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 $\sim50$ to $\sim120$ 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 $m_\mathrm{max}=40.8^{+11.8}{-4.4}\,M\odot$. Meanwhile, the BBH event GW190521 has a reported primary mass of $m_1=85^{+21}{-14}\,M\odot$, firmly above the inferred $m_\mathrm{max}$, and secondary mass $m_2=66^{+17}{-18}\,M\odot$. 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 $m_1$ and $m_2$, which assume that BH detector-frame masses are uniformly distributed, with this population-informed prior on $m_2$, finding $m_2<48\,M_\odot$ 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 \emph{above} the gap (39\% for $m_1 > 120\,M_\odot$, or 25\% probability for $m_1>130\,M_\odot$). 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.