2020ApJ...904L..26F

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 M_☉. Meanwhile, the BBH event GW190521 has a reported primary mass of m₁=85_–14⁺²¹ M_☉, firmly above the inferred m_{max}, and secondary mass m₂=66_–18⁺¹⁷ M_☉. 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 m₂< 48 M_☉ 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 M_☉, or 25% probability for m₁ > 130 M_☉). As long as the prior odds for a double-mass-gap BBH are smaller than ∼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.