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Abstract

GW190521 is the first confident observation of a binary black hole merger
with total mass $M > 100\,\mathrm{M}_{\odot}$. 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 $m_1^{\mathrm{src}} =
168_{-61}^{+15}\,\mathrm{M}_{\odot}$ and $m_2^{\mathrm{src}} =
16_{-3}^{+33}\,\mathrm{M}_{\odot}$. The uniform in component-mass prior yields
a bimodal posterior distribution. There is a low-mass-ratio mode ($q<4$) with
$m_1^{\mathrm{src}} = 100_{-18}^{+17}\,\mathrm{M}_{\odot}$ and
$m_2^{\mathrm{src}} = 57_{-16}^{+17}\,\mathrm{M}_{\odot}$ and a high-mass-ratio
mode ($q\geq4$) with $m_1^{\mathrm{src}} = 166_{-35}^{+16}\,\mathrm{M}_{\odot}$
and $m_2^{\mathrm{src}} = 16_{-3}^{+14}\,\mathrm{M}_{\odot}$. Although the two
modes have nearly equal posterior probability, the maximum-likelihood
parameters are in the high-mass ratio mode, with $m_1^{\rm src} =
171\,M_{\odot}$ and $m_2^{\rm src} = 16\,M_{\odot}$, and a signal-to-noise
ratio of $16$. These results are consistent with the proposed "mass gap"
produced by pair-instability in supernova. Our results are inconsistent with
those published in Abbott et al. (2020b). 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 (ZTF). We report parameters assuming a common origin; however, the
spatial agreement of GW190521 and the EM flare alone does not provide
convincing evidence for the association ($\ln\mathcal{B} \gtrsim -4$).