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

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) M-circle dot at redshift z = 1.37(-0.64)(0.93). The primary and secondary black hole masses are constrained to 94(-28)(+44) M-circle dot and 62(-25)(+30) M-circle dot, respectively, with 90% confidence. Across all signal models, we find greater than or similar to 70% probability for the effective spin parameter chi(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 (greater than or similar to 100M(circle dot)) in the current GW network.