The gamma-rays that accompanied GW170817 and the observational signature of a magnetic jet breaking out of NS merger ejecta

摘要

We present the first relativistic MHD numerical simulation of a magnetic jetthat propagates through and emerges from the dynamical ejecta of a binaryneutron star merger. Generated by the magnetized rotation of the mergerremnant, the jet propagates through the ejecta and produces an energetic cocoonthat expands at mildly relativistic velocities and breaks out of the ejecta. Weshow that if the ejecta has a low-mass ($\sim10^{-7} M_\odot$) high-velocity($v\sim0.85$ c) tail, the cocoon shock breakout will generate $\gamma$-rayemission that is comparable to the observed short GRB170817A that accompaniedthe recent gravitational wave event GW170817. Thus, we propose that this GRB,which is quite different from all other short GRBs observed before, wasproduced by a different mechanism. We expect, however, that such events arenumerous and many will be detected in coming LIGO-Virgo runs.