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Atmospheric Cerenkov telescopes are used to detect electromagnetic showers from primary gamma rays of energy > 300 GeV and to discriminate these from cascades due to hadrons using the shape and orientation of the Cerenkov images. The geomagnetic field affects the development of showers and diffuses and distorts the images. When the component of the field normal to the shower axis is sufficiently large (> 0.4 G) the performance of gamma ray telescopes may be affected.
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Atmospheric Cerenkov telescopes are used to detect electromagnetic showers from primary gamma rays of energy ~300 GeV - ~10 TeV and to discriminate these from cascades due to hadrons using the Cerenkov images. The geomagnetic field affects the development of showers and is shown to diffuse and distort the images. When the component of the field normal to the shower axis is sufficiently large (> 0.4 G) the performance of gamma ray telescopes may be affected, although corrections should be possible.
Three different analysis techniques for Atmospheric Imaging System are presented. The classical Hillas parameters based technique is shown to be robust and efficient, but more elaborate techniques can improve the sensitivity of the analysis. A comparison of the different analysis techniques shows that they use different information for gamma-hadron separation, and that it is possible to combine their qualities.
The hunt for cosmic TeV particle accelerators is prospering through Imaging Atmospheric Cerenkov Telescopes. We face challenges such as low light levels and MHz trigger rates, and the need to distinguish between particle air showers stemming from primary gamma rays and those due to the hadronic cosmic ray background. Our test beam is provided by the Crab Nebula, a steady accelerator of particles to energies beyond 20 TeV. Highly variable gamma-ray emission, coincident with flares at longer wavelengths, is revealing the particle acceleration mechanisms at work in the relativistic jets of Active Galaxies. These 200 GeV to 20 TeV photons propagating over cosmological distances allow us to place a limit on the infra-red background linked to galaxy formation and, some speculate, to the decay of massive relic neutrinos. Gamma rays produced in neutralino annihilation or the evaporation of primordial black holes may also be detectable. These phenomena and a zoo of astrophysical objects will be the targets of the next generation multi-national telescope facilities.
After the launch and successful beginning of operations of the FERMI satellite, the topics related to high-energy observations of gamma-ray bursts have obtained a considerable attention by the scientific community. Undoubtedly, the diagnostic power of high-energy observations in constraining the emission processes and the physical conditions of gamma-ray burst is relevant. We briefly discuss how gamma-ray burst observations with ground-based imaging array Cerenkov telescopes, in the GeV-TeV range, can compete and cooperate with FERMI observations, in the MeV-GeV range, to allow researchers to obtain a more detailed and complete picture of the prompt and afterglow phases of gamma-ray bursts.
The Pachmarhi Array of Cerenkov Telescopes consists of a distributed array of 25 telescopes that are used to sample the atmospheric Cerenkov Photon showers. Each telescope consists of 7 parabolic mirrors each viewed by a single photo-multiplier tube. Reconstruction of photon showers are carried out using fast timing information on the arrival of pulses at each PMT. The shower front is fitted to a plane and the direction of arrival of primary particle initiating the shower is obtained. The error in the determination of the arrival direction of the primary has been estimated using the {it split} array method. It is found to be $sim 2.4^prime ~$ for primaries of energy $ > 3 ~TeV$. The dependence of the angular resolution on the separation between the telescopes and the number of detectors are also obtained from the data.
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