3 years ago

Prompt gamma-ray emission of GRB 170817A associated to GW 170817: A consistent picture.

Houri Ziaeepour

The short GRB 170817A associated to the first detection of gravitation waves from a Binary Neutron Star (BNS) merger was in many ways unusual. Possible explanations are emission from cocoon at its break out, off-axis view of an structured jet, and on-axis ultra-relativistic jet with reduced density and Lorentz factor. Here we use a phenomenological model for generation and evolution of gamma-ray bursts to simulate prompt emission of GRB 170817A and test above proposals. We find that mildly relativistic cocoon models with characteristics considered in the literature generates too soft, too long and too bright prompt emission. Off-axis view of an structured jet with a Lorentz factor of about 10 can reproduce observations, but needs a very efficient transfer of kinetic energy to electrons in internal shocks, which is disfavored by particle in cell simulations. A diluted jet with a Lorentz factor about 100 seems the most plausible model. This is an evidence for intrinsic faintness of GRB 170817A. Based on this result and findings of relativistic magnetohydrodynamics simulations of BNS merger in the literature, we discuss physical and astronomical conditions, which may lead to such faint short GRBs. We identify small mass difference of neutron star progenitors, their old age and reduced magnetic field, and anti-alignment of spin-orbit angular momentum induced by environmental gravitational disturbances during the lifetime of the BNS as causes for the faintness of GRB 170817A, and predict that BNS mergers at lower redshifts generate on average fainter GRBs.

Publisher URL: http://arxiv.org/abs/1801.06124

DOI: arXiv:1801.06124v1

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