GRB 120521C at z ~ 6 and the Properties of High-redshift γ-Ray Bursts

Abstract

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 cm^−3. The radio observations reveal the presence of a jet break at t_jet ≈ 7 d, corresponding to a jet opening angle of θ_jet ≈ 3°. The beaming-corrected γ-ray and kinetic energies are E_γ ≈ E_K ≈ 3 × 10^50 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 gsim 6 with radio detections (GRBs 050904 and 090423). We find a jet break at t_jet ≈ 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.

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