Efficiency Crisis of Swift Gamma-Ray Bursts with Shallow X-ray Afterglows: Prior Activity or Time-Dependent Microphysics?

Most X-ray afterglows of gamma-ray bursts (GRBs) observed by the Swift satellite have a shallow decay phase ∝ t in the first thousands of seconds. We discuss that the shallow decay requires an unreasonably high gamma-ray efficiency, & 75–90%, within current models, which is difficult to be produced by internal shocks. Such a crisis may be avoided if a weak relativistic explosion occurs ∼ 10–10 s prior to the main burst or if the energy fraction that goes into electrons increases during the shallow decay, ǫe ∝ t . The former model predicts a very long precursor while either model would prefer dim optical flashes from the reverse shock as recently reported. Subject headings: gamma rays: bursts — gamma rays: theory — relativity