Neil Gehrels (NASA/GSFC) and Peter Michelson (Stanford University) on behalf of the GLAST Facility Science Team
This paper appears in the June 1999 Issue of Astroparticle Physics (Volume 11, Nos. 1-2) on pages 277 to 282.
The Gamma-ray Lararge Area Space Telescope (GLAST) is a high-energy (20 MeV - 300 GeV) gamma-ray astronomy mission planned for launch in 2005. The overarching scientific theme of the mission is studying sites of particle accleration in the Universe. Topics of interest include active galactic nuclei and their jets, extragalactic and Galactic diffuse emissions, dark matter, supernova remnants, pulsars, and the unidentified high-energy gamma-ray sources. The sensitivity is 2 x 10-9 photons cm-2 s-1 (> 100 MeV) for a 2-year all-sky survey, which is a factor of > 30 better than CGRO/EGRET. The GLAST program is currently in a high level of mission development activity.
The predecessor instruments of importance to GLAST are SAS-2 , COS-B , and especially EGRET . The first sensitive survey of the full gamma-ray sky was performed by EGRET between April 1991 and November 1992. This was followed by a phase where telescope pointings were chosen from peer reviewed proposals. The instrument was operated at full duty cycle until 1996. After 1996 it is being operated at reduced duty cycle and reduced field of view to conserve spark chamber gas. A map of the point sources in the recently released 3rd EGRET catalog  is shown in Fig. 1.
The most important EGRET finding is the class of Active Galactic Nuclei (AGN) that have high-energy gamma-ray emission. More than 70 AGN have now been detected, almost all of which fall in the blazar class . The prevailing view that blazars have jets aimed toward us indicates that the gamma radiation originates from particles acclerated in the jets. For pulsars, EGRET has tripled the number (from 2 to 6) of spin-down pulsars detected in high-energy gamma rays. In conjunction with ROSAT , EGRET  has found that Geminga is a gamma-ray pulsar with little or no radio emission. Concerning transients, EGRET has made the important discovery of long lasting high-energy emission from both gamma-ray bursts  and solar flares . GLAST will build on these findings and open a large area of new discovery space.
Fig. 1. Point sources found by EGRET. From .
The capabailities of GLAST compared to those of EGRET are listed in Table 1. The GLAST mission will be flown in low-Earth orbit and will operate in both zenith pointing mode and stare mode. The instrument views > 16% of the sky at a time, and in zenith pointing mode scans ~ 75% of the sky every orbit. Combining this field of view with the large effective area and good angular resolution gives a sensitivity of 2 x 10-9 photons cm-2 s-1 (> 100 MeV) for a 2-year all-sky survey. In the following sections we discuss some of the scientific topics that GLAST will address. Extragalactic science is emphasized in keeping with the theme of this conference. See also .
Table 1. List of GLAST instrument parameters compared to those of EGRET.
|Energy Range||20 MeV to 30 GeV||20 MeV to 300 GeV|
|Effective Area||1500 cm2||8000 cm2|
|Field of View||0.5 sr||> 2 sr|
|Angular Resolution||5.8o at 100 MeV||~ 3o at 100 MeV|
|~ 0.2o at 10 GeV|
|Sensitivity (> 100 MeV)||~ 10-7 cm-2 s-1||~ 2 x 10-9 cm-2 s-1|
|Source Location Accuracy||5 - 30 arcmin||30 arcsec - 5 arcmin|
Most of the high latitude sources that EGRET sees are blazars, namely objects identified at other wavelengths to be either flat spectrum radio quasars (FSRQs) or BL Lacs. They cover the distance range from z = 0.031 to 2.3, generally similar in distance distribution to the radio blazars. EGRET has shown that blazars can produce copious quantities of gamma rays, most likely from Compton upscattering in their relativistic particle jets . Beaming factors of ~ 10 are required to allow the gamma rays to escape without g g -> e+ e- absorption [10,11]. The peak in the nFn spectral energy distribution is typically seen to be in the high-energygamma-ray band.
Particularly important for studying the time varying spectrum of blazars has been multiwavelength coverage from radio through TeV gamma rays (e.g., ). From these we have an indication that the BL Lac subclass of blazars has a different characteristic multiwavelength spectrum than that of FSRQs. With the current poor statistics and small sample size from EGRET and the ground-based TeV observatories, it is difficult to model the spectra in detail and to understand differences in subclasses. GLAST, combined with new-generation TeV instruments such as VERITAS, will tremendously improve blazar spectral studies, filling in the band from 20 Mev to 10 TeV with high significance data for hundreds of AGN.
EGRET has found that the gamma-ray emission from blazars is highly variable. In almost all blazars seen, the emission is detected in only one or two observations and not seen in other good exposures. GLAST will have the capability to monitor most AGN in the sky at all times with its wide field of view. Also, its high sensitivity will allow the first observations of low-state emission from blazars. Based on the logN-logS plot shown in Fig. 2, the number of AGN that GLAST will detect at 5s is ~ 4000.
Fig. 2. logN-logS plot for estimating the GLAST detection rate of AGN. The jagged curve is the actual detections by EGRET. The straight line is an extrapolation of the EGRET slope. The curved line is calculated assuming that the luminsoity function of gamma-ray blazars is proportional to that of radio blazars.
High-energy photons propogating through intergalactic space are attenuated by photon-photon pair production interactions with the intergalactic infrared-optical-UV radiation field (extragalactic background light or EBL). The origin of the lower-energy intergalactic photons is starlight, predominantly from starburst activity during the epoch of galaxy formation. The effect of this attenuation may be discernible in the ground-based TeV observations of Mrk 421 . The combination of GLAST and future ground-based instruments will be a powerful tool for systematically studying the attenuation of AGN spectra and thereby measuring the EBL to redshifts of z ~ 4.
A key result from EGRET is that GRBs can have high-energy (> 50 MeV) emission during the burst extending for about an hour afterward [7,14]. The best example is shown in Fig. 3 for GRB940217. The sparsity of high-energy data points during the peak of the burst is due to the fact that EGRET was saturated. The remarkable result is that high-energy radiation continued for about 6000 seconds (interrupted by an Earth occultation). A very high-energy photon at 18 GeV is seen at 4700 seconds. The presence of high-energy gamma rays during and after the burst is of key importance in understanding the acceleration mechanisms during the burst and as the blastwave interacts with its surroundings. Although the EGRET findings are intriguing, they provide poor statistics with only 4 bursts detected during the main event and 1 burst detected in long-lasting afterglow. GLAST will significantly expand this data set by detecting ~ 100 bursts/yr.
Fig. 3. Time history of low-energy emission from Ulysses (jagged line peaking at left; axis on left) compared to the high-energy (> 100 MeV) emission from EGRET (axis on right). The EGRET data are shown as points for individual photons. Adapted from .
One of the leading candidates for the dark matter thought to dominate the Universe are stable, weakly-interacting massive particles (WIMPs). One candidate in supersymmetric extensions of the standard model is the neutralino, which might annihilate into gamma rays in the 30 - 300 GeV range covered by GLAST (e.g., ).
A number of young and middle-aged pulsars have their energy output dominated by gamma-ray emission. Because the gamma rays are directly related to the particles accelerated in the pulsar magnetospheres, they give specific information about physics in high magnetic and electric fields. Models based on the EGRET pulsars such as polar-cap  and outer-gap models  make specific predictions that will be testable with the larger number of GLAST pulsars . GLAST will detect approximately 50 radio pulsars, and will greatly expand the search for more radio-quiet, Geminga-type pulsars. These studies will lead to understanding of acceleration mechanisms in the pulsar magnetosphere and multiwavelength beaming.
Although a near-consensus can be found among scientists that cosmic rays originate in supernova remnants (SNRs), the proof of that hypothesis remains elusive. X-ray observations by ASCA give evidence for electron acceleration in SNRs such as SN1006 . GLAST could observe high-energy gamma rays from interactions producing pions and thereby provide the crucial observations of cosmic-ray nucleon acceleration. Some EGRET sources appear to be associated with SNRs, but the moderate spatial resolution and sensitivity make the identifications uncertain . GLAST will be able to search deeply for SNRs among the unidentified EGRET sources and resolve some of those detected.
Within the Galaxy, GLAST will explore diffuse radiation on scales from molecular clouds to Galactic arms, measuring the product of the cosmic-ray and gas densities . The improved angular resolution and sensitivity of GLAST compared to EGRET will allow much finer detail to be mapped in the diffuse emission along the Galactic plane. Also, contributions from currently undetected and unresolved point sources can be subtracted. At high latitude, GLAST will make high sensitivity observations of the extragalactic diffuse background. With better understanding of the Galactic emission, this component can be accurately subtracted. The combination of improved diffuse component observation and improved measurements of AGN will allow GLAST to determine if the extragalactic background is made up solely of unresolved AGN  or if there are cosmological contributions.
Over half of the sources detected by EGRET remain uncorrelated with known astrophysical objects . Candidates for these sources include molecular clouds , supernova remnants [24,20], massive stars , and radio-quiet pulsars [26,16,27]. In general, the EGRET error boxes are too large for spatial correlation, and the photon density is too small for detailed timing studies. Both of these limitations will be greatly alleviated with GLAST. In particular, the combination of GLAST with the next generation X-ray telescopes should resolve a large part of this long-standing mystery.
GLAST is currently in NASA's plans for "New Start" congressional approval in 2002 and launch in 2005. The GLAST mission is being managed at NASA's Goddard Space Flight Center. Scientific development of the mission is led by a Facility Science Team. The total NASA mission cost (including spacecraft, launch and 5 years of operations) is specified to be $325M.
NASA has competitively selected three groups to develop instrumentation and perform studies for GLAST. The two main instrument concepts being developed are Silicon GLAST and Fiber GLAST. These are briefly described below. Both are pair-conversion telescopes with three principal components: (1) a "tracker" in which pair conversion occurs in foils producing an electron and positron whose trajectories are detected by particle tracking detectors; (2) a "calorimeter" in which the electron and positron and shower particles/photons from them are stopped to provide a measurement of energy; and (3) an "anticoincidence shield" that discriminates particle backgrounds from the gamma-ray signal.
Silicon GLAST (PI: P. Michelson) uses thin silicon solid state detectors for tracking. The detectors are ~ 400 mm thick and have strip contacts of 240 mm pitch. The tracker (detectors plus converter foils) is about 0.5 radiation lengths thick. This is followed by a segmented CsI calorimeter. The thickness of the calorimeter is 10 radiation lengths. The instrument is surrounded by a segmented anticoincidence detector made of plastic scintillator.
Fiber GLAST (PI: G. Pendleton) uses scintillating fiber technology for both the tracker and calorimeter. The square cross section fibers (made of polysyrene) are ~ 0.75 mm on a side and are read out at their ends by multi-anode photomultiplier tubes. The tracker and calorimeter are 2.2 and 5 radiation lengths thick, respectively. The instrument is surrounded by an anticoincidence detector made of plastic scintillator.
In addition to the two instrument concepts, a third team (PI: A. Zych) is investigating the benefits and feasibility of enhancing the low-energy (10 - 100 MeV) response of GLAST. The main instrument for GLAST, plus a possible small secondary instrument, will be selected competitively in January 2000. Further information on the GLAST mission and instrument can be found at http://glast.gsfc.nasa.gov/.
Table 2. GLAST schedule.
|Instrument Technology NASA Research Announcement||January 1998|
|Technology Teams Selected||March 1998|
|GLAST NASA Announcement of Opportunity||June 1999|
|Instrument Selection||January 2000|
|Congressional "New Start"||2002|
|First Guest Observer Research Announcement||2004|
The authors gratefully acknowledge assitance from and useful discussions with D. Bertsch, E. Bloom, K. Derman, S. Digel, P. Leonard, D. Macomb, S. Ritz and D. Thompson.
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