Astrophys. J., 781, 1 (2014/January-3)
GRB 120521C at z ∼ 6 and the properties of high-redshift γ-ray bursts.
LASKAR T., BERGER E., TANVIR N., ZAUDERER B.A., MARGUTTI R., LEVAN A., PERLEY D., FONG W.-F., WIERSEMA K., MENTEN K. and HRUDKOVA M.
Abstract (from CDS):
We present optical, near-infrared, and radio observations of the afterglow of GRB 120521C. By modeling the multi-wavelength data set, we derive a photometric redshift of z ~ 6.0, which we confirm with a low signal-to-noise ratio spectrum of the afterglow. We find that a model with a constant-density environment provides a good fit to the afterglow data, with an inferred density of n ≲ 0.05/cm3. The radio observations reveal the presence of a jet break at tjet~ 7 d, corresponding to a jet opening angle of θjet~ 3°. The beaming-corrected γ-ray and kinetic energies are Eγ~ EK~ 3x1050 erg. We quantify the uncertainties in our results using a detailed Markov Chain Monte Carlo analysis, which allows us to uncover degeneracies between the physical parameters of the explosion. To compare GRB 120521C to other high-redshift bursts in a uniform manner we re-fit all available afterglow data for the two other bursts at z ≳ 6 with radio detections (GRBs 050904 and 090423). We find a jet break at tjet~ 15 d for GRB 090423, in contrast to previous work. Based on these three events, we find that γ-ray bursts (GRBs) at z ≳ 6 appear to explode in constant-density environments, and exhibit a wide range of energies and densities that span the range inferred for lower redshift bursts. On the other hand, we find a hint for narrower jets in the z ≳ 6 bursts, potentially indicating a larger true event rate at these redshifts. Overall, our results indicate that long GRBs share a common progenitor population at least to z ∼ 8.
gamma-ray burst: general - gamma-ray burst: individual (GRB 120521C, GRB 050904, GRB 090423)
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