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 γ-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 γ-rays that we have seen were a regular sGRB viewed off-axis. We revisit this question following recent refined analyses of the γ-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 $\lesssim 5^{\circ }$ and $E_{\mathrm{iso}}>10^{52}\mathrm{ergs}$. We show here that: (i) The region emitting the observed $\gamma$-rays must have been moving with a Lorentz factor $\Gamma \gtrsim 5$; (ii) The observed γ-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.

7 pages, 4 figures, submitted to MNRAS, comments welcome!!!

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• The collimation of relativistic jets in post-neutron star binary merger simulations
• Impact of anisotropic ejecta on jet dynamics and afterglow emission in binary neutron-star mergers
• Very High Energy Afterglow of Structured Jets: GW 170817 and Prospects for Future Detections
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• Could an X-ray Flare after GRB 170817A Originate from a Post-merger Slim Accretion Disc?
• How Do Nuclear Isomers Influence the Gamma-Ray Bursts in Binary Neutron Star Mergers?
• Detectability of Electromagnetic counterparts from Neutron Star mergers: prompt emission vs afterglow
• The non-thermal emission following GW170817 is consistent with a conical radially-stratified outflow with initial Lorentz factor $\lesssim 10$
• Low-efficiency long gamma-ray bursts: A case study with AT2020blt