2020ApJ...900...80U

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⁺⁵⁵M_☉ at redshift z=1.37_–0.64^+0.93. The primary and secondary black hole masses are constrained to 94_–28⁺⁴⁴M_☉ and 62_–25⁺³⁰M_☉, respectively, with 90% confidence. Across all signal models, we find >=70% probability for the effective spin parameter χ_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 (>=100 M_☉) in the current GW network.