Are short gamma-ray bursts collimated? GRB050709, a flare but no break

Abstract

From the small sample of afterglow lightcurves of short duration gamma-ray bursts (GRBs), the decays are rapid, roughly following a power-law in time. It has been assumed that the afterglow emission in short GRBs is collimated in jets in the same way as in long GRBs. An achromatic break in a short GRB afterglow lightcurve would therefore be strong evidence in favour of collimation in short GRBs. We examine the optical lightcurve of the afterglow of the short GRB 050709, the only short GRB where a jet break has been claimed from optical data. We show that (1) the decay follows a single power-law from 1.4 to 19 days after the burst and has a decay index alpha = 1.73_{-0.04}^{+0.11}, (2) that an optical flare at ~10 days is required by the data, roughly contemporaneous with a flare in the X-ray data, and (3) that there is no evidence for a break in the lightcurve. This means that so far there is no direct evidence for collimation in the outflows of short GRBs. The available limits on the collimation angles in short GRBs now strongly suggest much wider opening angles than found in long GRBs.