Efficiency crisis of swift gamma-ray bursts with shallow X-ray afterglows: prior activity or time-dependent microphysics?
Abstract
Context: .Most X-ray afterglows of gamma-ray bursts (GRBs) observed by the Swift satellite have a shallow decay phase proptot-1/2 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, ⪆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 10^3-106 s prior to the main burst or if the microphysical parameter of the electron energy increases during the shallow decay, ɛe propto t1/2. 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.
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, ɛB propto t-0.6.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- October 2006
- DOI:
- 10.1051/0004-6361:20064939
- arXiv:
- arXiv:astro-ph/0511749
- Bibcode:
- 2006A&A...458....7I
- Keywords:
-
- gamma rays: bursts;
- gamma rays: theory;
- relativity;
- acceleration of particles;
- shock waves;
- Astrophysics
- E-Print:
- 7 pages, 1 figure, accepted for publication in A&