The short GRB 170817A: Modelling the off-axis emission and implications on the ejecta magnetization

TL;DR

This paper models the off-axis emission of the short GRB 170817A, linking multi-wavelength observations to shock models and suggesting magnetic field amplification in the neutron star merger environment.

Contribution

It introduces a comprehensive off-axis synchrotron model for the GRB afterglow and gamma-ray emission, highlighting the role of magnetic field amplification in the merger ejecta.

Findings

X-ray, optical, and radio fluxes fit off-axis synchrotron forward-shock model.

Gamma-ray peak explained by internal and external shocks, possibly via synchrotron self-Compton.

Evidence suggests different regions for gamma-ray and afterglow emissions, with magnetic fields amplified in the merger environment.

Abstract

The short GRB 170817A, detected by the Fermi Gamma-ray Burst Monitor, orbiting satellites and ground-based telescopes, was the electromagnetic counterpart of a gravitational-wave transient (GW170817) from a binary neutron star merger. After this merger the $\gamma$-ray light curve exhibited a faint peak at $\sim$ 1.7s and the X-ray, optical and radio light curves displayed an extended emission which increased in brightness up to $\sim$ 160 days. In this paper, we show that the X-ray, optical and radio fluxes are consistent with the synchrotron forward-shock model viewed off-axis when the matter in the outflow is parametrized through a power law velocity distribution. We discuss the origin of the $\gamma$-ray peak in terms of internal and external shocks. We show that the $\gamma$-ray flux might be consistent with a synchrotron self-Compton reverse-shock model observed at high latitudes. Comparing the best-fit values obtained after describing the $\gamma$-ray, X-ray, optical and radio fluxes with our model, we find that the afterglow and $\gamma$-ray emission occurred in different regions and also evidence to propose that the progenitor environment was entrained with magnetic fields and therefore, we argue for the presence of the magnetic field amplification in the binary neutron star merger.