Minding the Gap: GW190521 as a Straddling Binary

Abstract Models for black hole (BH) formation from stellar evolution robustly predict the existence of a pair-instability supernova (PISN) mass gap in the range ∼50 to ∼120 solar masses. This theoretical prediction is supported by the binary black holes (BBHs) of LIGO/Virgo’s first two observing runs, whose component masses are well fit by a power law with a maximum mass cutoff at . Meanwhile, the BBH event GW190521 has a reported primary mass of , firmly above the inferred , and secondary mass . Rather than concluding that both components of GW190521 belong to a new population of mass-gap BHs, we explore the conservative scenario in which GW190521's secondary mass belongs to the previously observed population of BHs. We replace the default priors on m 1 and m 2 , which assume that BH detector-frame masses are uniformly distributed, with this population-informed prior on m 2 , finding at 90% credibility. Moreover, because the total mass of the system is better constrained than the individual masses, the population prior on m 2 automatically increases the inferred m 1 to sit above the gap (39% for m 1 > 120 , or 25% probability for m 1 > 130 ). As long as the prior odds for a double-mass-gap BBH are smaller than , it is more likely that GW190521 straddles the pair-instability gap. We argue that GW190521 may be the first example of a straddling binary black hole, composed of a conventional stellar mass BH and a BH from the “far side” of the PISN mass gap.