Gamma-ray astronomy activities in BARC - Past contributions and present challenges

Abstract

It was in the early seventies that we made a modest start in the Nuclear Research Laboratory (NRL) of the Bhabha Atomic Research Centre (BARC) in the then-budding field of observational gamma-ray astronomy by setting up an atmospheric scintillation experiment to search for prompt γ-ray emissions from supernovae explosions and primordial black-hole outbursts. This Gulmarg (Kashmir)-based experiment also permitted us to deploy , in a supplementary mode of operations, the atmospheric Cerenkov detection technique to investigate the cosmic- ray energy spectrum around the ‘knee’ position and more significantly, to obtain corroborative evidence for the reported ultra high energy γ-ray emission from the enigmatic galactic X-ray source Cyg X-3. This early, exploratory stage (1974-1984) was followed by the first consolidation phase (1984-1992) when we commissioned a multi-mirror atmospheric Cerenkov in Gulmarg and obtained evidence for possible TeV γ-ray emission from several γ-ray source candidates. With over 20 years of experience behind us in the related experimental techniques and theoretical concepts, we have recently embarked on a more ambitious second phase of consolidation (post-1992), with the principle aim of establishing a new, international class astronomy facility, GRACE in Mt. Abu, Rajasthan. Four high-sensitivity experiments are being set up under this project in order to comprehensively investigate nearly 10 decades of the γ-ray spectral window (10’s of keV – 100’s of TeV) from one location to operate in time –correlation with one another and with other major experiments in India and overseas, addressing other related spectral bands. In this largely ‘flash –back’ narrative, we retrace the path along which we have journeyed over the past 2 decades, referring on the way to some ‘bench-mark’ investigations successfully carried out over the years on various topical themes, by us and our colleagues in NRL unit of BARC.