Optical observations of GRB 060124 afterglow: a case for an injection break

Abstract

We present broad band optical afterglow observations of a long duration GRB 060124 using the 1.04-m Sampurnanand Telescope at ARIES, Nainital and the 2.01-m HCT at IAO, Hanle, including the earliest ground based observations in R band for this GRB. We determine the decay slope of the light curve at different bands and examine the reality of a proposed jet break. Methods. We use data from our observations as well as others reported in the literature to construct light curves in different bands and make power law fits to them. The spectral slope of the afterglow emission in the optical band is estimated. Results. Our first R-band observations were taken ~ 0.038 d after burst. We find that all available optical data after this epoch are well fit by a single power law, with a temporal flux decay index α~ 0.94. We do not find any evidence of a jet break within our data, which extend till ~ 2 d after the burst. The X-ray light curve, however, shows a distinct break around 0.6 day. We attribute this break to a steepening of the electron energy spectrum at high energies. We conclude that the above measurements are consistent with the picture of a standard fireball evolution with no jet break within t ~ 2 days after the burst. This sets a lower limit of 3 × 10⁵⁰ erg to the total energy released in the explosion.