Central engine of GRB170817A: Neutron star versus Kerr black hole based on multimessenger calorimetry and event timing

Abstract

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.)