Modeling GRB 050904: Autopsy of a Massive Stellar Explosion atz = 6.29

GRB 050904 at redshift z=6.29, discovered and observed by Swift and withspectroscopic redshift from the Subaru telescope, is the first gamma-ray burstto be identified from beyond the epoch of reionization. Since the progenitorsof long gamma-ray bursts have been identified as massive stars, this eventoffers a unique opportunity to investigate star formation environments at thisepoch. Apart from its record redshift, the burst is remarkable in two respects:first, it exhibits fast-evolving X-ray and optical flares that peaksimultaneously at t~470 s in the observer frame, and may thus originate in thesame emission region; and second, its afterglow exhibits an accelerated decayin the near-infrared (NIR) from t~10^4 s to t~3 10^4 s after the burst,coincident with repeated and energetic X-ray flaring activity. We make acomplete analysis of available X-ray, NIR, and radio observations, utilizingafterglow models that incorporate a range of physical effects not previouslyconsidered for this or any other GRB afterglow, and quantifying our modeluncertainties in detail via Markov Chain Monte Carlo analysis. In the process,we explore the possibility that the early optical and X-ray flare is due tosynchrotron and inverse Compton emission from the reverse shock regions of theoutflow. We suggest that the period of accelerated decay in the NIR may be dueto suppression of synchrotron radiation by inverse Compton interaction of X-rayflare photons with electrons in the forward shock; a subsequent interval ofslow decay would then be due to a progressive decline in this suppression. Therange of acceptable models demonstrates that the kinetic energy and circumburstdensity of GRB 050904 are well above the typical values found for low-redshiftGRBs.Comment: 45 pages, 7 figures, and ApJ accepted. Revised version, minor modifications and 1 extra figur