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

T. Sakamoto⁶, R. J. Smith¹, I. A. Steele¹, and N. R. Tanvir<sup>3, 7

1</sup>  Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD, UK
    e-mail: cristiano.guidorzi@brera.inaf.it
²  Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia
³  Department of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UK
⁴  INAF - Sezione di Palermo, via U. La Malfa 153, 90146 Palermo, Italy
⁵  ITC - IRST and INFN, Trento, via Sommarive 18, 38050 Povo (TN), Italy
⁶  NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁷  Centre for Astrophysics Research, University of Hertfordshire, Hatfield AL10 9AB, UK

(Received 5 July 2006 / Accepted 10 November 2006)

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 prompt -ray emission shows a classical FRED profile. The optical afterglow light curves are fitted with a broken power law, with to and a break time around 12 min after the GRB. Although contemporaneous X-ray observations were not taken, a power law connection between the -ray tail of the FRED temporal profile and the late XRT flux decay is feasible. Alternatively, if the X-ray afterglow tracks the optical decay, this would represent one of the first GRBs for which the canonical 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 energy distribution. From the absorption measured in the low X-ray band we find possible evidence for an overabundance of some  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 min 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 either to an energy injection episode or, less probably, to a stratified wind environment for the circumburst medium.