| 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 × 10 5 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) × 10 52 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.
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