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The complex multiwavelength emission of GRB afterglow 130427A (monitored inthe radio up to 10 days, in the optical and X-ray until 50 days, and at GeVenergies until 1 day) can be accounted for by a hybrid reverse-forward shocksynchrotron model, with inverse-Compton emerging only above a few GeV. The highratio of the early optical to late radio flux requires that the ambient mediumis a wind and that the forward-shock synchrotron spectrum peaks in the opticalat about 10 ks. The latter has two consequences: the wind must be very tenuousand the optical emission before 10 ks must arise from the reverse-shock, assuggested also by the bright optical flash that Raptor has monitored during theprompt emission phase (<100 s). The VLA radio emission is from thereverse-shock, the Swift X-ray emission is mostly from the forward-shock, butthe both shocks give comparable contributions to the Fermi GeV emission. Theweak wind implies a large blast-wave radius (8 t_{day}^{1/2} pc), whichrequires a very tenuous circumstellar medium, suggesting that the massivestellar progenitor of GRB 130427A resided in a super-bubble.

An external-shock model for GRB afterglow 130427A