Abstract
We follow the bright, highly energetic afterglow of Swift-discovered GRB 080721 at z = 2.591 out to 36 d or 3 x 10(6) s 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 theta(j) >= 7 degrees.3 and setting tight constraints on the total energy budget of the burst of E-gamma >= 9.9 x 10(51) 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.
Highlights
The discovery (Costa et al 1997; van Paradijs et al 1997) of afterglows to long-duration gamma-ray bursts (GRBs) showed that they occurred in star-forming galaxies at high redshifts
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 collimationcorrected energies could be accommodated with certain parameters of the standard massive star core-collapse models
We have investigated the alternative scenario in which an earlier break in the light curve at ∼0.02 d is the jet break and found that this cannot work within the fireball model
Summary
The discovery (Costa et al 1997; van Paradijs et al 1997) of afterglows to long-duration gamma-ray bursts (GRBs) showed that they occurred in star-forming galaxies at high redshifts. While the modest luminosity and low redshift of GRB 060729 (Eiso ∼ 7 × 1051 erg; Grupe et al 2007) make it unsuitable to test collapsar models to their limits, such observations provide a crucial guide to further theoretical developments If such high collimation-corrected energies continue to be inferred, the situation looks bleak for simple core-collapse models as the explanation for the high-luminosity long GRBs. it would suggest that a different progenitor is required for high- and lowluminosity long bursts. If late breaks are seen, the jet model is not proven this could provide evidence for an upper envelope to the energy tapped by GRBs which may still be consistent with that available in principle from core collapse, but would require a new understanding of how such high efficiencies of radiative emission could be achieved.
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