists to contact the appropriate executives at NASA HQ: Dr. Henry Brinton will head the "Research Program Management" Division, responsible for selecting and funding all research grants, suborbital programs, information systems, and MO&DA programs. Guenther Riegler will function as Deputy to Henry Brinton and will continue to be responsible for the MO&DA program of our operating satellites, and Vernon Jones will manage the suborbital program. Lou Kaluzienski will be Discipline Scientist for High Energy Astrophysics in Henry Brinton's Division.

SAX Announcement of Opportunity
Alan Bunner and Guenter Riegler, NASA Headquarters

The first Italian Announcement of Opportunity for SAX observations has been released, primarily to select the core observing program for the Italian/Dutch SAX Team. A second announcement is expected to be released after definition of the core observing program; guest observers may submit proposals for guest observations by March 15, 1996. The fraction of observing time allocated to Guest Observations is 20% during the first year of science operations, 40% during the second year, and 50% during the third year. Launch is currently planned for the mid-March to mid-April time frame, followed by a 2-month satellite checkout period and a 2-month instrument calibration period. U.S. scientists may propose to the Italian AO. At the present time, NASA does not have an identified budget set aside for SAX guest observations; however, NASA plans to include the U.S. Guest Observer program on SAX in the next "Senior Review" of Astrophysics MO&DA programs. This Senior Review is planned for mid-1996, after the planned selection of Guest Observations on SAX, and before the likely onset of actual observations. Successful U.S. scientists who are awarded research opportunities with SAX should notify Guenther Riegler and they will be contacted following the Senior Review regarding possible funding support.

HEASARC
Jesse Allen, HEASARC support staff

The High Energy Astrophysics Science Archive Research Center (HEASARC) at NASA/GSFC has recently put together a chronological record of the history of high energy astrophysics. This record has been placed online in our WWW services at the URL http://heasarc.gsfc.nasa.gov/docs/heasarc/headates/heahistory.html We have also compiled the complete history of high energy astrophysics satellites (for those which generated non-solar results), available from the URL http://heasarc.gsfc.nasa.gov/docs/heasarc/missions.html We would be very interested in comments, corrections, or additional information with regard to these pages. Please send comments etc. via e-mail to allen@prufrock.gsfc.nasa.gov. Burst Locations with an

ArcSecond Telescope (BLAST)
W. N.Johnson, C. D. Dermer, J. E. Grove, P. Hertz, R. L. Kinzer, R. A. Kroeger, J. D. Kurfess, M. Lovellette, G. Share, M. S. Strickman, K. Wood (Naval Research Laboratory), S. Inderhees, B. Phlips (Universities Space Research Association), D. Hartmann, M. D. Leising (Clemson University),  G. Fishman, C. Meegan (Marshall Space Flight Center),   and E. E. Fenimore (Los Alamos National Laboratory) Introduction Burst Locations with an Arc Second Telescope (BLAST) is one of several missions being studied in greater detail in the second phase of NASA's Medium Explorer (MIDEX) selection process. Two of the studied missions will be selected in March of 1996 for flight opportunities. The principal scientific objectives of the BLAST mission are (i) to localize gamma-ray burst (GRB) positions to arcsec accuracy (10 bursts/year with ~1 arcsec positions); (ii) to search for   enhancements in the rate of GRBs towards M31; and (iii) to conduct the most sensitive full-sky survey to date of X-ray sources in the 10 -150 keV regime.  A large field-of-view hard X-ray telescope with arc-second imaging provides the crucial tool both to identify GRB counterparts and to test galactic halo GRB models. With an order-of-magnitude improved sensitivity over the BATSE detector on the Compton Gamma Ray Observatory (CGRO), the BLAST mission will  also carry on the pioneering HEAO and CGRO studies of high energy emission from neutron star and black hole binaries, rotation-powered pulsars, supernova remnants, and active galactic nuclei. BLAST also holds the potential to discover new types of high-energy sources. Gamma Ray Bursts The origin of classical GRBs is one of the outstanding puzzles in astronomy.  These cosmic fireworks flare in the hard X-ray and gamma-ray regime from random directions in space for durations ranging from a fraction of a second to hundreds of seconds, and then fade away into multiwavelength obscurity.  Rather than solving the puzzle, the BATSE instrument on CGRO eliminated the favored model of galactic disk neutron stars by providing solid evidence that GRB directions are consistent with isotropy, but measured GRB peak count rates imply a spatial distribution which is strongly inhomogeneous. The only permitted spatial distribution for the bursters is therefore a bounded source population which is spherically symmetric about the Solar system. Hence the only models which survive the BATSE results are an extended halo or corona of high-velocity neutron stars leaving the gravitational field of the Milky Way or sources at cosmological distances.  The identification of counterparts to GRBs is generally recognized as the key to solving the GRB mystery, and the slow progress toward this goal is due to the relatively poor imaging capability of GRB detectors. The BLAST mission will provide arcsec imaging for ~10 GRBs per year, and 5 arcsecond imaging for observations within a short time following the event. Because there are about 5 sources per 10^2 arcsec^2 at magnitudes brighter than R ~ 27, an unambiguous source identification requires position accuracies of a few arcseconds if the bursters originate from sources at cosmological distances. If GRBs are associated with neutron stars in an extended galactic halo, they will probably not be detectable even with accurate positions (indeed, empty error boxes, when  sufficiently small, become an argument in favor of halo models). The discovery f an enhancement in the rate of GRBs towards M31 over the average rate easured in directions away from M31 is considered the definitive experiment to validate the galactic neutron star halo model of GRBs. Numerical simulations show that, given the minimum sampling distances of < 170 kpc to the faintest bursts observed with ATSE, then the 10 times greater sensitivity of BLAST will provide the necessary improvement to test galactic neutron star models. Hard X-ray Sky Survey he 10 - 150 keV hard X-ray band is an important spectral band for the study of   both  compact and diffuse astrophysical sources. The low energy end of this spectral band is dominated by thermal processes from 10^8-10^9 K plasmas in X-ray binary accretion disks, the shocked shells of supernova remnants, and the cores of clusters of galaxies. At the high energy end (>20 keV) are  non-thermal and Comptonized radiation sources such as X-ray novae, black hole candidates, and active galactic nuclei. Virtually all of the compact sources known at these energies are variable - a natural consequence of the small size of the emission region and the high energy processes present. BLAST will perform an all sky survey in hard X-rays over a one year period. The wide  field-of-view, which covers ~5% of the sky at FWHM, allows two-week (10^6 s) pointed observations of each survey field. By scaling from HEAO A-4 and OSSE results we estimate that ~300-700 extragalactic sources and 200-500 galactic sources will be detected. Because each source will be positioned to 1 arcmin, essentially all sources will be easily identified provided they have counterparts at other wavebands.  We can only speculate about some of the possible discoveries that a sensitive hard X-ray/soft gamma-ray all-sky survey might  produce. Nonthermal tails from SNRs could signal incipient cosmic-ray acceleration processes, long expected if cosmic rays are accelerated by supernovae. Hard X-ray mapping of SNRs could be correlated with radio maps to diagnose the density and magnetic field structures of these sources. The diffuse galactic and extragalactic X-ray  backgrounds might finally be fully resolved into discrete source contributions.  Previously unknown source classes could be discovered, such as isolated neutron stars or black holes accreting from the interstellar medium, extremely low luminosity X-ray binaries, new galactic black hole candidates, or strongly obscured compact objects. For example, obscured AGNs, buried behind large columns of dust and gas, could emerge in this energy band due to the reduced Compton and photoelectric opacity. A nonthermal component of galaxy cluster halo X- ray emission might also be discovered. BLAST Mission Concept The BLAST instrument is developed from technologies with proven space heritage, consisting of an array of position-sensitive scintillation detectors, a set of three aperture masks with supporting "telescope" structure and a data handling subsystem that collectively function as a hard X-ray imaging system covering the energy range 10 - 150 keV. BLAST is comprised of ~16000 cm^2 of detectors with aperture mask plane ~2 m above supported by a telescope  structure. Passive shielding and aperture masks define a relatively large field of view of approximately 1 steradian. The instrument utilizes a three-axis stabilized NASA MIDEX spacecraft and is launched into a 550 km, 28.5 degree inclination orbit. Accurate pointing and timing information are provided by three dedicated star trackers and a GPS receiver. A near real-time communication link is provided for the transmission of gamma ray burst positions to other observers.  BLAST can most easily be understood as two independent imaging systems in which a coded aperture telescope operating at energies >50 keV provides arcmin source localization. The aperture for lower energy X-rays (<50 keV) is defined by a Fourier grid mosaic which provides complementary arcsec information. This concept has been amply validated by the recent Yohkoh mission. The detectors view the sky through the grids and mask, placed in series. The detector array consists of position-sensitive NaI(Tl) scintillation detectors which provide a position accuracy of ~2 mm. The first mask is a modified uniformly redundant array coded aperture with ~6 mm pixels which is opaque to the highest energy measured, 150 keV, and forms the "coded aperture telescope". The mask is positioned ~2 m above the detector plane. Since the other mask arrays, the Fourier grids, are designed to be transparent above ~50 keV, the BLAST system effectively consists only of the URA mask and the detectors above that energy. The position-sensitive NaI detector array records shadows of the URA cast by the imaged field. Image reconstruction consists of standard URA inversion methods. The URA mask localizes bursts to 1 arcmin or better over the full field of view of the BLAST instrument. At low energies a mosaic of matched pairs of fine grids of the Fourier Transform type modulate the incident radiation. One of the pair is placed at the detector plane and the other is positioned with the coded aperture mask.  The upper and lower grids of a grid pair are nearly identical, consisting of parallel ribs of width 250 mm on ~500 mm centers. The spacing or pitch of the ribs in the upper grid differs slightly from that in its associated lower grid so that across a mask segment, the number of ribs in the upper grid array differs by one from that in the lower array. This arrangement causes a point source to cast a broad shadow or fringe on the detector plane such that a 1 arcsec shift in the location of a source in a plane perpendicular to the grid openings results in a translation of the fringe pattern by ~1 cm.  Determining the phase of this fringe in one coordinate on the detector plane gives the location of the source on the sky in one direction. Centroiding on a bright gamma-ray burst will reconstruct two orthogonal strips on the sky whose  widths are of the order of 1 arcsec. The Fourier grid positioning produces aliased positions at approximately 1 arcmin spacing; however, for all but the faintest bursts, only one of the positions lies in the error region given by the high-energy coded aperture mask described above. Combining information  from the coded aperture measurements at high-energy and the Fourier grids at low-energy localizes the source to a single region of order 1 arcsec.   Depending on the extrapolation of the BATSE logN/logP distribution for peak burst intensities down to the BLAST detection threshold of ~0.03 photons/cm^2-s, BLAST can be expected to see anywhere from 55 to 115 bursts per year, with the number nearer the lower value if the observed BATSE flattening of the LogN/LogP curve near their threshold sensitivity is real. BLAST will position bursts with an intensity of 1 photon/cm^2-s to ~1 arcsec. We estimate that 10 events per year will be located to a position of ~1 arcsec, and about 35 events per year can be positioned to better than 5 arcsec. These precise locations will permit deep exposures at optical and radio wavelengths for   counterparts, and thereby provide a high probability of determining the host objects that are responsible for gamma ray bursts. 

Hot Interstellar Medium Spectrometer (HIMS)
Wilt Sanders, University of Wisconsin

1. HIMS Summary

The Hot Interstellar Medium Spectrometer (HIMS) experiment is designed to perform a high-resolution spectroscopic all-sky survey of diffuse emission in the 50 - 1000 eV range. This spectral exploration of the diffuse background with high resolution resolves important questions about the role of hot gas in the interstellar medium that cannot be answered in any other way. The diffuse X-ray background arises from a combination of Galactic and extragalactic emission; the relative magnitudes are functions of energy and Galactic latitude. The observational objective of HIMS is to spectroscopically separate these components over the entire sky on  as fine an angular scale as possible. The survey has ~ 5 eV FWHM spectral resolution and angular resolution of 4 arcminutes, although the achievable angular scale is  limited by statistics, depending on the sky brightness and depth of  the exposure. For analysis the data are binned into pixels that are no larger than 200 square degrees for the survey phase, are eight  times smaller at high ecliptic latitude, and are several times  smaller yet in selected interesting directions in the deep exposure  phase.  The survey is performed with a 6 x 6 array of cryogenic  microcalorimeters on a spin-stabilized satellite. The spin axis is  aligned with the Sun, and the detectors look 90 degrees to the spin   axis through a 50 cm diameter 1-meter focal length conical-foil mirror which gives 4-arcminute angular resolution and a 1/2 degree x 1/2 degree field of view. The instrument scans great circles that advance by ~ 4 arcminutes each orbit, scanning the entire sky twice in twelve months. The final six to twelve months of the mission are devoted to deep exposures along selected ecliptic meridians with the spin axis fixed for up to 20 days at a time.

2. HIMS Scientific Objectives

The immediate solar neighborhood, i.e., within 100 pc or so, is  filled with hot, tenuous gas which 1) produces substantial thermal  soft X-ray emission below 1/4 keV; 2) has a substantial thermal  pressure and is a dominant factor in the energy budget of the local  ISM; and 3) is difficult to observe outside the soft X-ray band.   Despite general agreement on these properties, the temperature  distribution, emission measure distribution, ionization  distributions, and metallicity of the gas are unknown. Spectroscopic  observations with resolution sufficient to measure line strengths,  ionization states, and atomic abundance ratios are necessary to   constrain properties of the local hot ISM, to distinguish between models.