Predicting the broad lines polarization emitted by supermassive binary
  black holes
  Some Type-1 active galactic nuclei (AGNs) are showing extremely asymmetric
Balmer lines with the broad peak redshifted or blueshifted by thousands of
km/s. These AGNs may be good candidates for supermassive binary black holes
(SMBBHs). The complex line shapes can be very well due to the complex
kinematics of the two broad line regions (BLRs). Therefore another methods
should be applied to confirm the SMBBHs. One of them is spectropolarimetry. We
rely on numerical modeling of the polarimetry of binary black holes systems
since polarimetry is sensitive to geometry, in order to find specific influence
of supermassive binary black hole (SMBBH) geometry and dynamics on polarized
parameters across the broad line profiles. We apply our method to SMBBHs in
which both components are assumed to be AGNs with distances at the sub-pc
scale. We use a Monte Carlo radiative transfer code that simulates the
geometry, dynamics and emission of a binary system where two black holes are
getting increasingly closer. Each gravitational well is accompanied by its own
BLR and the whole system is surrounded by an accretion flow from the distant
torus. We examine the emission line deformation and predict the polarization
that could be observed. We model broad line polarization for various BLR
geometries with complex kinematics. We find that the presence of SMBBHs can
produce complex polarization angle profiles and strongly affect the polarized
and unpolarized line profiles. Depending on the phase of the SMBBH, the
resulting double-peaked emission lines either show red or blue peak dominance,
or both the peak can have the same intensity. In some cases, the whole line
profile appears as a single Gaussian line, hiding the true nature of the
source. Our results suggest that observations with the high resolution
spectropolarimetry of optical broad emission lines could play an important role
in detecting sub-pc SMBBHs.
