Inferring Parameters of GW170502: The Loudest Intermediate-mass Black Hole Trigger in LIGO's O1/O2 data

TL;DR

This paper estimates the parameters of the loudest intermediate-mass black hole candidate GW170502 using advanced models and techniques, providing insights into its mass, spin, and the role of higher-order modes in gravitational wave data.

Contribution

It introduces a novel parameter estimation approach for intermediate-mass black hole mergers, improving mass and spin constraints and assessing higher-order mode significance.

Findings

GW170502 has a total mass of approximately 157 solar masses.

There is over 70% probability that the effective spin parameter is greater than 0.1.

Including higher-order modes reduces the primary mass confidence region by 10%.

Abstract

Gravitational wave (GW) measurements provide the most robust constraints of the mass of astrophysical black holes. Using state-of-the-art GW signal models and a unique parameter estimation technique, we infer the source parameters of the loudest marginal trigger, GW170502, found by LIGO from 2015 to 2017. If this trigger is assumed to be a binary black hole merger, we find it corresponds to a total mass in the source frame of $157^{+55}_{-41}~\rm{M}_\odot$ at redshift $z=1.37^{+0.93}_{-0.64}$. The primary and secondary black hole masses are constrained to $94^{+44}_{-28}~\rm{M}_{\odot}$ and $62^{+30}_{-25}~\rm{M}_{\odot}$ respectively, with 90\% confidence. Across all signal models, we find $\gtrsim 70\%$ probability for the effective spin parameter $\chi_\mathrm{eff}>0.1$. Furthermore, we find that the inclusion of higher-order modes in the analysis narrows the confidence region for the primary black hole mass by 10\%, however, the evidence for these modes in the data remains negligible. The techniques outlined in this study could lead to robust inference of the physical parameters for all intermediate-mass black hole binary candidates $(\gtrsim100~\mathrm{M}_\odot)$ in the current GW network.