Observations with the High Altitude GAmma-Ray (HAGAR) telescope array in the Indian Himalayas

Abstract

The High Altitude GAmma-Ray (HAGAR) ar- ray is a wavefront sampling array of 7 telescopes, set-up at Hanle, at 4270 m amsl, in the Ladakh region of the Hi- malayas (Northern India). It constitutes the first phase of the HImalayan Gamma-Ray Observatory (HIGRO) project. HA- GAR is the first array of atmospheric Cherenkov telescopes established at a so high altitude, and was designed to reach a relatively low threshold (currently around 200 GeV) with quite a low mirror area (31 m 2 ). Regular source observations are running since September 2008. Estimation of the sensi- tivity of the experiment is undergoing using several hours of data from the direction of Crab nebula, the standard candle source of TeV gamma-ray astronomy, and from dark regions. Data were acquired using the On-source/Off-source tracking mode, and by comparing these sky regions the strength of the gamma-ray signal could be estimated. Gamma-ray events arrive close to telescope axis direction while the cosmic-ray background events arrive from the whole field of view. We discuss our analysis procedures for the estimate of arrival di- rection, estimate of gamma ray flux from Crab nebula, and the sensitivity of the HAGAR system, in this paper.