Astronomy and Astrophysics – Astrophysics
Scientific paper
2005-11-27
Astronomy and Astrophysics
Astrophysics
7 pages, 1 figure, accepted for publication in A&A
Scientific paper
10.1051/0004-6361:20064939
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 few hours. This is not predicted by the standard afterglow model and needs an explanation. We discuss that the shallow decay requires an unreasonably high gamma-ray efficiency, >75-90%, within current models, which is difficult to produce by internal shocks. Such a crisis may be avoided if a weak relativistic explosion occurs ~10^3-10^6 s prior to the main burst or if the microphysical parameter of the electron energy increases during the shallow decay, \epsilon_e ~ t^{1/2}. 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. 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, \epsilon_B ~ t^{-0.6}.
Ioka Kunihito
Nakamura Takashi
Toma Kenji
Yamazaki Ryo
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