Minding the Gap: GW190521 as a Straddling Binary

Models for black hole (BH) formation from stellar evolution robustly predict the existence of a pair-instability supernova (PISN) mass gap in the range similar to 50 to similar 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 m(max) = 40.8(-4.4)(+11.8) M-circle dot. Meanwhile, the BBH event GW190521 has a reported primary mass of m(1) = 85(-14)(+21) M-circle dot, firmly above the inferred m(max), and secondary mass m(2) = 66(-18)(+17) M-circle dot. 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 m(2) < 48 M-circle dot 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 M-circle dot, or 25% probability for m(1) > 130 M-circle dot). As long as the prior odds for a double-mass-gap BBH are smaller than similar to 1 : 15, 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.