In LIGO ’ s O1 and O2 observational runs , the detectors were sensitive to stellar mass binary black hole coalescences with component masses up to 100 M _ { \odot } , with binaries with primary masses above 40 M _ { \odot } representing \gtrsim 90 \% of the total accessible sensitive volume .
Nonetheless , of the 5.9 detections ( GW150914 , LVT151012 , GW151226 , GW170104 , GW170608 , GW170814 ) reported by LIGO-Virgo , the most massive binary detected was GW150914 with a primary component mass of \sim 36 M _ { \odot } , far below the detection mass limit .
Furthermore , there are theoretical arguments in favor of an upper mass gap , predicting an absence of black holes in the mass range 50 \lesssim M \lesssim 135 M _ { \odot } .
We argue that the absence of detected binary systems with component masses heavier than \sim 40 M _ { \odot } may be preliminary evidence for this upper mass gap .
By allowing for the presence of a mass gap , we find weaker constraints on the shape of the underlying mass distribution of binary black holes .
We fit a power-law distribution with an upper mass cutoff to real and simulated BBH mass measurements , finding that the first 3.9 BBHs favor shallow power law slopes \alpha \lesssim 3 and an upper mass cutoff M _ { \mathrm { max } } \sim 40 M _ { \odot } .
This inferred distribution is entirely consistent with the two recently reported detections , GW170608 and GW170814 .
We show that with \sim 10 additional LIGO-Virgo BBH detections , fitting the BH mass distribution will provide strong evidence for an upper mass gap if one exists .