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-1/2 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 ～ 103-106 s prior to the main burst or if the energy fraction that goes into electrons increases during the shallow decay, e t1/2. The former model predicts a very long precursor while either model would prefer dim optical flashes from the reverse shock as recently reported