Astronomy and Astrophysics of Extreme Universe 1 The Gamma Ray Large Area Space Telescope (GLAST) Tsunefumi Mizuno, 1 on behalf of the GLAST LAT team (1) Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8526, Japan Abstract The Gamma Ray Large Area Space Telescope (GLAST) consists of two instru- ments, Large Area Telescope (LAT) and GLAST Burst Monitor (GBM), and is being developed by an international collaboration. The main instrument LAT is a pair-conversion gamma-ray telescope which observes gamma-ray sky with wide field-of-view ( ∼ 2 . 4 sr) and high sensitivity in ∼ 20 MeV–300 GeV. The secondary instrument GBM covers energy range below 30 MeV and monitor gamma-ray bursts with even wider field-of-view ( ∼ 9 sr). The GLAST will survey gamma-ray sky and provide vital information on almost all kinds of gamma-ray astronomical sources. The GLAST is scheduled to be launched in December 2007. 1. GLAST Mission Overview The Gamma Ray Large Area Space Telescope (GLAST) is a next generation high-energy gamma-ray observatory developed under an international collabora- tion among United States, Japan and European countries. The main instrument, the Large Area Telescope (LAT), follows the footsteps of the CGRO-EGRET ex- periment [1] [2] and will provide unprecedented sensitivity to gamma-ray sources in the energy range of ∼ 20 MeV–300 GeV. The GLAST Burst Monitor (GBM), a successor to BATSE experiment on-board CGRO, was selected as a complemen- tary instrument for the GLAST LAT and will monitor gamma-ray bursts (GRBs) in the energies between ∼ 10 keV to ∼ 30 MeV. The GLAST satellite will be launched in late 2007 or early 2008 by a Delta-II rocket from Kennedy Space Center in Florida. 2. Instrumentation 2.1. Large Area Telescope Overview The GLAST LAT [3] has been developed and built by an international col- laboration among United States, Japan, France, Italy and Sweden. The LAT is designed to measure the direction of gamma-rays incident over a wide energy range of ∼ 20 MeV–300 GeV and a wide field-of-view (FoV) of ∼ 2 . 4 sr, while rejecting background from cosmic-rays. The LAT is a pair-conversion telescope with a Si-strip tracker (TKR) and a CsI hodoscopic array of calorimeter (CAL), each consisting of a 4 x 4 array of 16 modules called ”towers”. A segmented anti- coincidence detector shield (ACD) made of plastic scintillators covers the tracker array, and data acquisition system (DAQ) utilizes prompt signals from the TKR, CAL and ACD subsystems to issue a trigger. Upon triggering, the DAQ initiates the read-out of three subsystems and utilizes on-board event processing to reduce number of cosmic-ray events to fit to the band width of available down-link. A prototype single tower had been tested and validated through a series of beam tests and a balloon flight [4] [5] [6] [7]. The LAT instruments is schematically shown in Figure 2.1.. � 2007 by Universal Academy Press, Inc. c
2 Fig. 1. A schematic drawing of the GLAST LAT composed of the TKR, CAL and ACD subsystems. Note that the instrumental design has been updated (e.g., converters of flight model TKR are tungsten instead of Lead). Tracker Each TKR module has 18 tracking planes, each consisting of two layers (to measure x and y direction) of single-sided 400 µ m-thick and 228 µ m-pitch silicon strip detectors [8] and tungsten converters. The support structure for the detec- tors and converter foils is composed of a stack of 19 composite panels (trays) of about 3 cm thickness made of carbon-composite assembly. All trays are of similar construction, although the top and bottom ones are special, with detectors on only a single face. An x,y measurement plane consists of a layer of detectors on the bottom of one tray together with an orthogonal detector layer on the top of the tray just below, with a 2 mm separation. The tungsten converter foils lie immediately above the upper detector layer. The strips on the top and bottom of a given tray are parallel, while alternate trays are rotated 90 ◦ with respect to each other. There are 16 x,y planes at the top of the tracker with converter foils, among which upper 12 converters are thinner (3% radiation length) to maintain good an- gular resolution, and next four converters are thicker (18% radiation length) to achieve high effective area within a limited size of the TKR module. The lowest two x,y planes have no converter. Thanks to its fine position resolution (228 µ m), silicon-strip TKR has much improved position accuracy and angular resolution than those of EGRET. It also makes it possible for the LAT to have much larger effective area, wider FoV and smaller dead time. Details of the GLAST TKR can be found in [9]. Calorimeter Each CAL module consists of 96 CsI(Tl) scintillators, with each crystal of size 2.7 cm (width) × 2.0 cm (height) × 32.6 cm (length). The crystals are optically isolated from each other and are arranged horizontally in 8 layers of 12 crystals each, giving the total depth of 8.6 radiation lengths (for a total instrument depth Each calorimeter module layer is aligned 90 ◦ with of 10.1 radiation lengths). respect to its neighbors, forming an x and y (hodoscopic) array. The segmentation
3 allows spatial imaging of the electromagnetic shower and accurate reconstruction of its direction, because each CsI crystal provides three spatial coordinates for the energy deposited within: two discrete coordinates from the physical location of the crystal and the third coordinate determined by measuring the light yield asymmetry at the ends of the crystal along its long dimension. The calorimeter’s shower imaging capability contributes significantly to the background rejection, and enables the shower leakage correction which results in the high-energy reach of the LAT up to ∼ 300 GeV. Thanks to its large collection area, the CAL also works as a highly efficient cosmic electron detector (e.g., [10] [11]). For more detail of the CAL, refer to [12]. Anticoincidence detector The role of the ACD is to provide charged particle background rejection; there- fore its main requirement is to have very high efficiency (of 0.9997 when averaged over the whole area) for detection of singly-charged particles entering the tracking detector from the top or side of the LAT. The ACD veto signal on-board reduces the trigger rate to a level compatible to the data transmission rate to the ground. The ACD data will also be used during off-line analysis to achieve an ultimate background rejection efficiency. In order for the LAT to measure gamma-rays with energies up to 300 GeV, one has to take care of a problem called backsplach effect: a small fraction of secondary particles (mostly 100-1000 keV photons) from the electromagnetic shower created by the incident high energy photon in the CAL travel backward through the tracker and create veto signals in the ACD. This effect was present in the EGRET experiment and limited the sensitivity at ∼ 10 GeV. To minimize these false vetos and maintain sufficient sensitivity for gamma-rays above 100 GeV, the LAT ACD is segmented into 89 tiles and only ACD segments in the projected path of the incident photon is considered for vetoing, thereby dramatically reducing the area of ACD that can contribute to backsplash. See [13] for more details of the ACD instrumentation and testing. Expected performance A combination of three sub-systems provides us with an excellent performance of the LAT and unprecedented sensitivity for gamma-ray objects. The key pa- rameters of the LAT performance are summarized in Figure 2. and Table 1.. A better angular resolution and larger effective area (compared to those of EGRET) result in much improved sensitivity for point sources and diffuse emission. A much larger FoV allows us to monitor gamma-ray sky and search for transient sources and flare of known objects continuously. A better timing resolution is a great advantage in searching for gamma-ray pulsars. 2.2. GLAST Burst Monitor Aside from the main instrument LAT, a secondary instrument GBM (GLAST Burst Monitor) is equipped to the GLAST observatory. The GBM is designed to be complementary to the LAT: it has even wider FoV ( ∼ 9 sr) and sensitivity for low-energy photons down to ∼ 10 keV in order to increase the detection rate of the gamma-ray bursts (GRBs) and transients, and extend the energy coverage for the spectroscopic study. The GBM was developed by a collaboration between United States and Germany. Like CGRO-BATSE, the GBM design is based on the use of two types cylindrical scintillation detectors. One is an array of 12 sodium iodide (NaI) detectors having sensitivity in the energy range of ∼ 10 keV–1 MeV covering a typical spectral
4 Expected performance of the GLAST LAT. Angular resolution, ef- Fig. 2. fective area and relative effective area as a function of incident angle (i.e., FoV) are given by left, middle and right panel, respectively. See http://www-glast.slac.stanford.edu/software/IS/glast_lat_performance.htm for updated information. Table 1. GLAST-LAT expected performance based on computer simulation and com- parison with EGRET GLAST LAT EGRET Energy Range 20 MeV–300 GeV 20 MeV–30 GeV Energy Resolution ≤ 10% 10% ≥ 8000 cm − 2 1500 cm − 2 Effective area Field of view ≥ 2 . 4 sr 0.5 sr Angular Resolution 3 . 5 ◦ @100 MeV/0 . 15 ◦ @10 GeV 5 . 8 ◦ @100 MeV 4 × 10 − 9 ph s − 1 cm − 2 10 − 7 ph s − 1 cm − 2 Sensitivity ( ≥ 100 MeV) Deadtime ≤ 100 µ s 100 ms
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