GRB 000301C with peculiar afterglow emission

Abstract

The CCD magnitudes 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 emissions 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 steepening 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 cause of variability is not well understood, though it has occurred simultaneously with similar amplitude in all the filters. After removing the superposed flux due to variability, we derive the early and late time flux decay constants using jet model. 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-band 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 > 20 keV indicates, if isotropic, ≥10⁵³ ergs of release of energy. The enormous amount of released energy will be reduced, if the radiation is beamed which is the case for this event. Using a jet break time of 7.6 days, we infer a jet opening angle of ~ 0.15 radian. This means the energy released is reduced by a factor of ~ 90 relative to the isotropic value.