Gamma rays, interacting with the Earth’s atmosphere create showers of photos (smallest units of light with zero mass and no electric charge). As a result, a cascade of photons and secondary electrons travel down the atmosphere, very close to the speed of light in vacuum but faster than its speed in a medium like air. The shower of electrons leads to the creation of Cerenkov light (an electromagnetic shock wave), named after a Russian physicist who discovered it: Gamma ray photons and cosmic ray particles produce electron-positron pairs when they strike the atmosphere. Electrons and positrons recombine to produce secondary gamma rays, which again produce the electron-positron pairs, leading to Cerenkov radiation.
The radiation takes the form of a cone at an altitude of 10 km above the seal level. The radius of the cone increases as it nears the telescope. At Hanle it is possible to intercept the cone at only 5 km from the source of the secondary rays. The light is scattered and absorbed by the atmosphere. The detection threshold at Hanle is low, because of its high altitude. The number of Cernenkov photons is considerable even at low energies. An energy threshold of 50 or 60 GeV can be reached at Hanle, as against 200 GeV at pachmarhi.
The ‘pool of light’ created would be 200 m in diameter but only 1 m or so in thickness. The wavelengths of gamma rays are six billion nanometers. The telescopes should therefore be extremely sensitive to catch the Cerenkov radiation that passes through in nano seconds. Moreover, it is necessary to differentiate gamma rays from the more abundant cosmic rays. Fortunately, there is a fundamental difference between the particle showers initiated by gamma rays and those due to cosmic rays (mostly protons). The motion of electrically charged particles like protons is affected by the magnetic fields in the galaxy and in intergalactic fields. Hence it is not possible to trace their point of origin. In contrast, the electrically neutral photons of gamma rays can be correlated with the direction of their source.
Very High Energy Gamma Rays from both galactic and extra-galactic sources were clearly observed around 1987 using the Air Cerenkov Technique (ACT). Ground-based telescopes have an inherent advantage in detecting them over those in space. The latter have photon-collecting area of only a few square meters, which cannot detect rays with energies above say 10 GeV, as their ‘flux’ decreases with energies. Ground-based telescopes rely on the Earth’s atmosphere to spread out the interaction of the rays, which on impact with the air, generate a cascade of particles.
The cascade transforms a single high-energy gamma ray into a large number of charged particles and spread them over a huge area. The particles generate Cerenkov light. The particles, both charged and neutral, can reach ground level and can be detected by air shower telescopes (AST). Thus, a gamma ray can convert itself into either a light flash or a swarm of particles. However, the telescopes need dark nights.
A Cerenkov telescope has a large optical mirror, typically five or more meters in diameter. The high-speed camera photographs an image of the cascade. The Cerenkov flashes last only a few nanoseconds. The shape of the image of a gamma ray is such that it corresponds to such a phenomenon. For example, the Whipple telescope at Mt Hopkins in Arizona is able to reject most of the cosmic ray showers. The technique can give good angular resolution; hence sources that are not point-like can be rejected. The Whipple telescope detected two extra-galactic sources, the Blazars Mrk 421 and 501 (super-massive black holes), which were observed at Pachmarhi too.
The other type of telescopes is built at high altitudes, as the maximum number of shower particles occurs high up in the atmosphere. For example, the Tibet Array at about 4600 m at yanbajing, a few hours from Lasha can detect air shower particles corresponding to very high energy (in TeV) gamma rays. A new type of air shower telescope is coming up in New Mexico, where a large pond detects gamma rays day and night.