2019ApJ...876L...2V

GW170817/GRB 170817A probably marks a double neutron star (NS) coalescence. Extended emission t_s≃(0.67±0.03) s post-merger shows an estimated energy output E ≃(3.5±1) % M_☉c² determined by response curves to power-law signal injections, where c is the velocity of light. It provides calorimetric evidence for a rotating black hole of ∼3M_☉, inheriting the angular momentum J of the merged hyper-massive NS in the immediate aftermath of GW170817 following core-collapse about or prior to t_s. Core-collapse greatly increases the central energy reservoir to E_J≲1M_☉c², accounting for E even at modest efficiencies in radiating gravitational waves through a non-axisymmetric thick torus. The associated multi-messenger output in ultra-relativistic outflows and sub-relativistic mass-ejecta is consistent with observational constraints from the gamma-ray burst afterglow emission of GRB 170817A and accompanying kilonova.