EDP Sciences
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Volume 454, Number 3, August II 2006
Page(s) L123 - L126
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20065380

A&A 454, L123-L126 (2006)
DOI: 10.1051/0004-6361:20065380


Are short $\gamma$-ray bursts collimated? GRB 050709, a flare but no break

D. Watson, J. Hjorth, P. Jakobsson, D. Xu, J. P. U. Fynbo, J. Sollerman, C. C. Thöne and K. Pedersen

Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
    e-mail: [darach;jens;pallja;dong;jfynbo;jesper;cthoene;kp]@astro.ku.dk

(Received 6 April 2006 / Accepted 2 June 2006)

Context.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.
Aims.An achromatic break in a short GRB afterglow lightcurve would therefore be strong evidence in favour of collimation in short GRBs.
Methods.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.
Results.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 ${\sim}10$ 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.
Conclusions.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.

Key words: gamma rays: bursts -- stars: neutron

© ESO 2006