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.