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

Abstract

The CCD Johnson BV and Cousins RI photomertric magnitudes are determined for 20 stars in the field of GRB 000926. They are used to calibrate the present R as well as published BVRI photometric magnitudes of the GRB 000926 afterglow. Optical light curves of the afterglow emission are obtained in B and R passbands. They show a steepening of the flux decay as expected of an anisotropic fire ball 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 1.7 days after the burst. Negligible Galactic but relatively large intrinsic extinction amounting E(B - V) = 0.03 and 0.36 ± 0.02 mag respectively are derived in the direction of GRB 000926. The value of the spectral index in the X-ray-optical-near-infrared region is ~ -0.9. The determination of the redshift z = 2.0369. indicates a cosmological origin of the burst at a luminosity distance of 16.6 Gpc. The observed fluence in the energy range 20-100 keV indicates, if isotropic, the release of ≥10⁵³ ergs of energy. Attributing the observed break in the light curve at 1.7 days to the onset of sideways expansion of a jet-like ejecta, we infer an initial jet opening angle of ~ 0.14 radian. This indicates a large anisotropy in the original emission and the amount of released energy is reduced by factor of ~ 100 relative to the isotropic value, which can be understood in terms of the currently popular stellar death models.