Insights gained from GW170817/GRB 170817A: BNS merger remnant and outflow structure

                                                                                                                           The first-ever gravitational wave detection from a binary neutron star (NS) merger in GW170817 was rich in electromagnetic counterparts. Each provided invaluable insights into the properties of the merger remnant and post-merger outflows, which also featured a relativistic jet. First, the slightly delayed (by 1.74 sec) onset of the prompt gamma-ray emission from GRB170817A along with the properties of the kilonova emission hinted at a massive NS remnant and delayed collapse to a black hole. Second, the sub-luminous and only slightly softer prompt gamma-ray emission was used to show that the relativistic jet that powered the prompt emission was viewed off-axis. Third, the peculiar afterglow emission, with a long-lasting rise in flux to its peak at ~150 days, and its post-peak properties ultimately revealed that the afterglow was powered by a structured jet with a compact relativistic core. In this talk, I will argue that the merger remnant of GW170817 was a hyper-massive NS and present new findings on its collapse time to a black hole based on two distinct lines of arguments, one accounting for the blue kilonova ejecta mass and the other accounting for the delay in jet launching and breakout. I will present results from both 2D hydrodynamical numerical simulations and semi-analytic models that were successfully used to explain the off-axis afterglow lightcurve. In particular, I will highlight that even an initially top-hat jet can explain the afterglow lightcurve and image properties of this event. Finally, I’ll emphasize the importance of three diagnostics, namely the afterglow flux-centroid motion, image size, and polarization, that can be instrumental in pinning down the outflow structure and properties of the post-shock magnetic field in future events.