A simulation of high-energy gamma-ray observations with imaging Cherenkov telescopes at high altitudes

The performance of a single gamma-ray telescope is discussed based on the Imaging Atmospheric Cherenkov Technique (IACTech) at high altitudes. Observations at a high altitude are quite suitable for the IACTech because the density of Cherenkov light caused by gamma-rays is several-times larger than at sea level. To observe gamma-ray pulsars at high altitude, a gamma-ray camera, designated CheSS (Cherenkov light detecting System on Subaru) based on the IACTech, has been developed for the Subaru infrared-optical telescope at an altitude of 4200 m. Using CheSS, the Crab nebula/pulsar was observed for three days in December 2001. From a comparison between the simulated and observed shower events, it was found that the night-sky background (N.S.B.) and muon showers have significant effects on the detection rate below 80 GeV in spite of the small field of view (FOV) of CheSS (0.75°). The simulated spectrum, which includes muon shower events, is consistent with the observed spectrum. on this study, the performance of a camera with a larger FOV of 3.0° is discussed. Although the effects of muon showers and the N.S.B. become serious, the simulation results indicate that an 8.2 m Imaging Atmospheric Cherenkov Telescope (IACT) with a FOV of 3.0° at an altitude of 4200 m could detect 15 GeV energy gamma-rays.