GW190521 may be an intermediate mass ratio inspiral
Nitz, A., Capano, C.
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 ($\\ln\\mathcal{B} \\gtrsim -4$).