GRB 050502B optical afterglow: A jet-break at high redshift

Abstract

Aims. Swift GRB 050502B is well known for the very bright flare displayed in its X-ray light curve. Despite extensive studies, however, the optical light curve has never been discussed and its redshift is unconstrained. Possible correlations between optical and X-ray data are analysed. Methods. Photometric data from TNG in the R and I bands were used to compare the optical afterglow with the X-ray light curve. The HyperZ package and a late-time VLT host observation were used to derive redshift estimates. Results. The I-band afterglow decay followed a power law of index α = 2.1±0.6, after a late break at ~ 1.3×105 s. The R-I colour is remarkably red and the broadband spectral index βOX = 0.9±0.1 is consistent with the X-ray spectral slope βX. Although a photometric redshift of z > 4 is the most conservative result to consider, a photometric redshift of z = 5.2±0.3 is suggested with no extinction in the host, based on which an isotropic energy Eγ,iso = (3.8±0.7)×1052 erg and a jet opening angle θ ~ 3.7° are subsequently derived. Conclusions. The combined X-ray and optical data suggest an achromatic break, which we interpret as a jet-break. The post jet-break slope roughly obeys the closure relation for the jet's slow cooling model. Because of the afterglow's very red colour, in order for the redshift to be low (z < 1), extinction must be significantly high if present in the host. Since the optical-to-X-ray index is consistent with the X-ray spectrum, and there is no XRT evidence for excess NH, GRB  050502B was likely at high redshift. © 2011 ESO.