Microlensing of Sub-parsec Massive Binary Black Holes in Lensed QSOs: Light Curves and Size-Wavelength Relation

Abstract

Sub-parsec binary massive black holes (BBHs) are long anticipated to exist in many QSOs but remain observationally elusive. In this paper, we propose a novel method to probe sub-parsec BBHs through microlensing of lensed QSOs. If a QSO hosts a sub-parsec BBH in its center, it is expected that the BBH is surrounded by a circum-binary disk, each component of the BBH is surrounded by a small accretion disk, and a gap is opened by the secondary component in between the circum-binary disk and the two small disks. Assuming such a BBH structure, we generate mock microlensing light curves for some QSO systems that host BBHs with typical physical parameters. We show that microlensing light curves of a BBH QSO system at the infrared-optical-UV bands can be significantly different from those of corresponding QSO system with a single massive black hole (MBH), mainly because of the existence of the gap and the rotation of the BBH (and its associated small disks) around the center of mass. We estimate the half-light radii of the emission region at different wavelengths from mock light curves and find that the obtained half-light radius vs. wavelength relations of BBH QSO systems can be much flatter than those of single MBH QSO systems at a wavelength range determined by the BBH parameters, such as the total mass, mass ratio, separation, accretion rates, etc. The difference is primarily due to the existence of the gap. Such unique features on the light curves and half-light radius-wavelength relations of BBH QSO systems can be used to select and probe sub-parsec BBHs in a large number of lensed QSOs to be discovered by current and future surveys, including the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), the Large Synoptic Survey telescope (LSST) and Euclid.

18 pages, 17 figures, accepted for publication in the Astrophysical Journal

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• A probable Milli-Parsec Supermassive Binary Black Hole in the Nearest Quasar Mrk 231
• Evidence for a milliparsec-separation Supermassive Binary Black Hole with quasar microlensing
• Detecting subparsec super-massive binary black holes: Long termmonitoring perspective
• Diffractive lensing of nano-Hertz gravitational waves emitted from supermassive binary black holes by intervening galaxies
• Apparent quasar disc sizes in the "bird's nest" paradigm
• Predicting High-magnification Events in Microlensed Quasars in the Era of LSST using Recurrent Neural Networks
• Gravitational Lensing of Gravitational Waves from Astrophysical Sources: Theory, Detection, and Applications
• Simulating quasar microlensing light curves: High magnification events
• Multiwavelength periodic microlensing signatures of macrolensed supermassive binary black holes
• An equal mass ratio supermassive binary black holes in Q J0158-4325 with periodic microlensing signature?