  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.
