Ground-based - γ ray astronomy : Present status and future prospects

Abstract

In this article, we review the observational scene in ground based γ-ray astronomy as it has changed from a rather ambiguous exploratory-phase in the eighties to a more unequivocal consolidation–phase in the recent times. We suggest that the detection-claims, made in the eighties in the TeV-PeV photon regime with regard to Cyg X-3 and Her X-1, may be genuine and reconcile these results to the present non-detections by presenting two plausible time-variability scenarios. Finally, we refer to the upsurge that is being witnessed in the field currently on the experimental front and comment on the progress that is likely to result, particularly in the tens of GeV – tens of TeV energy regime. An epoch-making result presented here, is the first-ever detection of a TeV γ-ray signal from a cosmic γ-ray source (BL –Lac object Mkn-501), concurrently by a global network of 4 imaging γ-ray telescopes, viz., Whipple (USA), CAT Imager (France), C-HEGRA(Germany) and TACTIC(India).