Emission signatures from sub-parsec binary supermassive black holes

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Nguyen, Khai Huu
Bogdanović, Tamara
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Motivated by advances in observational searches for sub-parsec supermassive black hole binaries (SBHBs) made in the past few years, we develop a semi-analytic model to describe spectral broad emission-line (BEL) signatures of these systems. The goal of this study is to aid the interpretation of spectroscopic searches for binaries and to help test one of the leading models of binary accretion flows in the literature: SBHB in a circumbinary disk. We model the accretion flows as a set of three accretion disks: two mini-disks that are gravitationally bound to the individual black holes, and a circumbinary disk. The model also includes treatment of radiative transfer by taking into account effects of radiatively driven accretion disk wind on the properties of the BEL profiles. Given a physically motivated parameter space occupied by sub-parsec SBHBs, we calculate a synthetic database of nearly 42.5 million BEL profiles and explore the dependence of the profile shapes on characteristic properties of SBHBs. We find that the modeled profiles show distinct statistical properties as a function of the semimajor axis, mass ratio, eccentricity of the binary, and the degree of alignment of the triple disk system. This suggests that the broad emission-line profiles from SBHB systems can in principle be used to infer the distribution of these parameters. We also present a method for comparison of the BEL profiles of observed candidates for SBHBs with those calculated from the model of sub-parsec binaries in circumbinary disks. Using this approach we infer the values of the binary parameters for the observed SBHB candidates and evaluate the parameter degeneracies, representative of the uncertainties intrinsic to such measurements. We find that as a population, the SBHB candidates favor the average value of the semimajor axis corresponding to log(a/M ) ≈ 4.20 ± 0.42 and comparable mass ratios, q > 0.5. If the considered candidates are true binaries, this result would suggest that there is a physical process that allows initially unequal mass systems to evolve toward comparable mass ratios. Our method also indicates that the candidates do not favor configurations in which the mini-disks are coplanar with the binary orbital plane. This would indicate the presence of a physical mechanism that maintains misalignment of the mini-disks (or causes them to be warped) down to sub-parsec binary separations. Finally, our method can in principle be used to interpret the observed BEL profiles once a sample of confirmed SBHBs is available, but cannot be used as a conclusive test of binarity.
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