An external-shock model for gamma-ray burst afterglow 130427A

Abstract

The complex multiwavelength emission of gamma-ray burst (GRB) afterglow 130427A (monitored in the radio up to 10 d, in the optical and X-ray until 50 d, and at GeV energies until 1 d) can be accounted for by a hybrid reverse-forward shock synchrotron model, with inverse- Compton emerging only above a few GeV. The high ratio of the early optical to late radio flux requires that the ambient medium is a wind and that the forward-shock synchrotron spectrum peaks in the optical at about 10 ks. The latter has two consequences: the wind must be very tenuous and the optical emission before 10 ks must arise from the reverse-shock, as suggested also by the bright optical flash that Raptor has monitored during the prompt emission phase (<100 s). The Very Large Array radio emission is from the reverse-shock, the Swift X-ray emission is mostly from the forward-shock, but the both shocks give comparable contributions to the Fermi GeV emission. The weak wind implies a large blast-wave radius (8 t1/2day pc), which requires a very tenuous circumstellar medium, suggesting that the massive stellar progenitor of GRB 130427A resided in a superbubble. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.