Historical aspects of Gamma-ray astronomy

Abstract

Over the entire 20th century, Cosmic Rays proved to be the watershed of fundamental knowledge from which poured out several streams that made us familiar with aspects of the universe that could never have been known through optical and radio astronomies alone. Cosmic ray interaction studies opened up the field of elementary particles and high energy physical processes. Gamma-ray astronomy enabled us to study celestial environments characterised by the dominance of high energy particles and their interactions with matter, magnetic and electric fields in the neighbourhood of these special environments. While neutrino astronomy is still in its infancy, it has the potential of becoming the most exciting field of study in the current century. Gamma-ray astronomy has had a chequered career. In the early part of the 20th century, Millikan proposed that cosmic rays are merely gamma rays. This was disproved by Compton, through the establishment of the latitude effect. The soviet astrophysicist Shklovskii pointed out at the III International conference on cosmic rays held at Guanjuato, Mexico, the possibility of supernova remants like the Crab Nebula being sources of TeV gamma rays. This was based on his realisation that the high degree of polarized light from the Crab could be due to Synchrotron emission by TeV energy electrons spiralling round the filamentary magnetic fields of the nebula. He argued that the same mechanism that accelerated electrons could also accelerate the protons which through their interaction with the surrounding matter generate pi-zero measons that would immediately decay into gamma rays. However, the efforts by the soviet experimentalists, who used the night air cerenkov technique for detection of the TeV gamma rays, proved negative; only upper limits could be set on the fluxew of TeV gamma rays from several of the SN-remnants; the negative results were first reported at the 7th ICRC held at Jaipur, India in 1963. High energy gamma ray astronomy had a remarkable revival with the discovery of Pulsars in 1967 and their identification with Neutron stars. The field has thrived since then has been extended even to the PeV range. Beginning with 1965, gamma ray astronomy in the energy range MeV to tens of GeV has also been successfully pursued with ballons, and satellites. The most perplexing in this energy range has been the discovery of the Gamma ray bursts. In the keynote address the historical aspects of this field will be covered with some references to the work in India.