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

Abstract

The CCD Johnson BV and Cousins RI photometric 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 passband. They show a steepening of the flux Lorentz factor. We drive 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 bursts. 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 bursts 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.