The effect of the geomagnetic field on TeV gamma -ray detection

Very high energy (VHE) gamma -ray astronomy using the ground based atmospheric Cerenkov technique is an important new field of high energy astrophysics and a range of VHE gamma -ray sources has been detected at various levels of significance by a number of experimental groups. Outstanding evidence for gamma -rays from the Crab Nebula was obtained by the Whipple collaboration. This involved the use of a sophisticated technique based on computer simulations to select gamma -ray-like events from the proton induced background When this same technique was applied to data from other sources detected using an earlier version of the Whipple telescope, the evidence for a gamma -ray signal disappeared. The inability to reconcile the Whipple result from the Crab Nebula with both the failure of the Whipple camera to detect other objects and the detections made with other non-imaging telescopes has caused some concern at a critical time in the evolution of this branch of high energy astrophysics. In an attempt to reconcile these results, the authors have made a detailed study of the effects of the geomagnetic field on the electron-photon cascades. They have simulated the Cerenkov light produced by individual protons and gamma -rays at different orientations in the atmosphere, and made measurements of the sensitivity of the observations to the geomagnetic field. Indications are that these geomagnetic effects may be a cause of the differing results.