Very-high energy (VHE) gamma quanta contribute only a minuscule fraction - below one per million - to the flux of cosmic rays. Nevertheless, being neutral particles they are currently the best "messengers" of processes from the relativistic/ultra-relativistic Universe because they can be extrapolated back to their origin. The window of VHE gamma rays was opened only in 1989 by the Whipple collaboration, reporting the observation of TeV gamma rays from the Crab nebula. After a slow start, this new field of research is now rapidly expanding with the discovery of more than 150 VHE gamma-ray emitting sources. Progress is intimately related with the steady improvement of detectors and rapidly increasing computing power. We give an overview of the early attempts before and around 1989 and the progress after the pioneering work of the Whipple collaboration. The main focus of this article is on the development of experimental techniques for Earth-bound gamma-ray detectors; consequently, more emphasis is given to those experiments that made an initial breakthrough rather than to the successors which often had and have a similar (sometimes even higher) scientific output as the pioneering experiments. The considered energy threshold is about 30 GeV. At lower energies, observations can presently only be performed with balloon or satellite-borne detectors. Irrespective of the stormy experimental progress, the success story could not have been called a success story without a broad scientific output. Therefore we conclude this article with a summary of the scientific rationales and main results achieved over the last two decades.

Very-high energy gamma-ray astronomy: A 23-year success story in high-energy astroparticle physics

The techniques, the status and prospects of very high energy (VHE) gamma ray astronomy with ground based telescopes will be reviewed

Very high energy gamma ray astronomy

The field of ground-based γ-ray astronomy has made very significant advances over the last three decades with the extremely successful operations of several atmospheric Cherenkov telescopes worldwide. The advent of the imaging Cherenkov technique for indirect detection of cosmic γ rays has immensely contributed to this field with the discovery of more than 220 γ-ray sources in the Universe. This has greatly improved our understanding of the various astrophysical processes involved in the non-thermal emission at energies above 100 GeV. In this paper, we summarize the important results achieved by the Indian γ-ray astronomers from the GeV-TeV observations using imaging Cherenkov telescopes over the last two decades. We mainly emphasize the results obtained from the observations of active galactic nuclei with the TACTIC (TeV Atmospheric Cherenkov Telescope with Imaging Camera) telescope, which has been operational since 1997 at Mount Abu, India. We also discuss the future plans of the Indian γ-ray astronomy program with special focus on the scientific objectives of the recently installed 21 m diameter MACE (Major Atmospheric Cherenkov Experiment) telescope at Hanle, India.

20 Years of Indian Gamma Ray Astronomy Using Imaging Cherenkov Telescopes and Road Ahead

An optimized method is presented for the reconstruction of air shower directions for scintillator arrays. The method takes into account that both the expectation value and the spread of the measured arrival times not only depend on the distance of a counter from the shower axis, but also on the number of particles registered in that counter. It also takes into account that the distributions of the measured arrival times are not Gaussian. For showers recorded with the HEGRA scintillator array above the threshold energy of E thres = 20 TeV the mean angular resolution obtained with this method is 〈 σ 63% θ 〉 = 1.0°, and above the threshold E thres = 50 TeV it is 〈 σ 63% θ 〉 = 0.6°. Comparing the new procedure with the HEGRA standard procedure the angular resolution has improved on average by a factor of 1.33. The mis-pointing has been determined with an accuracy of 0.15°. The method is developed using experimental data.

An optimized method for the reconstruction of the direction of air showers for scintillator arrays

http://www.cosmicray-ocu.jp/papers/2010APh....33...97O.pdf

The angular resolution of the GRAPES-3 array from the shadows of the Moon and the Sun

Search for Steady and Sporadic Emission of Neutral Radiation Above 50 TeV with the HEGRA Array

Extension of the HEGRA Experiment at La Palma

Methods to determine the angular resolution of the HEGRA extended air shower scintillator array

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Search for Steady and Sporadic Emission of Neutral Radiation Above 50 TeV with the HEGRA Array

Extension of the HEGRA Experiment at La Palma

Methods to determine the angular resolution of the HEGRA extended air shower scintillator array