GRB 000301C with peculiar afterglow emission

Abstract

The CCD magnitude in Johnson V and Cousins R and I photometric passbands are determined for GRB 000301C afterglow starting ~ 1.5 day after the γ – ray burst. In fact we provide the earliest optical observations for this burst. Light curves of the afterglow emission in U, B, V, R, I, J and K' passbands are obtained by combining the present measurements with the published data. Flux decay shows a very uncommon variation relative to other well observed GRBs. Overall, there is a steeping of the optical and near-infrared flux decay caused by a geometric and sideways expanding jet. This is superimposed by a short term variability especially during early time (Δt < 8 days). The late time flux decay is the steepest amongst the GRB OTs observed so far with α ~ 3. Steepening in the flux decay seems to have started simultaneously around Δt ~ 7.6 day in all passbands. On the other hand no such variations are observed in the quasi-simultaneous broad-bands photometric spectral energy distributions of the afterglow. The value of spectral index in the optical-near IR region is ~ -1.0. Redshift determination with z = 2.0335 indicates cosmological origin of the GRB having a luminosity distance of 16.6 Gpc. Thus it becomes the second farthest amongst the GRBs with known distances. An indirect estimate of the fluence > 20KeV indicates, if isotropic, ≥ 10⁵³ ergs of release of energy. Using a jet break time of 7.6 days, we infer a jet opening angles of ~ 0.15 radian. This means the energy released is reduced by a factor of ~ 90 relative to the isotropic value.