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

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 $157_{-41}^{+55}{\mathrm{M}}_{\odot }$ at redshift $z=1.37_{-0.64}^{+0.93}$. The primary and secondary black hole masses are constrained to $94_{-28}^{+44}{\mathrm{M}}_{\odot }$ and $62_{-25}^{+30}{\mathrm{M}}_{\odot }$ 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 $(\gtrsim 100{\mathrm{M}}_{\odot })$ in the current GW network.

6 pages, 4 figures, 3 tables; Accepted for publication in the Astrophysical Journal