Multi-messenger Extended Emission from the Compact Remnant in GW170817

GW170817/GRB 170817A probably marks a double neutron star (NS) coalescence. Extended emission ${t}_{s}\simeq (0.67\pm 0.03)$ s post-merger shows an estimated energy output ${ \mathcal E }\simeq (3.5\pm 1) \% {M}_{\odot }{c}^{2}$ 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 $\sim 3{M}_{\odot }$, inheriting the angular momentum J of the merged hyper-massive NS in the immediate aftermath of GW170817 following core-collapse about or prior to ts. Core-collapse greatly increases the central energy reservoir to ${E}_{J}\lesssim 1{M}_{\odot }{c}^{2}$, accounting for ${ \mathcal E }$ 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.