GW190521 May Be an Intermediate-mass Ratio Inspiral

GW190521 is the first confident observation of a binary black hole merger with total mass M > 100 M-circle dot. Given the lack of observational constraints at these masses, we analyze GW190521 considering two different priors for the binary's masses: uniform in mass ratio and source-frame total mass, and uniform in source-frame component masses. For the uniform in mass-ratio prior, we find that the component masses are m(1)(src) = 168(-61)(+15) M-circle dot and m(2)(src) = 16(-3)(+33) M-circle dot. The uniform in component-mass prior yields a bimodal posterior distribution. There is a low-mass-ratio mode (q < 4) with m(1)(src) = 100(-18)(+17) M-circle dot and m(2)(src) = 57(-16)(+17) M-circle dot and a high-mass-ratio mode (q >= 4) with m(1)(src) = 166(-35)(+16) M-circle dot and m(2)(src) = 16(-3)(+14) M-circle dot. Although the two modes have nearly equal posterior probability, the maximum-likelihood parameters are in the high-mass-ratio mode, with m(1)(src) = 171 M-circle dot and m(2)(src) = 16 M-circle dot, and a signal-to-noise ratio of 16. These results are consistent with the proposed mass gap produced by pair-instability in supernovae. Our results differ from those published in Abbott et al. We find that a combination of the prior used and the constraints applied may have prevented that analysis from sampling the high-mass-ratio mode. An accretion flare in AGN J124942.3+344929 was observed in possible coincidence with GW190521 by the Zwicky Transient Facility. We report parameters assuming a common origin; however, the spatial agreement of GW190521 and the electromagnetic flare alone does not provide convincing evidence for the association (ln B greater than or similar to -4).

Automated summary

GW190521 is the first confident observation of a binary black hole merger with total mass exceeding 100 solar masses. Different priors led to a bimodal posterior distribution in mass, with the high-mass-ratio mode dominating in maximum-likelihood parameters. Results differ from previous studies, suggesting the presence of a mass gap and emphasizing the importance of considering all possible scenarios in analyses.