Predicting the broad lines polarization emitted by supermassive binary black holes

Abstract: 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.