Off-axis afterglow light curves and images from 2D hydrodynamic simulations of double-sided GRB jets in a stratified external medium

Abstract

Gamma-ray burst (GRB) jets are narrow, and thus typically point away from us. They are initially ultra-relativistic, causing their prompt $\gamma$-ray and early afterglow emission to be beamed away from us. However, as the jet gradually decelerates its beaming cone widens and eventually reaches our line of sight and the afterglow emission may be detected. Such orphan afterglows were not clearly detected so far. Nevertheless, they should be detected in upcoming optical or radio surveys, and it would be challenging to clearly distinguish between them and other types of transients. Therefore, we perform detailed, realistic calculations of the expected afterglow emission from GRB jets viewed at different angles from the jet's symmetry axis. The dynamics are calculated using 2D relativistic hydrodynamics simulations of jets propagating into different power-law external density profiles, $\rho_{\rm ext}\propto{}R^{-k}$ for $k=0,\,1,\,1.5,\,2$, ranging from a uniform ISM-like medium ($k=0$) to a stratified steady stellar-wind like profile ($k=2$). We calculate radio, optical and X-ray lightcurves, and the evolution of the radio afterglow image size, shape and flux centroid. This may help identify misaligned relativistic jets, whether initially ultra-relativistic and producing a GRB for observers within their beam, or (possibly intrinsically more common) moderately relativistic, in either (i) nearby supernovae Ib/c (some of which are associated with long duration GRBs), or (ii) in binary neutron star mergers, which may produce short duration GRBs, and may also be detected in gravitational waves (e.g. GW$\,$170827/GRB$\,$170817A with a weak prompt $\gamma$-ray emission may harbor an off-axis jet).