Constraining the energy budget of GRB080721
Howarth, I. D.; Rol, E.; Fynbo, J. P. U.; O'Brien, P. T.; van der Horst, A. J.; Wiersema, K.; Osborne, J. P.; Page, K. L.; Levan, A. J.; Starling, R. L. C.; Tanvir, N. R.; Curran, P. A.; Jakobsson, P.; de Ugarte Postigo, A.; Ledoux, C.; Guidorzi, C.; Yoon, S. -C.; Pal'Shin, V.; Svinkin, D.; Oosting, T.
United Kingdom, Netherlands, United States, Russia, Chile, Denmark, Iceland, Italy
Abstract
We follow the bright, highly energetic afterglow of Swift-discovered GRB080721 at z = 2.591 out to 36 d or 3 × 106s since the trigger in the optical and X-ray bands. We do not detect a break in the late-time light curve inferring a limit on the opening angle of and setting tight constraints on the total energy budget of the burst of Eγ >= 9.9 × 1051 erg within the fireball model. To obey the fireball model closure relations, the gamma-ray burst (GRB) jet must be expanding into a homogeneous surrounding medium and likely lies behind a significant column of dust. The energy constraint we derive can be used as an observational input for models of the progenitors of long GRBs: we discuss how such high collimation-corrected energies could be accommodated with certain parameters of the standard massive star core-collapse models. We can, however, most probably rule out a magnetar progenitor for this GRB which would require 100 per cent efficiency to reach the observed total energy.