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

Free access article

Issue		A&A Volume 458, Number 1, October IV 2006


Page(s)		7 - 12
Section		Astrophysical processes
DOI		http://dx.doi.org/10.1051/0004-6361:20064939

A&A 458, 7-12 (2006)
DOI: 10.1051/0004-6361:20064939

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

K. Ioka¹, K. Toma¹, R. Yamazaki² and T. Nakamura¹

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 $\propto$ t^-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, $\ga$ $75{-}90\%$ , within current models, which is difficult to produce by internal shocks. Such a crisis may be avoided if a weak relativistic explosion occurs ~ 10³-10⁶ s prior to the main burst or if the microphysical parameter of the electron energy increases during the shallow decay, $\epsilon_{\rm e} \propto t^{1/2}$ . 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.
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, $\epsilon_B \propto t^{-0.6}$ .

Key words: gamma rays: bursts -- gamma rays: theory -- relativity -- acceleration of particles -- shock waves

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