Performance Limits for Cherenkov Instruments

Imaging Cherenkov telescopes and in particular stereoscopic arrays of Cherenkov telescopes such as HEGRA or H.E.S.S. have emerged as the prime instrument for gamma-ray astronomy in the TeV energy regime. H.E.S.S., for example, reaches an energy threshold around 100 GeV, an angular resolution for individual gamma rays of 0.1◦ and a sensitivity of less then 1% of the Crab flux for 25 h of exposure. Considering the sensitivity as a function of energy, one finds three different regimes. At low energy, the detection threshold is basically determined by the number of Cherenkov photons detected, i.e. by the product of mirror area and photodetection efficiency; typically, about 50 photoelectrons are required to trigger a telescope and to generate a reconstructible image. At the high energy end, instruments are limited by the number of detected gamma rays, i.e. by the area over which showers are detected. In the intermediate energy range, background from hadronand electron-induced cosmic ray showers limits the sensitivity. For point sources, the number of background events under a signal is proportional to the square of the angular resolution times the rejection efficiency for cosmic rays; an improved angular resolution is therefore a key element in increasing the sensitivity of instruments. Angular resolution is also of significant interest to unravel the processes in extended gamma-ray sources such as supernova remnants. Recent H.E.S.S. measurements have for the first time resolved the structure of supernova remnants in very high energy gamma rays [1, 2]; however, Chandra and XMM X-ray images of remnants show much finer structures than currently resolved by Cherenkov instruments [3, 4, 5]. The ability to detect such structures at TeV energies would help in identifying the origin of the gamma rays: Inverse Compton scattering of electrons should show narrow structures comparable to the X-ray structures – governed by the rapid cooling of the radiating electrons – whereas hadronic interactions are expected to generate much smoother structures [6].