Observations of GRB afterglows from Nainital : Evidence in favour of jet model

Abstract

Broad band optical photometric observations of five Gamma-ray burst (GRB) afterglows (GRB 990123, GRB 991208, GRB 991216, GRB 000301c and GRB 000926) are taken from Nainital during 1999 and 2000. These in combination with the published observations including at other wavelengths are used to study their flux decays, broad spectral energy distributions and energetics in the light of fireball plus blast wave synchrotron emission models. Their early time flux decay constants is ~ 1.1 which becomes ? 2 at later times. All are having a relatively flat spectral index with values ranging from 0.73 to 0.95. Observations of these afterglows support the model of non-isotropic synchrotron emission from the centre of these GRBs which reduces the budget of isotropic energy emission from > 1053 erg to ? 1052 erg. This, as well as the other observations indicate that GRBs originate most likely from the collapse of massive stars rather than the merging of compact objects.