Gravitational-wave observation of the spin-down of a Kerr black hole in GRB170817A

We report on an un-modeled search for the trigger signaling the birth of the central engine of GRB170817A in gravitational-wave analysis of merged and independent H1- and L1-spectrograms. For the trigger time t_s, we derive the respective Probability Density Functions (PDFs) of a descending chirp in gravitational radiation contemporaneous with GRB170817A. In energy and frequency, this output is identified with the spin-down of a Kerr black hole formed in delayed gravitational collapse of the initial hyper-massive neutron star produced in the progenitor GW170817. These PDFs are obtained at high-resolution by exascale high-performance computing. Event timing inferred from the PDFs of t_s produces two statistically independent Probabilities of False Alarm (PFAs) based on causality and, respectively, consistency in H1- and L1-observations, the combined result of which establishes an observation at 5.5σ C.L. By university of Kerr black holes, similar output may be expected from long gamma-ray bursts (GRB)s and their parent population of core-collapse supernovae of type Ib/c. (van Putten & Della Valle, 2023, A&A, 669, A36, https://doi.org/10.1051/0004-6361/202142974)