Minding the Gap: GW190521 as a Straddling Binary
Abstract
Models for black hole (BH) formation from stellar evolution robustly predict the existence of a pairinstability 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 massgap 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 detectorframe masses are uniformly distributed, with this populationinformed prior on m_{2}, 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 m_{2} automatically increases the inferred m_{1} to sit above the gap (39% for m_{1} > 120 ${\text{}}{M}_{\odot }$ , or 25% probability for m_{1} > 130 ${\text{}}{M}_{\odot }$ ). As long as the prior odds for a doublemassgap BBH are smaller than $\sim 1\,:\,15$ , it is more likely that GW190521 straddles the pairinstability 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.
 Publication:

The Astrophysical Journal
 Pub Date:
 December 2020
 DOI:
 10.3847/20418213/abc827
 arXiv:
 arXiv:2009.05472
 Bibcode:
 2020ApJ...904L..26F
 Keywords:

 Astrophysical black holes;
 Gravitational waves;
 Gravitational wave astronomy;
 Bayesian statistics;
 Intermediatemass black holes;
 98;
 678;
 675;
 1900;
 816;
 Astrophysics  High Energy Astrophysical Phenomena;
 General Relativity and Quantum Cosmology
 EPrint:
 5 pages main text, 3 figures, 3 page appendix. Accepted for publication in ApJL