GRB 050502B optical afterglow: a jet-break at high redshift^⋆

¹ Max-Planck-Institut für extraterrestriche Physik, Giessenbachstraße 1, 85748 Garching, Germany
e-mail: pafonso@mpe.mpg.de
² Osservatorio Astronomico di Trieste, Istituto Nazionale di Astrofisica, via G.B. Tiepolo 11, 34131 Trieste, Italy
³ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
⁴ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
⁵ Osservatorio Astronomico di Brera, Istituto Nazionale di Astrofisica, via Brera 28, 20121 Milano, Italy
⁶ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
⁷ Universe Cluster, Technische Universität München, Boltzmannstraße 2, 85748 Garching, Germany
⁸ University College of Dublin, Science Centre, School of Physics, Belfield, Dublin 4, Ireland
⁹ Princeton University, Department of Astrophysical Sciences, Peyton Hall, Princeton, NJ 08544-1001, USA

Received: 24 December 2009
Accepted: 5 October 2010

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 β_OX = 0.9 ± 0.1 is consistent with the X-ray spectral slope β_X. 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 E_γ,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 N_H, GRB 050502B was likely at high redshift.