- Email: [email protected]
GLAST: the next-generation high energy gamma-ray astronomy mission
Ast roparticle Physics Astroparticle Physics I1 ( 1999) 277-282 www.elsevier.nl/locate/astropart
GLAST: the next-generation high energy gamma-ray astronomy mission Neil Gehrels a, Peter Michelson b a NASA/GSFC Greenbelt, MD, USA b Stanford University, Stanford, CA, USA on
behalf of the GLAST Facility Science Team
Abstract The Gamma Ray Large Area Space Telescope (GLAST) is a high energy (20-300 GeV) gamma-ray astronomy mission planned for launch in 2005. The overarching scientific theme of the mission is studying sites of particle acceleration 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 lo-’ photons cm-* s-’ (>I00 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. @ 1999 Elsevier Science B.V. All rights reserved. Keyword.s: Gamma-ray astronomy; Space astrophysics; Future missions
1. Introduction The predecessor instruments of importance to GLAST are SAS-2 [ 11, COS-B [ 21 and especially EGRET [3]. 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 [4] is shown in Fig. 1. The most important EGRET finding is the class of Active Galactic Nuclei (AGN) that have high en-
ergy gamma-ray emission. More than 70 AGN have now been detected, almost all of which fall in the blazar class [ 41. The prevailing view that blazars have jets aimed toward us indicates that the gamma radiation originates from particles accelerated 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 [5], EGRET [6] has found that Geminga is a gammaray 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 [ 71 and solar flares [ 81. GLAST will build on these findings and open a large area of new discovery space.
0927-6505/99/$ - see front matter @ 1999 Elsevier Science B.V. All rights PII SO927-6505(99)00066-3
reserved.
278
N. Gehrek,
F! Michelson /Astroparticle
Physics 11 (1999) 277-282
Third EGRET Catalog E > 100 MeV +90
-180
Sources
sr Pulsars LMC
Solar FLare Fig. 1. Point sources
2. GLAST Overview The capabilities of GLAST compared to those of EGRET are listed in Table I. The GLAST mission will be flown in low-Earth orbit and will operate in both a zenith pointing mode and a stare mode. The instrument views >16% of the sky at a time, and in zenith pointing mode scans N 15% 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 lop9 photons cm-* s-’ (> 100 MeV) for a 2-year all-sky survey. In the following sections we discuss some of scientific topics that GLAST will address. Extragalactic science is emphasized in keeping with the theme of this conference. See also [ 81.
3. Active galactic nuclei 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
found by EGRET. From 141.
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 [ 91. Beaming factors of N 10 are required to allow the gamma rays to escape without yy + e+e- absorption [ 10,111. The peak in the VFU spectral energy distribution is typically seen to be in the high energy gamma-ray band. Particularly important for studying the time varying spectrum of blazars has been multiwavelength coverage from radio through TeV gamma rays (e.g., [ 121) . 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 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 newgeneration TeV instruments such as VERITAS, will tremendously improve blazar spectral studies, filling in the band from 20 MeV to 10 TeV with high signif-
N. Gehrels. I? Michelson/Astroparticle Table I List of GLAST
instrument
parameters
compared
Physics II (1999) 277-282
119
to those of EGRET
Parameter
EGRET
GLAST
Energy range Energy resolution Effective area Field of view Angular resolution
20 MeV to 30 GeV 10% 1500 cm? 0.5 sr 5.8” at 100 MeV
Sensitivity ( > 100 MeV) Source location accuracy
5-30 arcmin
20 MeV to 300 GeV 10% 8000 cm? >2 sr - 3O at 100 MeV - 0.2’ at 10 GeV - 2 x IO-’ cm-’ 8-l 30 arcsec-5 arcmin
N tOv7 cm-?
s-t
4. Extragalactic
1o-9
1o-’
S (source flux Ey > 100 MeV)
background
light
High energy photons propagating through intergalactic space are attenuated by photon-photon pair production interactions with the intergalactic infraredoptical-W 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 observation of Mrk 421 [ 131. 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 N 4.
(ph cmm2s-l) Fig. 2. log N-log S plot for estimating the GLAST detection rate of AGN. The jagged curved is the actual detections by EGRET. The straight line is an extrapolation of the EGRET slope. The curved line is calculated assuming that the luminosity function of gamma-ray blazars is proportional to that of radio blazars.
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 lowstate emission from blazars. Based on the log N-log S plot shown in Fig. 2, the number of AGN that GLAST will detect at Sa is -4000.
icance
5. Gamma-ray
bursts
A key result from EGRET is that GRBs can have high energy (> 50 MeV) emission during the burst extending for about an hour afterward [ 7,141. 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 bursts 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
N. Gehrels, I? Michelson/Astroparticle
280
Physics 11 (1999) 277-282
FEBRUARY 17,1994 BURST
1200
n
ULYSSES 25-50 keV
TIME (SECONDS)
Fig. 3. Time history of low energy emission from (>I00 MeV) emission from GLAST (axis on right).
Ulysses (jagged line peaking at left; axis on left) compared The GLAST data are shown as points for individual
long-lasting afterglow. GLAST will significantly pand this data set by detecting -100 bursts/yr.
ex-
6. Dark matter One of the leading candidates for the dark matter thought to dominate the Universe are stable, weaklyinteracting massive particles (WIMPS). One candidate in supersymmetric extensions of the standard model in particle physics is the neutralino, which might annihilate into gamma rays in the 30-300 GeV range covered by GLAST (e.g., [ 151) .
7. Pulsars 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 [ 161 and outer gap models [ 173 make specific predictions that will be testable with the larger number of GLAST pulsars [ 181. 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.
8. Supernova
remnants
photons.
to the high energy Adapted from 171.
and origin of cosmic rays
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 [ 191. 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 [ 201. GLAST will be able to search deeply for SNRs among the unidentified EGRET sources and resolve some of those detected.
9. Diffuse gamma radiation 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 [ 211. 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
N. Gehrels, P MichelsonIAstroparticle
Physics I I (1999) 277-282
281
Table 2 GLAST schedule Item
Date
Instrument technology NASA Research Announcement Technology teams selected GLAST NASA Announcement of Opportunity release Instrument selection Spacecraft procurement Congressional “New Start” First Guest Observer Research Announcement Launch
January 1998 March 1998 June 1999 January 2000 2002 2002 2004 2005
subtracted. The combination of improved diffuse component observations and improved measurements of AGN will allow GLAST to determine if the extragalactic background is made up solely of unresolved AGN [ 221 or if there are cosmological contributions.
10. Unidentified
sources
Over half of the sources detected by EGRET remain uncorrelated with known astrophysical objects [ 41. Candidates for these sources include molecular clouds [ 231, supernova remnants [ 24,201, massive stars [ 251, 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.
11. GLAST mission 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 $325 M. NASA has competitively selected three groups to develop instrumentation and perform studies for GLAST. The two main instrument concepts being de-
veloped 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 ,um thick and have strip contacts of 240 ,um 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 (lo100 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 instru-
N. Gehrels, P Michelson/Astroparticle
282
mentation
can be found at http://glast.gsfc.nasa.gov.
Acknowledgements The authors gratefully acknowledge assistance from and useful discussions with D. Bertsch, E. Bloom, K. Derman, S. Diegel, P. Leonard, D. Macomb, S. Ritz and D. Thompson.
References [I] M. Derdeyn et al., NIM 1998 (1972) 557. [ 21 G. Bignami et al., Space Sci. Instrum. 1 (1975) 245. [3] 141 [5] [6] [ 71 [ 81
D.J. Thompson et al., ApJS 86 (1993) 629. R.C. Hartman et al.. ApJS (1999). accepted. J.P. Halpem, S.S. Holt, Nature 357 ( 1992) 222. D. Bertsch et al., Nature 357 (1992) 306. K. Hurley et al., Nature 372 (1994) 652. T. Kamae. T. Ohsugi, D.J. Thompson, K. Watanabe, COSPAR ‘98 Symposium El.1 (1998), in press. [9] R. Blandford, M.J. Rees, in: Pittsburgh Conference on BL Lac Objects, A.M. Wolfe, ed. (Pittsburgh University Press, Pitsburg, 1978) p. 328.
Physics I1 (1999) 277-282 [lo] [I I] [ 121 [ 131 [ 141
[ 151 [ 161 [ 171 [ 181
[ 191 [20] [21] 1221 [23] [24] 1251 [26] 1271
J. Mattox et al., ApJ 410 (1993) 609. C.D. Dermer, N. Gehrels, ApJ 447 (1995) 103. A. Wehrle et al., ApJ 197 (1998) 178. F.W. Stecker, O.C. De Jager, ApJ 476 (1997) 712. B.L. Dingus, J.R. Catelli, E.J. Schneid, in: 25th ICRC Durban, South Africa, Vol. 3, M.S. Potgieter, B.C. Raubenheiner, D.J. van der Walt, eds. (ICRC Press, 1997) p. 29. G. Jungman, M. Kamionkowski, K. Griest, Phys. Rep. 267 (1996) 195. R.W. Romani, ApJ 470 ( 1996) 469. J.K. Daugherty, A.K. Harding, ApJ 458 (1996) 278. D.J. Thompson, A.K. Harding, W. Hermsen, M.P Ulmer, Fourth Compton Symposium, CD. Dermer, M.S. Strickman. J.D. Km-fess (AIP, New York, 1997) p. 39. K. Koyama et al.. Nature 378 (1995) 255. J.A. Esposito et al., ApJ 461 ( 1996) 820. S.D. Hunter et al., ApJ 481 (1997) 481. EW. Stecker, M.H. Salamon, ApJ 464 (1996) 600. B.P. Houston, A.W. Wolfendale, A&A 126 (1983) 22. S.J. Stumer, C.D. Dermer, ApJ 293 (1995) Ll7. T. Montmerle, ApJ 231 (1979) 95. D.J. Thompson et al., ApJ 436 (1994) 229. P Kaaret, J. Cottam, ApJ 462 (1996) L35.
GLAST: the next-generation high energy gamma-ray astronomy mission Neil Gehrels a, Peter Michelson b a NASA/GSFC Greenbelt, MD, USA b Stanford University, Stanford, CA, USA on
behalf of the GLAST Facility Science Team
Abstract The Gamma Ray Large Area Space Telescope (GLAST) is a high energy (20-300 GeV) gamma-ray astronomy mission planned for launch in 2005. The overarching scientific theme of the mission is studying sites of particle acceleration 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 lo-’ photons cm-* s-’ (>I00 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. @ 1999 Elsevier Science B.V. All rights reserved. Keyword.s: Gamma-ray astronomy; Space astrophysics; Future missions
1. Introduction The predecessor instruments of importance to GLAST are SAS-2 [ 11, COS-B [ 21 and especially EGRET [3]. 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 [4] is shown in Fig. 1. The most important EGRET finding is the class of Active Galactic Nuclei (AGN) that have high en-
ergy gamma-ray emission. More than 70 AGN have now been detected, almost all of which fall in the blazar class [ 41. The prevailing view that blazars have jets aimed toward us indicates that the gamma radiation originates from particles accelerated 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 [5], EGRET [6] has found that Geminga is a gammaray 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 [ 71 and solar flares [ 81. GLAST will build on these findings and open a large area of new discovery space.
0927-6505/99/$ - see front matter @ 1999 Elsevier Science B.V. All rights PII SO927-6505(99)00066-3
reserved.
278
N. Gehrek,
F! Michelson /Astroparticle
Physics 11 (1999) 277-282
Third EGRET Catalog E > 100 MeV +90
-180
Sources
sr Pulsars LMC
Solar FLare Fig. 1. Point sources
2. GLAST Overview The capabilities of GLAST compared to those of EGRET are listed in Table I. The GLAST mission will be flown in low-Earth orbit and will operate in both a zenith pointing mode and a stare mode. The instrument views >16% of the sky at a time, and in zenith pointing mode scans N 15% 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 lop9 photons cm-* s-’ (> 100 MeV) for a 2-year all-sky survey. In the following sections we discuss some of scientific topics that GLAST will address. Extragalactic science is emphasized in keeping with the theme of this conference. See also [ 81.
3. Active galactic nuclei 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
found by EGRET. From 141.
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 [ 91. Beaming factors of N 10 are required to allow the gamma rays to escape without yy + e+e- absorption [ 10,111. The peak in the VFU spectral energy distribution is typically seen to be in the high energy gamma-ray band. Particularly important for studying the time varying spectrum of blazars has been multiwavelength coverage from radio through TeV gamma rays (e.g., [ 121) . 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 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 newgeneration TeV instruments such as VERITAS, will tremendously improve blazar spectral studies, filling in the band from 20 MeV to 10 TeV with high signif-
N. Gehrels. I? Michelson/Astroparticle Table I List of GLAST
instrument
parameters
compared
Physics II (1999) 277-282
119
to those of EGRET
Parameter
EGRET
GLAST
Energy range Energy resolution Effective area Field of view Angular resolution
20 MeV to 30 GeV 10% 1500 cm? 0.5 sr 5.8” at 100 MeV
Sensitivity ( > 100 MeV) Source location accuracy
5-30 arcmin
20 MeV to 300 GeV 10% 8000 cm? >2 sr - 3O at 100 MeV - 0.2’ at 10 GeV - 2 x IO-’ cm-’ 8-l 30 arcsec-5 arcmin
N tOv7 cm-?
s-t
4. Extragalactic
1o-9
1o-’
S (source flux Ey > 100 MeV)
background
light
High energy photons propagating through intergalactic space are attenuated by photon-photon pair production interactions with the intergalactic infraredoptical-W 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 observation of Mrk 421 [ 131. 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 N 4.
(ph cmm2s-l) Fig. 2. log N-log S plot for estimating the GLAST detection rate of AGN. The jagged curved is the actual detections by EGRET. The straight line is an extrapolation of the EGRET slope. The curved line is calculated assuming that the luminosity function of gamma-ray blazars is proportional to that of radio blazars.
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 lowstate emission from blazars. Based on the log N-log S plot shown in Fig. 2, the number of AGN that GLAST will detect at Sa is -4000.
icance
5. Gamma-ray
bursts
A key result from EGRET is that GRBs can have high energy (> 50 MeV) emission during the burst extending for about an hour afterward [ 7,141. 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 bursts 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
N. Gehrels, I? Michelson/Astroparticle
280
Physics 11 (1999) 277-282
FEBRUARY 17,1994 BURST
1200
n
ULYSSES 25-50 keV
TIME (SECONDS)
Fig. 3. Time history of low energy emission from (>I00 MeV) emission from GLAST (axis on right).
Ulysses (jagged line peaking at left; axis on left) compared The GLAST data are shown as points for individual
long-lasting afterglow. GLAST will significantly pand this data set by detecting -100 bursts/yr.
ex-
6. Dark matter One of the leading candidates for the dark matter thought to dominate the Universe are stable, weaklyinteracting massive particles (WIMPS). One candidate in supersymmetric extensions of the standard model in particle physics is the neutralino, which might annihilate into gamma rays in the 30-300 GeV range covered by GLAST (e.g., [ 151) .
7. Pulsars 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 [ 161 and outer gap models [ 173 make specific predictions that will be testable with the larger number of GLAST pulsars [ 181. 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.
8. Supernova
remnants
photons.
to the high energy Adapted from 171.
and origin of cosmic rays
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 [ 191. 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 [ 201. GLAST will be able to search deeply for SNRs among the unidentified EGRET sources and resolve some of those detected.
9. Diffuse gamma radiation 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 [ 211. 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
N. Gehrels, P MichelsonIAstroparticle
Physics I I (1999) 277-282
281
Table 2 GLAST schedule Item
Date
Instrument technology NASA Research Announcement Technology teams selected GLAST NASA Announcement of Opportunity release Instrument selection Spacecraft procurement Congressional “New Start” First Guest Observer Research Announcement Launch
January 1998 March 1998 June 1999 January 2000 2002 2002 2004 2005
subtracted. The combination of improved diffuse component observations and improved measurements of AGN will allow GLAST to determine if the extragalactic background is made up solely of unresolved AGN [ 221 or if there are cosmological contributions.
10. Unidentified
sources
Over half of the sources detected by EGRET remain uncorrelated with known astrophysical objects [ 41. Candidates for these sources include molecular clouds [ 231, supernova remnants [ 24,201, massive stars [ 251, 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.
11. GLAST mission 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 $325 M. NASA has competitively selected three groups to develop instrumentation and perform studies for GLAST. The two main instrument concepts being de-
veloped 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 ,um thick and have strip contacts of 240 ,um 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 (lo100 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 instru-
N. Gehrels, P Michelson/Astroparticle
282
mentation
can be found at http://glast.gsfc.nasa.gov.
Acknowledgements The authors gratefully acknowledge assistance from and useful discussions with D. Bertsch, E. Bloom, K. Derman, S. Diegel, P. Leonard, D. Macomb, S. Ritz and D. Thompson.
References [I] M. Derdeyn et al., NIM 1998 (1972) 557. [ 21 G. Bignami et al., Space Sci. Instrum. 1 (1975) 245. [3] 141 [5] [6] [ 71 [ 81
D.J. Thompson et al., ApJS 86 (1993) 629. R.C. Hartman et al.. ApJS (1999). accepted. J.P. Halpem, S.S. Holt, Nature 357 ( 1992) 222. D. Bertsch et al., Nature 357 (1992) 306. K. Hurley et al., Nature 372 (1994) 652. T. Kamae. T. Ohsugi, D.J. Thompson, K. Watanabe, COSPAR ‘98 Symposium El.1 (1998), in press. [9] R. Blandford, M.J. Rees, in: Pittsburgh Conference on BL Lac Objects, A.M. Wolfe, ed. (Pittsburgh University Press, Pitsburg, 1978) p. 328.
Physics I1 (1999) 277-282 [lo] [I I] [ 121 [ 131 [ 141
[ 151 [ 161 [ 171 [ 181
[ 191 [20] [21] 1221 [23] [24] 1251 [26] 1271
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