Observation of 7 TeV gamma rays from the Crab using the large zenith angle air Čerenkov imaging technique

Abstract

We have observed the Crab pulsar/nebula from 1992 December to 1993 January to search for very high energy gamma rays, using an imaging air Cerenkov telescope of 3.8 m diameter, located at Woomera, Australia. Since the observation was carried out in the southern hemisphere, the zenith angle of the object is large, which results in a maximum sensitivity of approximately 10-13/cm2/sec with approximately 7 TeV threshold energy for detected gamma rays. With high angular resolution, the Cerenkov imaging technique is found to be useful at zenith angles 53 to 56 deg. We report here possible evidence of detection with 4 sigma significance of very high energy gamma-rays above approximately 7 TeV from the Crab pulsar/nebula. The integral flux is estimated to be (7.6 plus or minus 1.9) x 10-13/cm2/sec. The result is consistent with the revised spectrum of the Crab recently reported by the Whipple group (Lewis et al. 1993).