Probing Extremal Gravitational-Wave Events with Coarse-Grained Likelihoods

Kavli Affiliate: Daniel E. Holz

| First 5 Authors: Reed Essick, Amanda Farah, Shanika Galaudage, Colm Talbot, Maya Fishbach

| Summary:

As catalogs of gravitational-wave transients grow, new records are set for
the most extreme systems observed to date. The most massive observed black
holes probe the physics of pair instability supernovae while providing clues
about the environments in which binary black hole systems are assembled. The
least massive black holes, meanwhile, allow us to investigate the purported
neutron star-black hole mass gap, and binaries with unusually asymmetric mass
ratios or large spins inform our understanding of binary and stellar evolution.
Existing outlier tests generally implement leave-one-out analyses, but these do
not account for the fact that the event being left out was by definition an
extreme member of the population. This results in a bias in the evaluation of
outliers. We correct for this bias by introducing a coarse-graining framework
to investigate whether these extremal events are true outliers or whether they
are consistent with the rest of the observed population. Our method enables us
to study extremal events while testing for population model misspecification.
We show that this ameliorates biases present in the leave-one-out analyses
commonly used within the gravitational-wave community. Applying our method to
results from the second LIGO–Virgo transient catalog, we find qualitative
agreement with the conclusions of Abbott et al, ApJL 913 L7 (2021). GW190814 is
an outlier because of its small secondary mass. We find that neither GW190412
nor GW190521 are outliers.

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