Low energy ?-ray events in imaging ?erenkov telescope arrays

Abstract

One of the major goals in VHE-γ-ray astronomy is to open the energy range below 100 GeV with earthbound detectors. This paper demonstrates a new method for analyzing Cerenkov light of a shower in a Cerenkov telescope array. This method is successful for showers in this low energy regime where previous techniques (e.g. alpha analysis) are not applicable. A Monte Carlo simulation is applied to a system of 19 Whipple type [3, Cawley 1990] Imaging Atmospheric Cerenkov telescopes (IACT), each built as a 10 m diameter reflector and equipped with a 109 photomultiplier tube camera. The energy threshold for a single detector of this type is given [5, Kerrick et al. 1995] as 250 GeV. Analysis of simulated coincident events of the system for those events not having enough light to apply a standard imaging analysis [4, Hillas 1985], leads to a considerably lower threshold of 85 GeV. With a new analysis method of these events it is shown that it should be possible to distinguish between γ-ray induced and proton induced showers. The improvement of sensitivity (Q = figure of merit) of this analysis method is found to be Q=2.9.