A Possible High Altitude High Energy Gamma Ray Observatory in India

Recently an Indian Astronomical Observatory has been set up at Hanle (32$^\circ$ 46$^\prime$ 46$^{\prime\prime}$ N, 78$^\circ$ 57$^\prime$ 51$^{\prime\prime}$ E, 4515m amsl) situated in the high altitude cold desert in the Himalayas. The Observatory has 2-m aperture optical-infrared telescope, recently built by the Indian Institute of Astrophysics. We have carried out systematic simulations for this observation level to study the nature of \v{C}erenkov light pool generated by gamma ray and proton primaries incident vertically at the top of the atmosphere. The differences in the shape of the lateral distributions of \v{C}erenkov light with respect to that at lower altitudes is striking. This arises primarily due to the proximity of the shower maximum to the observation site. The limited lateral spread of the \v{C}erenkov light pool and near 90% atmospheric transmission at this high altitude location makes it an ideal site for a gamma ray observatory. This results in a decrease in the gamma ray energy threshold by a factor of 2.9 compared to that at sea-level. Several parameters based on density and timing information of \v{C}erenkov photons, including local and medium range photon density fluctuations as well as photon arrival time jitter could be efficiently used to discriminate gamma rays from more abundant cosmic rays at tens of GeV energies.