Minding the gap: GW190521 as a straddling binary

Maya Fishbach; Daniel E. Holz

doi:10.3847/2041-8213/abc827

paper

Minding the gap: GW190521 as a straddling binary

arXiv:2009.05472 · doi:10.3847/2041-8213/abc827

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 $\sim 50$ to $\sim 120$ 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 $m_{1} = 8 5_{- 14}^{+ 21} M_{⊙}$ , firmly above the inferred $m_{max}$ , and secondary mass $m_{2} = 6 6_{- 18}^{+ 17} M_{⊙}$ . 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 $m_{1}$ and $m_{2}$ , which assume that BH detector-frame masses are uniformly distributed, with this population-informed prior on $m_{2}$ , finding $m_{2} < 48 M_{⊙}$ 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 $m_{1} > 120 M_{⊙}$ , or 25\% probability for $m_{1} > 130 M_{⊙}$ ). 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.

5 pages main text, 3 figures, 3 page appendix. Accepted for publication in ApJL

Minding the gap: GW190521 as a straddling binary

Abstract

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