Multi-messenger Extended Emission from the Compact Remnant in GW170817

GW170817/GRB170817A probably marks a double neutron star coalescence. Extended Emission $t_s\simeq (0.67\pm0.03)$\,s post-merger shows an estimated energy output ${\cal E}\simeq (3.5\pm1)\%M_\odot c^2$ 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 $\sim 3M_\odot$, inheriting the angular momentum $J$ of the merged hyper-massive neutron star in the immediate aftermath of GW170817 following core-collapse about or prior to $t_s$. Core collapse greatly increases the central energy reservoir to $E_J\lesssim 1M_\odot c^2$, accounting for ${\cal E}$ 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 GRB-afterglow emission of GRB170817A and accompanying kilonova.