Gravitational wave measurements provide the most robust constraints on the mass of astrophysical black holes .
Using state-of-the-art parameter estimation techniques and gravitational waveform models , we infer the source parameters of the loudest marginal trigger , 170502 , found in LIGO from 2015-2017 .
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 } ~ { } M _ { \odot } .
This is a factor of \sim 3 / 2 higher than that of the heaviest confirmed detection , GW170729 .
The primary and secondary black hole masses are constrained to 94 ^ { +44 } _ { -28 } ~ { } M _ { \odot } and 62 ^ { +30 } _ { -25 } ~ { } M _ { \odot } respectively with 90 % confidence .
We find \chi _ { \mathrm { eff } } = 0.49 ^ { +0.31 } _ { -0.63 } , the effective spin aligned with the total angular momentum vector , which has most of its support at larger values than the other O1/O2 events .
Further , we find the inclusion of higher order harmonics narrows the confidence region for the total binary mass by 10 % .
Our study illustrates the necessary techniques for inferring the physical parameters of intermediate mass black hole binary candidates ( \gtrsim 100 ~ { } \mathrm { M } _ { \odot } ) in the current gravitational wave network .