  The central engine of GRB170817A post-merger to GW170817 is probed by
GW-calorimetry and event timing, applied to a post-merger descending chirp
which can potentially break the degeneracy in spin-down of a neutron star or
black hole remnant by the relatively large energy reservoir in the angular
momentum, $E_J$, of the latter according to the Kerr metric. This analysis
derives from model-agnostic spectrograms with equal sensitivity to ascending
and descending chirps generated by time-symmetric butterfly matched filtering.
The sensitivity was calibrated by response curves generated by software
injection experiments. The statistical significance for candidate emission from
the central engine of GRB170817A is expressed by probabilities of false alarm
(PFA; type I errors) derived from an event-timing analysis. PDFs were derived
for start-time $t_s$, identified via high-resolution image analyses of the
available spectrograms. For merged (H1,L1)-spectrograms of the LIGO detectors,
a PFA $p_1$ derives from causality in $t_s$ given GW170817-GRB17081A. A
statistically independent confirmation is presented in individual H1 and L1
analyses, in a second PFA $p_2$ of consistency in their respective observations
of $t_s$. A combined PFA derives from their product since mean and
(respectively) difference in timing are statistically independent. Applied to
GW170817-GRB170817A, PFAs of event timing in $t_s$ produce $p_1=8.3\times
10^{-4}$ and $p_2=4.9\times 10^{-5}$ of a post-merger output ${\cal
E}_{GW}\simeq 3.5\%M_\odot c^2$ ($p_1p_2=4.1\times 10^{-8}$, equivalent
$Z$-score 5.48). ${\cal E}_{GW}$ exceeds $E_J$ of the hyper-massive neutron
star in the immediate aftermath of GW170817, yet it is consistent with $E_J$
rejuvenated in delayed gravitational collapse to a Kerr black hole. Similar
emission may be expected from energetic core-collapse supernovae producing
black holes. (Abbr.)
