The circumburst environment of a FRED GRB: study of the prompt emission and X-ray/optical afterglow of GRB 051111

Abstract

Aims. We report a multi-wavelength analysis of the prompt emission and early afterglow of GRB 051111 and discuss its properties in the context of current fireball models. Methods. The detection of GRB 051111 by the Burst Alert Telescope on-board Swift triggered early BVRi’ observations with the 2-m robotic Faulkes Telescope North in Hawaii, as well as X-ray observations with the Swift X-Ray Telescope. Results. The promptγ-ray emission shows a classical FRED profile. The optical aft erglow light curves are fitted with a broken power law, withα1 = 0.35 toα2 = 1.35 and a break time around 12 minutes after the GRB. Although contemporaneous X-ray observations were not taken, a power law connection between theγ-ray tail of the FRED temporal profile and the late XRT flux deca y is feasible. Alternatively, if the X-ray afterglow tracks the optical de cay, this would represent one of the first GRBs for which the ca nonical steep-shallow-normal decay typical of early X-ray afterglows has been monitored optically. We present a detailed analysis of the intrinsic extinction, elemental abundances and spectral e nergy distribution. From the absorption measured in the low X-ray band we find possible evidence for an overabundance of some α elements such as oxygen, [O/Zn]=0.7± 0.3, or, alternatively, for a significant presence of molecular gas. The IR-to-X-ray Spectral Energy Distribution measured at 80 minutes after the burst is consistent with the cooling break lying between the optical and X-ray bands. Extensive modelling of the intrinsic extinction suggests dust with big grains or grey extinction profiles. The early optical break is due ei ther to an energy injection episode or, less probably, to a st ratified wind environment for the circumburst medium.