Martin Lemoine, Zhuo Li, Xiang-Yu Wang
The origin of magnetic fields that permeate the blast waves of gamma-ray bursts is a long-standing problem. The present paper argues that in four GRBs revealing extended emission at >100 MeV, with follow-up in the radio, optical and X-ray domains at later times, this magnetisation can be described as the partial decay of the microturbulence that is generated in the shock precursor. Assuming that the extended high energy emission can be interpreted as synchrotron emission of shock accelerated electrons, we model the multi-wavelength light curves of GRB 090902B, GRB 090323, GRB 090328 and GRB 110731A, using a simplified then a full synchrotron calculation with power law decaying microturbulence \epsilon_B \propto t^{\alpha_t} (t denotes the time since injection through the shock, in the comoving blast frame). We find that these models point to a consistent value of the decay exponent -0.5 < \alpha_t < -0.4.
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http://arxiv.org/abs/1305.3689
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