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Spectral evolution in gamma-ray bursts: predictions of the internal shock model and comparison to observations. (arXiv:1404.4577v1 [astro-ph.HE])

April 18th, 2014

Z. Bosnjak (1,2,3), F. Daigne (2) ((1) CEA-Saclay, (2) UPMC-CNRS, Institut d'Astrophysique de Paris, (3) University of Rijeka)

Several trends have been identified in the prompt gamma-ray burst (GRB)
emission: e.g. hard-to-soft evolution, pulse width evolution with energy, time
lags, hardness-intensity/-fluence correlations. Recently Fermi has
significantly extended the spectral coverage of GRB observations and improved
the characterization of this spectral evolution. We study how internal shocks
can reproduce these observations. In this model the emission comes from the
synchrotron radiation of shock accelerated electrons, and the spectral
evolution is governed by the evolution of the physical conditions in the
shocked regions. We present a comprehensive set of simulations of a single
pulse and investigate the impact of the model parameters, related to the shock
microphysics and to the initial conditions in the ejecta.

We find a general qualitative agreement between the model and the various
observations used for the comparison. All these properties or relations are
governed by the evolution of the peak energy and photon indices of the
spectrum. In addition, we identify the conditions for a quantitative agreement.
We find that the best agreement is obtained for (i) steep electron slopes
(p>~2.7), (ii) microphysics parameters varying with shock conditions so that
more electrons are accelerated in stronger shocks, (iii) steep variations of
the initial Lorentz factor in the ejecta. When simulating short GRBs by
contracting all timescales, all other parameters being unchanged, we show that
the hardness-duration correlation is reproduced, as well as the evolution with
duration of the pulse properties. Finally, we investigate the signature at high
energy of these different scenarios and find distinct properties - delayed
onset, longer emission, and flat spectrum in some cases - suggesting that
internal shocks could have a significant contribution to the prompt LAT
emission. [abridged]


http://arxiv.org/abs/1404.4577


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