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2.1. Space Telescopes

The physics involved in the interaction of photons with matter is well established. At energies above 10 MeV the predominant interaction is pair production in which a gamma-ray is converted into an electron-positron pair in the presence of a nucleus: Egamma -> me+ c2 + me- c2. The electron and positron carry information about the direction, energy, and polarization of the primary gamma-ray, and hence detection methods focus on the observation of these secondary particles. The interaction length of photons in matter is 30 g cm2 and the Earth's atmosphere is 1030 g cm2 thick, so the preferred (only) method of unambiguously detecting high-energy gamma-rays is from space vehicles (from balloons initially but now from satellites).

The basic elements of a space-borne gamma-ray detector are therefore (1) a particle detector in which the gamma-ray interacts and the resulting electron pair tracks are recorded, (2) a calorimeter in which the electrons are absorbed and their total energy recorded, and (3) an anticoincidence shield surrounding the tracking detector which registers (and rejects) the incidence of charged cosmic-ray particles (which are about 10,000 times more numerous than the gamma-rays). In most of the telescopes flown to date, the tracking detector has been a spark chamber, the calorimeter a sodium iodide crystal, and the anticoincidence detector a thin sheet of scintillator. The resulting telescope is a sophisticated device which can unambiguously identify gamma-rays and has an energy resolution of about 15%, an angular resolution of 1°, and a field of view of 20°-40° half-angle. Unfortunately, this sophistication does not come cheaply; the effective collection area is only a small fraction of the total telescope area so that in the largest telescope flown to date, the hugely successful Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory (CGRO) (Thompson et al. 1995; Hartman et al. 1999), the effective collection area was only 1500 cm2 (about the size of two pages of this journal!) whereas the actual physical instrument was about the size of a compact car. All of the gamma-ray telescopes flown to date (the SAS 2 telescope in 1973, the COS B telescope in 1975, EGRET in 1991) have had the same functional form; the next-generation space telescope, the Gamma-Ray Large Area Space Telescope (GLAST), scheduled for launch in 2005 (Gehrels & Michelson 1999), has the same general features but will use solid state detectors with a factor of 10-30 improvement in sensitivity.

The Third EGRET catalog (based on some 4 years of observation by EGRET) (Hartman et al. 1999) contains a listing of more than 250 sources of higher than 100 MeV gamma-rays, more than half of which are unidentified with any known astronomical object. In addition to a detailed map of the diffuse emission along the Galactic plane, there is evidence for more than 100 Galactic sources, a small number of which have been identified with pulsars and possibly with supernova remnants. The bulk of the sources away from the plane are extragalactic and have been identified with active galactic nuclei (AGNs), almost all of which are blazars. Some of the sources are variable, indicating that the high-energy gamma-ray sky is a dynamic place. Many of the identified sources (pulsars, AGNs) have very flat spectra and have luminosities that peak in the high-energy region of the spectrum. The EGRET sensitivity extends to 10 GeV but is limited by the calorimeter (at the highest energies the cascade from the electron-positron pair is not contained and charged particles from the cascade can reach the anticoincidence detector, causing a veto of the event). Since the source flux almost invariably decreases as energy increases, it is only possible to extend observations by building larger telescopes. Even for a flat-spectrum source (power law with differential spectral index -2.0), the power sensitivity of EGRET falls off with energy.

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