GW190521 may be an intermediate mass ratio inspiral

TL;DR

This paper analyzes GW190521 under different priors, revealing possible high-mass-ratio configurations and discussing implications for black hole mass gaps, while also considering potential electromagnetic counterparts.

Contribution

It introduces a detailed Bayesian analysis of GW190521 with alternative priors, highlighting the possibility of high-mass-ratio black hole mergers previously underexplored.

Findings

High-mass-ratio mode with component masses around 171 and 16 solar masses.

Bimodal posterior distribution depending on prior assumptions.

No strong evidence for electromagnetic counterpart association.

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 differ from 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 ($\ln\mathcal{B} \gtrsim -4$).