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.