GRB 170817A as a jet counterpart to gravitational wave trigger GW 170817

Abstract

{\it Fermi}/GBM (Gamma-ray Burst Monitor) and INTEGRAL (the International Gamma-ray Astrophysics Laboratory) reported the detection of the $\gamma$-ray counterpart, GRB 170817A, to the LIGO (Light Interferometer Gravitational-wave Observatory)/{\it Virgo} gravitational wave detected binary neutron star merger, GW 170817. GRB 170817A is likely to have an internal jet or another origin such as cocoon emission, shock-breakout, or a flare from a viscous disc. In this paper, we assume that the $\gamma$-ray emission is caused by energy dissipation within a relativistic jet and we model the afterglow synchrotron emission from a reverse- and forward-shock in the outflow. We show the afterglow for a low-luminosity $\gamma$-ray burst (GRB) jet with a high Lorentz-factor ($\Gamma$); a low-$\Gamma$ and low-kinetic energy jet; a low-$\Gamma$, high kinetic energy jet; structured jets viewed at an inclination within the jet-half-opening angle; and an off-axis `typical' GRB jet. All jet models will produce observable afterglows on various timescales. The late-time afterglow from 10-110 days can be fit by a Gaussian structured jet viewed at a moderate inclination, however, the GRB is not directly reproduced by this model. These jet afterglow models can be used for future GW detected NS merger counterparts with a jet afterglow origin.