Gamma ray astronomy

Gamma-ray astronomy is the astronomical observation of gamma rays, the most energetic form of electromagnetic radiation, with photon energies above 100 keV. Radiation below 100 keV is classified as X-rays and is the subject of X-ray astronomy.

Gamma rays in the MeV range are generated in solar flares (and even in the Earth's atmosphere), but gamma rays in the GeV range do not originate in the Solar System and are important in the study of extrasolar, and especially extra-galactic astronomy. The mechanisms emitting gamma rays are diverse, mostly identical with those emitting X-rays but at higher energies, including electron-positron annihilation, the Inverse Compton Effect, and in some cases also the decay of radioactive material (gamma decay) in space reflecting extreme events such as supernovae and hypernovae, and the behaviour of matter under extreme conditions, as in pulsars and blazars. The highest photon energies measured to date are in the TeV range, the record being held by the Crab Pulsar in 2004, yielding photons with as much as 80 TeV.

Observation of gamma rays first became possible in the 1960s. Their observation is much more problematic than that of X-rays or of visible light, because gamma-rays are comparatively rare, even a "bright" source needing an observation time of several minutes before it is even detected, and because gamma rays are difficult to focus, resulting in a very low resolution. The most recent generation of gamma-ray telescopes (2000s) have a resolution of the order of 6 arc minutes in the GeV range (seeing the Crab Nebula as a single "pixel"), compared to 0.5 arc seconds seen in the low energy X-ray (1 keV) range by the Chandra X-ray Observatory (1999), and about 1.5 arc minutes in the high energy X-ray (100 keV) range seen by High-Energy Focusing Telescope (2005).

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