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

¹ Department of Physics, Kyoto University, Kyoto 606-8502, Japan e-mail: ioka@tap.scphys.kyoto-u.ac.jp
² Department of Physics, Hiroshima University, Higashi-Hiroshima 739-8526, Japan

Received: 31 January 2006
Accepted: 29 June 2006

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, .