We present multiwavelength observations of the afterglow of GRB 130427A, the brightest (in total fluence) gamma-ray burst of the past 29 years. Optical spectroscopy from Gemini-North reveals the redshift of the GRB to be z = 0.340, indicating that its unprecedented brightness is primarily the result of its relatively close proximity to Earth; the intrinsic luminosities of both the GRB and its afterglow are not extreme in comparison to other bright GRBs. We present a large suite of multiwavelength observations spanning from 300 s to 130 d after the burst and demonstrate that the afterglow shows relatively simple, smooth evolution at all frequencies, with no significant latetime flaring or rebrightening activity. The entire dataset from 1 GHz to 10 GeV can be modeled as synchrotron emission from a combination of reverse and forward shocks in good agreement with the standard afterglow model, providing strong support to the applicability of the underlying theory and clarifying the nature of the GeV emission observed to last for minutes to hours following other very bright GRBs. A tenuous, wind-stratified circumburst density profile is required by the observations, suggesting a massive-star progenitor with a low mass-loss rate, perhaps due to low metallicity. GRBs similar in nature to GRB 130427A, inhabiting low-density media and exhibiting strong reverse shocks, are probably not uncommon but may have been difficult to recognize in the past owing to their relatively faint late-time radio emission; more such events should be found in abundance by the new generation of sensitive radio and millimeter instruments. 25 Here and elsewhere we assume a standard ΛCDM cosmological model with Ω Λ = 0.7, Ωm = 0.3, h = 0.7.

Aims.A model of jet precession driven by a neutrino-cooled disk around a spinning black hole is presented to explain the temporal structure and spectral evolution of gamma-ray bursts (GRBs). Methods. The differential rotation of the outer part of a neutrino-dominated accretion disk may result in precession of the inner part of the disk and the central black hole, hence driving a precessed jet via neutrino annihilation around the inner part of the disk. Results. Both analytic and numeric results for our model are presented. Our calculations show that a black-hole, accretion-disk system with the black hole mass M 3.66 M , accretion rateṀ 0.54 M s −1 , spin parameter a = 0.9, and viscosity parameter α = 0.01 may drive a precessed jet with period P = 1 s and luminosity L = 10 51 erg s −1 , corresponding to the scenario for long GRBs. A precessed jet with P = 0.1 s and L = 10 50 erg s −1 may be powered by a system with M 5.59 M ,Ṁ 0.74 M s −1 , a = 0.1, and α = 0.01, and is possibly responsible for the short GRBs. Both the temporal and spectral evolution in GRB pulse may be explained with our model. Conclusions. GRB central engines most likely power a precessed jet driven by a neutrino-cooled disk. The global GRB lightcurves thus could be modulated by the jet precession during the accretion timescale of the GRB central engine. Both the temporal and spectral evolution in GRB pulse may stem from a viewing effect of the jet precession.