Oral Presentation

Microlensing of Sub-parsec Massive Binary Black Holes in Lensed QSOs

Changshuo Yan (NAOC)

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.