GLAST, a detector for cosmic gamma rays in the range from 20 MeV to 300 GeV,
will be launched in space in 2005. Breakthroughs are expected in particular in
the study of particle acceleration mechanisms in space and of gamma ray bursts,
and maybe on the search for cold dark matter; but of course the most exciting
discoveries could come from the unexpected.Comment: 6 pages, 3 figures. Invited talk at the hird International Workshop
  "New Worlds in Astroparticle Physics", September 2000, University of the
  Algarve. Faro, Portugal. To be published in the Proceeding

de Angelis, Alessandro

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DE ANGELIS, Alessandro

Archivio istituzionale della ricerca - Università degli Studi di Udine

GLAST, THE GAMMA RAY LARGE AREA SPACE TELESCOPE.

ALESSANDRO DE ANGELIS

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GLAST, THE GAMMA-RAY LARGE AREA SPACE TELESCOPE

GLAST, a detector for cosmic gamma rays in the range from 20 MeV to 300 GeV, will be launched in space in 2005. Breakthroughs are expected in particular in the study of particle acceleration mechanisms in space and of gamma ray bursts, and maybe on the search for cold dark matter; but of course the most exciting discoveries could come from the unexpected

De Angelis, A

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GLAST, THE GAMMA-RAY LARGE AREA SPACETELESCOPEALESSANDRO DE ANGELISUniversita` di Udine and INFN Trieste, via delle Scienze 208, I-33100 Udine (Italy)∗E-mail: deangelis@ud.infn.itGLAST, a detector for cosmic γ rays in the range from 20 MeV to 300 GeV, will belaunched in space in 2005. Breakthroughs are expected in particular in the studyof particle acceleration mechanisms in space and of gamma ray bursts, and maybeon the search for cold dark matter; but of course the most exciting discoveriescould come from the unexpected.1 IntroductionThe exchange between laboratory physics and astrophysics has demonstratedthrough centuries of science to be at the origin of fundamental discoveries.GLAST1, a detector for cosmic γ rays in the range from 20 MeV to 300 GeV, isbuilt around this idea: both the design of the instrument and the guidelines forscience originate from a partnership of researchers from High Energy Physics(HEP) and High Energy Astrophysics (HEA). The collaboration involves theUS (NASA in particular), France, Italy, Japan and Sweden.The study of γ rays is fundamental for our understanding of the universe2:γ rays probe the most energetic phenomena occurring in nature, and severalsignatures of new physics are associated with the emission of γ rays. Pho-tons can travel essentially undeflected and unabsorbed in space, and thus theypoint with excellent approximation to the source of their emission. Moreover,we have the technology for building high-performance γ detectors. The de-sign of GLAST, which will be better described in the next section, is rathertypical for a modern HEP detector: essentially a silicon tracker followed by acalorimeter.GLAST comes after the scientic success of the Energetic Gamma RayExperiment Telescope (EGRET) instrument on the Compton Gamma RayObservatory3. Launched in 1991, EGRET made the rst complete survey ofthe sky above 30 MeV. EGRET increased the number of identied γ sourcesproducing a catalog which is a reference. Although EGRET answered manyquestions, it opened as many mysteries, which GLAST hopes to solve. GLASTmaintains the same design philosophy as EGRET, but will improve substan-∗also at Instituto Superior Te´cnico, Lisboa.University of Udine Report UDPHIR 00/02/AA, September 2000 1Figure 1. GLAST in detail.tially the EGRET performance by using up-to-date technology from HEP.GLAST will be sent in space in 2005; it should orbit at a height of 550 Kmand make a full sky coverage in 2-3 orbits, sending data at a rate of 0.3 Mb/s(which could triplicate if the rapidly growing telecommunication technologywill allow it). The detector is expected to take data for at least ve years,with the possibility of extending the mission to ten years. The launch by aDelta 2 vector limits the weight of GLAST to around 4 tons and its size to2x2x1 m3. The total power consumption is limited to 650 W.2 The detectorGLAST is an array of 4x4 identical towers (Figure 1), embedded in a plasticscintillator shield which serves as an anticoincidence trigger. Each tower,about 40x40 cm2 in surface, comprises a tracker, a calorimeter and a dataacquisition module.The tracking detector consists of 18 xy planes of silicon strip detectors,with pitch of about 200 m. On 16 of them a converter will increase theradiation length so to increase the probability of γ conversion; the electron-positron pair coming from such conversion can be tracked to reconstruct withUniversity of Udine Report UDPHIR 00/02/AA, September 2000 2calorimeter γ raye+ e-particle- tracking detectorsconversion foilscharged-particle anticoincidence shieldpair-Figure 2. Principle of photon detection in GLAST.accuracy the direction of the incoming photon. The choice of silicon guaran-tees high signal/noise ratio, radiation hardness and low power consumption.The system is mechanically stiened by carbon walls; electronics is placed onthe sides to minimize the gaps.The calorimeter in each tower consists of 12 CsI bars in a hodoscopicarrangement, read out by double photodiodes; the total thickness is 10 radia-tion lengths. The hodoscopic geometry, with alternate x and y planes, allowstting the shower prole, so to be able to compute the leakage and overcomethe consequent deterioration of the energy resolution.The setup consisting of a tracker and a calorimeter is a standard forthe modern HEP experiments: the tracker detects the conversion and allowstting the photon direction, while the calorimeter measures the total energy(Figure 2). The anticoincidence detector helps in suppressing the backgroundfrom charged cosmic rays, which is ve orders of magnitude larger than the γsignal. The height/width ratio of ≈ 0:4 guarantees a large eld of view; thearrangement in 16 complete towers guarantees modularity.As a result GLAST, built around the same design philosophy as EGRET,can increase substantially EGRET’s sensitivity (Table 1).GLAST is complementary to ground-based experiments, and is a key ofthe γ astrophysics program for the new century (Figure 3).University of Udine Report UDPHIR 00/02/AA, September 2000 3EGRET GLASTEnergy range 0.02{30 GeV 0.02{300 GeVEnergy resolution E=E 10% (E > 100 MeV) 10% (E > 100 MeV)Single γ angular resolution 5.8◦ (100 MeV) ' 3◦ (100 MeV)' 0:15◦ (10 GeV)Peak e. area 0.15 m2 1 m2Field of view 0.5 sr 2.4 srTime resolution 100 s 10 sTable 1. Selected performance parameters for GLAST and EGRET.Figure 3. Sensitivity of present and future high energy γ detectors.3 Key science objectivesGLAST has a wide range of physics objectives, ranging from gamma astro-physics to fundamental physics. For a more complete review, see1 and refer-ences therein. I try here just to give a short summary.• Resolving the γ ray sky: AGN, diuse emission and unidentied sourcesEGRET has discovered ' 80 Active Galactic Nuclei (AGN) at γ rayenergies, and GLAST will discover several thousands; the γ part of theUniversity of Udine Report UDPHIR 00/02/AA, September 2000 4spectrum could be dominant, and the variability in time of the sourceswill provide important information.Most of the gamma-ray sources detected by EGRET remain unidentied;the accuracy of EGRET could allow identifying such sources.Besides the AGNs, EGRET discovered a diuse background radiation. Isit really diuse, and thus produced at a very early epoch, or a flux fromunresolved sources? GLAST will improve the angular resolution and thesensitivity to weak sources, possibly giving an answer.• Acceleration mechanisms of Cosmic Rays (CR)The signal of 0 decaying into γγ could indicate the dominant role ofthe acceleration of nuclei. The angular resolution of GLAST will allowto possibly associate CR sources to supernova remnants.• Gamma-Ray Bursts (GRB)EGRET detected high energy afterglows (which can last for more thanone hour) in 2-3 out of 6 GRBs observed; GLAST can provide measure-ments over a new energy range for 50-100 GRB/year. The notication ofGRBs can be given by an on-board trigger (Gamma-Ray Burst Monitor)in a few seconds, and from earth in a few minutes.In addition, recent quantum gravity models predict that the speed of pho-tons depends on their energy4; this eect could cause delays of O(100ms)in the arrival time of photons from GRBs, and thus be detectable.• Solar flaresGLAST could image these unexplained solar explosions, measuring thesurface interested.• Probing dark matter: WIMPsIf dark matter is made by Majorana particles weakly interacting andwith thermic energies, the annihilation of such particles should result innarrow γ lines. This is in particular true in SUSY, where the LightestSupersymmetric Particle (LSP) can annihilate with itself producing γγor Zγ pairs. GLAST can be sensitive to a non-negligible part of the spaceof the Minimal Supersymmetric Standard Model (MSSM) parameters fora LSP in the range from 30 to 100 GeV/c.• Last but not least, the totally unexpectedcould come from the newly opened exploration regimes.University of Udine Report UDPHIR 00/02/AA, September 2000 54 Status of GLASTThe GLAST design has been extensively studied by means of simulations,and test beams are well on the way since three years to verify the expectedperformance.In particular, the possibility of the calorimeter to reach a relative energyresolution E=E ' 10% by tting the shower prole has been successfullyveried by dedicated tests in 1997.A full GLAST tower has been tested at SLAC in 1999/2000 with incidentelectron, photon and hadron beams. It has been veried in particular thata rejection factor of 105 against hadrons is realistic, still keeping 80% of thephotons.The software for the simulation, the analysis and the distribution of datais being developed along the line of modern data handling technologies; simu-lation and reconstruction are well advanced and being used on the test data.A particular mention to the simulation: the detector project has been donewith detailed simulations based on GISMO5, and now a simulation is beingdeveloped between Europe and Japan in close collaboration with the CERNGeant46 development team, which prots also of the interaction with ESA.5 Summary and conclusionsGLAST will provide an important step forward in γ astronomy, increasingthe sensitivity of EGRET by one-two orders of magnitude.The progress is in schedule for the launch in 2005; beam tests and softwaredevelopement are well on the way.We are condent that this partnership between HEP and HEA will bethe source of new discoveries over a wide range of subjects.References1. See http://glast.gsfc.nasa.gov/ and references therein.2. For a review, see for example C.M. Homan, C. Sinnis, P. Fleury and M.Punch, Rev. Mod. Phys 71 (1999) 897.3. See http://cossc.gsfc.nasa.gov/cossc/egret/ and references therein.4. G. Amelino-Camelia et al., Nature 393 (1998) 763.5. T. Burnett and B. Atwood, http://glast.phys.washington.edu/ bur-nett/gismo/6. http://wwwinfo.cern.ch/asd/geant4/geant4.htmlUniversity of Udine Report UDPHIR 00/02/AA, September 2000 6

GLAST, the Gamma-ray Large Area Space Telescope

GLAST, the Gamma-ray Large Area Space Telescope

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