Efficiency crisis of swift gamma-ray bursts with shallow X-ray afterglows: prior activity or time-dependent microphysics?

Open Access
Publisher Website
Google Scholar
Abstract
Context.Most X-ray afterglows of gamma-ray bursts (GRBs) observed by the Swift satellite have a shallow decay phase in the first few hours.Aims.This is not predicted by the standard afterglow model and needs an explanation.Methods.We discuss that the shallow decay requires an unreasonably high gamma-ray efficiency, , within current models, which is difficult to produce by internal shocks. Such a crisis may be avoided if a weak relativistic explosion occurs ~ s prior to the main burst or if the microphysical parameter of the electron energy increases during the shallow decay, . The former explanation predicts a very long precursor, while both prefer dim optical flashes from the reverse shock, as was recently reported. We also calculate the multi-wavelength afterglows and compare them with observations.Results.No optical break at the end of the shallow X-ray decay indicates a preference for the time-dependent microphysics model with additionally decaying magnetic fields, .