2023A&A...669A..36V

Context. LIGO-Virgo-KAGRA observations may identify the remnant of compact binary coalescence and core-collapse supernovae associated with gamma-ray bursts. The multimessenger event GW170817-GRB170817A appears ripe for this purpose thanks to its fortuitous close proximity at 40 Mpc. Its post-merger emission, ℰ_GW, in a descending chirp 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.
Aims. The complex merger sequence of GW170817 is probed for the central engine of GRB170817A by multimessenger calorimetry and event timing.
Methods. We used 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, covering a broad range in energies and timescales. 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. Probability density functions (PDF) 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 p₁ derives from causality in t_s given GW170817-GRB17081A (contextual). A statistically independent confirmation is presented in individual H1 and L1 analyses, quantified by a second PFA p₂ of consistency in their respective observations of t_s (acontextual). A combined PFA derives from their product since the mean and (respectively) the difference in timing are statistically independent.
Results. Applied to GW170817-GRB170817A, PFAs of event timing in t_s produce p₁ = 8.3 × 10^–4 and p₂ = 4.9 × 10^–5 of a post-merger output ℰ_GW ≃ 3.5% M_⊙c² (p₁p₂ = 4.1 × 10^–8, equivalent Z-score 5.48). 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 gravitational collapse to a Kerr black hole. Similar emission may be expected from energetic core-collapse supernovae producing black holes of interest to upcoming observational runs by LIGO-Virgo-KAGRA.