GW190521 May Be an Intermediate-mass Ratio Inspiral

GW190521 is the first confident observation of a binary black hole merger with total mass $M\gt 100\,{M}_{\odot }$. 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 ${m}_{1}^{\mathrm{src}}={168}_{-61}^{+15}\,{M}_{\odot }$ and ${m}_{2}^{\mathrm{src}}={16}_{-3}^{+33}\,{M}_{\odot }$. The uniform in component-mass prior yields a bimodal posterior distribution. There is a low-mass-ratio mode ($q\lt 4$) with ${m}_{1}^{\mathrm{src}}={100}_{-18}^{+17}\,{M}_{\odot }$ and ${m}_{2}^{\mathrm{src}}={57}_{-16}^{+17}\,{M}_{\odot }$ and a high-mass-ratio mode ($q\geqslant 4$) with ${m}_{1}^{\mathrm{src}}={166}_{-35}^{+16}\,{M}_{\odot }$ and ${m}_{2}^{\mathrm{src}}={16}_{-3}^{+14}\,{M}_{\odot }$. Although the two modes have nearly equal posterior probability, the maximum-likelihood parameters are in the high-mass-ratio mode, with ${m}_{1}^{\mathrm{src}}=171\,{M}_{\odot }$ and ${m}_{2}^{\mathrm{src}}=16\,{M}_{\odot }$, 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 ($\mathrm{ln}{ \mathcal B }\gtrsim -4$).