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 ∼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 ${m}_{\max }={40.8}_{-4.4}^{+11.8}\,{\text{}}{M}_{\odot }$. Meanwhile, the BBH event GW190521 has a reported primary mass of ${m}_{1}={85}_{-14}^{+21}\,{\text{}}{M}_{\odot }$, firmly above the inferred ${m}_{\max }$, and secondary mass ${m}_{2}={66}_{-18}^{+17}\ {\text{}}{M}_{\odot }$. 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 m1 and m2, which assume that BH detector-frame masses are uniformly distributed, with this population-informed prior on m2, finding ${m}_{2}\lt 48\,{\text{}}{M}_{\odot }$ at 90% credibility. Moreover, because the total mass of the system is better constrained than the individual masses, the population prior on m2 automatically increases the inferred m1 to sit above the gap (39% for m1 > 120 ${\text{}}{M}_{\odot }$, or 25% probability for m1 > 130 ${\text{}}{M}_{\odot }$). As long as the prior odds for a double-mass-gap BBH are smaller than $\sim 1\,:\,15$, 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.