GW190521 may be an intermediate mass ratio inspiral

New analysis of public LIGO/Virgo data provides alternative explanation for gravitational-wave signal

January 15, 2021

GW190521 is a somewhat mysterious gravitational-wave signal published by the LIGO and Virgo collaborations in September 2020. Their interpretation of the data seemed to point at the merger of two black holes of 66 and 85 times the mass of the Sun, respectively, merging to the first observed intermediate mass black hole. The mystery: How came the 85 solar mass black hole to be? It sits right in the proposed “mass gap” produced by pair-instability in supernova, where no black holes should exists. New analysis of the same data by AEI researchers now shows that the truth may be even more exciting: They find that the signal could be explained by the merger of a 16 solar mass black hole with an intermediate mass black hole of 171 solar masses.

Paper abstract

GW190521 is the first confident observation of a binary black hole merger with total mass M>100M. 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 msrc1=168+15−61M and msrc2=16+33−3M. The uniform in component-mass prior yields a bimodal posterior distribution. There is a low-mass-ratio mode (q<4) with msrc1=100+17−18M and msrc2=57+17−16M and a high-mass-ratio mode (q≥4) with msrc1=166+16−35M and msrc2=16+14−3M. Although the two modes have nearly equal posterior probability, the maximum-likelihood parameters are in the high-mass ratio mode, with msrc1=171M and msrc2=16M, 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 (lnB≳−4).

Comparison between our reweighted posteriors and posteriors published by the LIGO and Virgo collaborations (LVC) in Abbott et al. (2020b). The left column shows detector-frame masses; right column shows source-frame masses. The top row shows the result of reweighting our samples from a prior uniform in source-frame total mass and mass ratio to uniform in source-frame component masses. The bottom row shows the result when we additionally reweight from a prior uniform in comoving volume and source-frame masses to a prior uniform in the cube of the luminosity distance and uniform in detector- frame masses; this is the prior most similar to what was used in the LVC analysis. The median and 90% credible interval on each parameter is reported above and to the side of the marginal distributions. Grayed regions indicate constraints used in the LVC analysis. The dotted, dashed, and solid boundaries correspond to the chirp mass, total mass, and mass ratio constraints that were used. The combination of the constraints excludes the region in which we find the maximum-likelihood waveform (black cross), and which has the largest posterior support assuming a prior uniform in mass ratio, source-frame total mass, and comoving volume.

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