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Gamma-ray astronomy is the astronomical observation of gamma rays,〔Astronomical literature generally hyphenates "gamma-ray" when used as an adjective, but uses "gamma ray" without a hyphen for the noun.〕 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 our 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〔for example, supernova SN 1987A emitted an "afterglow" of gamma-ray photons from the decay of newly made radioactive cobalt-56 ejected into space in a cloud, by the explosion. (【引用サイトリンク】 The Electromagnetic Spectrum - Gamma-rays )〕 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. 〔(THE CRAB NEBULA AND PULSAR BETWEEN 500 GeV AND 80 TeV: OBSERVATIONS WITH THE HEGRA STEREOSCOPIC AIR CERENKOV TELESCOPES, The Astrophysical Journal, 614:897–913, 2004 October 20 )〕 ==Detector technology== 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). Very energetic gamma rays, with photon energies over ~30 GeV, can also be detected by ground based experiments. The extremely low photon fluxes at such high energies require detector effective areas that are impractically large for current space-based instruments. Fortunately, such high-energy photons produce extensive showers of secondary particles in the atmosphere that can be observed on the ground, both directly by radiation counters and optically via the Cherenkov light which the ultra-relativistic shower particles emit. The Imaging Atmospheric Cherenkov Telescope technique currently achieves the highest sensitivity. Gamma radiation in the TeV range emanating from the Crab Nebula was first detected in 1989 by the Whipple Observatory at Mt. Hopkins, in Arizona in the USA. Modern Cherenkov telescope experiments like H.E.S.S., VERITAS, MAGIC, and CANGAROO III can detect the Crab Nebula in a few minutes. The most energetic photons (up to 16 TeV) observed from an extragalactic object originate from the blazar, Markarian 501 (Mrk 501). These measurements were done by the High-Energy-Gamma-Ray Astronomy (HEGRA) air Cherenkov telescopes. Gamma-ray astronomy observations are still limited by non-gamma-ray backgrounds at lower energies, and, at higher energy, by the number of photons that can be detected. Larger area detectors and better background suppression are essential for progress in the field. A discovery in 2012 may allow focusing gamma-ray telescopes.〔 At photon energies greater than 700 keV, the index of refraction starts to increase again.〔(Tim Wogan - Silicon 'prism' bends gamma rays (May 2012) - PhysicsWorld.com )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Gamma-ray astronomy」の詳細全文を読む スポンサード リンク
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