Data analysis method for the search of point sources of gamma rays with the HAGAR telescope array

Abstract

The High Altitude GAmma-Ray (HAGAR) experiment is the highest altitude atmospheric Cherenkov sampling array, set up at 4300 m amsl in the Himalayas (Northern India). It constitutes 7 telescopes, each one with seven 90 cm-diameter mirrors, a field of view of 3 degrees, and was designed to reach a relatively low threshold (currently around 200 GeV) with quite a low total mirror area (31 m^2). In order to remove the strong isotropic background of charged cosmic rays, data are collected by tracking separately ON-source followed by OFF-source regions, or vice-versa. Typical observations period is about 30-40 min. ON-OFF data pairs are then selected according to quality parameters such as stability of the trigger rate and the comparison of average trigger rates between ON and OFF-source data sets. Signal extraction from point sources is done by performing analysis cuts on the count rate excess, rejecting off-axis events. Validation of method and systematics are evaluated through the analysis of fake sources (OFF-OFF pairs) located at similar declination as the observed point sources. Spurious signal, if any, would show up in this study.