GRB 000926 and its optical afterglow: Another evidence for non-isotropic emission

The CCD magnitudes in Cousins R photometric passbands are determined for GRB 000926 afterglow ∼ 2.6 day after trigger of the γ−ray burst. Optical light curves of the afterglow emission are obtained by combining the present measurements with the published ones in B and R passbands. They show a steepening of the flux decay as expected of an anisotropic fireball losing collimation with the fall of the bulk Lorentz factor. We derive the early and late time flux decay constants as 1.4± 0.1 and 2.6± 0.06 respectively. Steepening in the flux decay seems to have started around 1.6 days after the burst. The value of the spectral index in the X-ray–optical region is ∼ −0.8. The determination of the redshift z = 2.066 indicates a cosmological origin of the burst at a luminosity distance of 16.9 Gpc. Thus it becomes the second farthest among the GRBs with known distances. The observed fluence in the energy range 20–100 keV indicates, if isotropic, the release of ≥ 10 ergs of energy. Attributing the observed break in the light curve at 1.6 days to the onset of sideways expansion of a jet-like ejecta, we infer an initial jet opening angle of ∼0.09 radian. This indicates a large anisotropy in the original emission and the amount of released energy is reduced by a factor of ∼ 250 relative to the isotropic value, which can be understood in terms of the currently popular stellar death models.