Many celestial sources of Very High Energy (VHE) -rays have now been detected by exploiting the ground-based atmospheric Cherenkov technique. Most of these detections have been made with systems operating at energy thresholds of a few hundred GeV.
While, the satellite based EGRET on board CGRO detected gamma ray pulsars at MeV-GeV energies, there were no convincing detections of these by the ground based systems operating at much higher energies(> 100 GeV ), implying steepening of the energy spectrum or a spectral cutoff. Thus, the energy band between 10 GeV and 100 GeV, which still largely remains to be explored, is expected to shed light on spectral cut-offs in spectra of pulsars and AGNs and also expected to lead to astrophysical discoveries involving gamma-ray bursts, Supernova remnants, plerions and un identified EGRET sources.
The energy threshold of ground-based set-ups could be lowered by installing them at higher altitudes where the photon density of atmospheric Cherenkov events is higher. Thus we have set up the High Altitude Gamma Ray Observatory (HAGAR) at Hanle (32◦.8 N, 78◦.9 E, 4300 m amsl) in the Ladakh region to address this important energy range using the technique of “wave-front sampling” of detecting atmospheric Cherenkov photon showers produced by gamma-rays.
The site offers an average of about 260 uniformly distributed spectroscopic nights per year which is a major advantage in terms of sky coverage for source observations. Located closer to the shower maximum, the Cherenkov photon density at Hanle is a factor of about 4 -5 more than at sea level. Using the high altitude and low night sky background of the site to advantage, the two telescope systems will access the important region of very high energy gamma-rays in the GeV energy range.
Study of VHE gamma rays from celestial sources is carried out using ground-based atmospheric Cherenkov technique. Energy thresholds of previous generation atmospheric Cherenkov telescopes were of the order of few hundred GeVs or higher. There are strong astrophysical motivations for lowering energy thresholds of such setups to below 100 GeV and have overlap in energy with satellite based detectors. This will enable the study of cutoffs in the spectra of AGNs as well as pulsars.
High Altitude GAmma Ray (HAGAR) experiment is an effort in setting up an array of small telescopes at very high altitude so that even modest size telescopes can achieve lower energy threshold.
The HAGAR array consists of seven telescopes with six of them deployed in the form of a hexagon. The seventh telescope is located at the centre of the array.The spacing between neighbouring telescopes is 50 m.
Each telescope consists of seven mirrors of diameter 0.9 m each. They are made by forming 10 mm thick float glass sheets into parabolic shapes of f/d ratio unity. At the focus of each mirror one fast UV sensitive phototube of the type Photonis XP2268B is mounted.
High voltages fed to photo-tubes are controlled and monitored using CAEN controller (model SY1527). Pulses from photo-tubes are brought to the control room situated below the central telescope via coaxial cables of length 85 m and of types LMR-ultraflex-400 (30 m) and RG 213 (55 m). For generating trigger, the pulses from 7 photo-tubes of a telescope are added linearly to form a telescope pulse. Event trigger is generated on coincidence of at least 4 out of 7 telescope pulses above a preset threshold within a resolving time of 150 ns.
Measurement of data to be done in this experiment are: