Emissions from GRBs and their afterglows: Indian observations

Abstract

The study of Gamma-ray bursts, electromagnetically the most luminous events in the Universe though only for a few seconds, was revolutionized in 1997 when the Italian-Dutch X-ray satellite BeppoSAX started providing the positions of some events with an accuracy of a few arcminutes within a few hours after the burst. Indian contributions in this ares are summarized. While mainly ISRO contributed to studies of GRB emission, a number of research organizations contributed to the optical observations of GRB afterglows. So far successful optical observations have been obtained for five afterglows namely GRB 990123, GRB 991208, GRB 991216, GRB 000301C and GRB 000926. The light curves, spectral energy distributions and energetics of these are discussed in the light of recent fireball plus blast wave theoretical models of GRBs. Their early-time flux decay constant is ~ 1.1 which become ~ 2.4 at later times. All are having relatively flat spectral index with its value ranges from 0.73 to 0.95. Observations of these afterglows support the model of non-isotropic synchrotron emission from the centre of the GRBs which reduces the budget of isotropic energy emission from the ≥ 10⁵³ ergs to < 10⁵² ergs, a value which is comparable with the current popular model of the origin of GRBs.