Constraints on the emitting region of the gamma-rays observed in GW170817

Abstract: The gravitational waves from the neutron star merger event GW170817 were accompanied by an unusually weak short GRB 170817A, by an optical/IR macronova/kilonova and by a long lasting radio to X-rays counterpart. While association of short GRBs with mergers was predicted a long time ago, the luminosity of this prompt {\gamma}-ray emission was weaker by a few orders of magnitude than all known previous sGRBs and it was softer than typical sGRBs. This raise the question whether the {\gamma}-rays that we have seen were a regular sGRB viewed off-axis. We revisit this question following recent refined analyses of the {\gamma}-ray signal and the VLBI observations that revealed the angular structure of the relativistic outflow: observing angle of $\sim\,20^\circ$, a narrow jet with core $\lesssim5^\circ$ and $E_{\rm iso}>10^{52}\,{\rm ergs}$. We show here that: (i) The region emitting the observed $\gamma$-rays must have been moving with a Lorentz factor $\Gamma\gtrsim5$; (ii) The observed {\gamma}-rays were not "off-axis" emission (viewing angle $>1/\Gamma$) emerging from the core of the jet, where a regular sGRB was most likely produced; (iii) The $\gamma$-ray emission region was either "on-axis" (at an angle $<1/\Gamma$) or if it was "off-axis" then the observing angle must have been small ($<5^\circ$) and the on-axis emission from this region was too faint and too hard to resemble a regular sGRB.