Very-high-energy gamma ray
Very-high-energy gamma ray (VHEGR) denotes gamma radiation with photon energies of 100 GeV to 100 TeV, i.e., 1011 to 1014electronvolts. This is approximately equal to wavelengths between 10−17 and 10−20 meters, or frequencies of 2 × 1025 to 2 × 1028 Hz. Such energy levels have been detected from emissions from astronomical sources such as some binary star systems containing a compact object. For example, radiation emitted from Cygnus X-3 has been measured at exaelectronvolt-levels. Other astronomical sources include BL Lacertae,3C 66AMarkarian 421 and Markarian 501. Various other sources exist that are not associated with known bodies. For example, the H.E.S.S. catalog contains 64 sources in November 2011.
Instruments to detect this radiation commonly measure the Cherenkov radiation produced by secondary particles generated from an energetic photon entering the Earth's atmosphere. This method is called imaging atmospheric Cherenkov technique or IACT. A high-energy photon produces a cone of light confined to 1° of the original photon direction. About 10,000 m2 of the earth's surface is lit by each cone of light. A flux of 10−7 photons per square meter per second can be detected with current technology, provided the energy is above 0.1 TeV. Instruments include the planned Cherenkov Telescope Array, GT-48 in Crimea, MAGIC on La Palma, High Energy Stereoscopic System (HESS) in NamibiaVERITAS and Chicago Air Shower Array which closed in 2001. Cosmic rays also produce similar flashes of light, but can be distinguished based on the shape of the light flash. Also having more than one telescope simultaneously observing the same spot can help exclude cosmic rays.Extensive air showers of particles can be detected for gamma rays above 100 TeV. Water scintillation detectors or dense arrays of particle detectors can be used to detect these particle showers.
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