Altitude effect in Čerenkov light flashes of low energy gamma-ray-induced atmospheric showers

Abstract

At present the field of ground-based very high energy (VHE) gamma-ray astronomy is racing to complete construction of a number of modern gamma-ray detectors, i.e. CANGAROO III, MAGIC, HESS and VERITAS. They should be fully operational in a year's time. After much debate, the further development of this gamma-ray astronomy in the foreseeable future must be widely anticipated to proceed with the designing and building of a new instrumentation, which is primarily intended for the further drastic reduction of the energy threshold in gamma-ray observations down to about 10 GeV. On the ground one can hardly reach such low energy thresholds without considerably larger, up to 30 m diameter, optical telescopes, which might be able to collect sufficient amount of Čerenkov light from the atmospheric gamma-ray showers of that low energy. If not taken off the ground entirely (like GLAST), then it seems to be profitable to mount future low energy Čerenkov telescopes at higher altitudes in the atmosphere in order that they will be able to detect substantially more unabsorbed Čerenkov light from a shower. There are a few sites high up on mountains of roughly 5 km height worldwide, which can be used for such a venture. However, one has to remember that actual time profile, and in particular a two-dimensional distribution (image) of Čerenkov light flash from atmospheric gamma-ray showers, undergoes a rapid change after an increase in the observational level. This paper briefly describes the results of a topological analysis of Čerenkov light images calculated at both conventional and desirable altitudes of 2.2 and 5 km above the sea, respectively. A discussion on major upgrades of image topology at high altitude is also given.