http://arxiv.org/abs/1806.11161
The gamma-ray burst that followed the first detection of gravitational waves from the merger of a Binary Neutron Stars (BNS) and its low energy counterparts were in many respects unusual. In a previous work we used a phenomenological formulation of relativistic shocks and synchrotron emission to analyze the prompt gamma-ray emission of GW/GRB 170817A. We showed that it was most probably intrinsically faint due to the old age of the progenitor neutron stars and their reduced magnetic field. Here we use the same model to analyze late afterglows of this event. We find that a collision between a mildly relativistic outflow from the merger, also called the cocoon, with the circumburst material can explain observations if synchrotron self-absorption of radio emission and local extinction of optical/IR photons are taken into account. These processes are overlooked in the literature. The outflow is presumably the remnant of polar dynamical ejecta during the collision of the neutron stars and at the time of its encounter with circumburst material could have been still magnetized. The most plausible origin for optical extinction is an old faint star cluster surrounding the BNS, which additionally had helped its formation and merger. Such an environment is consistent with our conclusions about evolution and properties of the progenitor neutron stars and may evade present observational constraints. Alternatively, if the synchrotron emission was produced internally through collisions of density shells in the cocoon, the extinction has occurred inside the outflow itself rather than externally. Without a significant extinction of optical/IR emission an additional source of X-ray would be necessary. Decay of medium and heavy nuclides produced by the kilonova, including r-processes, and recombination of electrons may be the missing source of X-ray excess.
H. Ziaeepour
Mon, 2 Jul 18
37/70
Comments: 21 pags, 4 figures
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