2013A&A...557A..85R

If we assume that nuclei of extragalactic radio sources contain binary black hole systems, the two black holes can eject VLBI components, in which case two families of different VLBI trajectories will be observed. Another important consequence of a binary black hole system is that the VLBI core is associated with one black hole, and if a VLBI component is ejected by the second black hole, one expects to be able to detect the offset of the origin of the VLBI component ejected by the black hole that is not associated with the VLBI core. The ejection of VLBI components is perturbed by the precession of the accretion disk and the motion of the black holes around the center of gravity of the binary black hole system. We modeled the ejection of the component taking into account the two pertubations and present a method to fit the coordinates of a VLBI component and to deduce the characteristics of the binary black hole system. Specifically, this is the ratio T_p/T_b where T_p is the precession period of the accretion disk and T_b is the orbital period of the binary black hole system, the mass ratio M₁/M₂, and the radius of the binary black hole system R_bin. From the variations of the coordinates as a function of time of the ejected VLBI component, we estimated the inclination angle i_o and the bulk Lorentz factor γ of the modeled component. We applied the method to component S1 of 1823+568 and to component C5 of 3C279, which presents a large offset of the space origin from the VLBI core. We found that 1823+568 contains a binary black hole system whose size is R_bin ≃60µas (µas is a microarcsecond) and 3C279 contains a binary black hole system whose size is R_bin ≃420µas. We calculated the separation of the two black holes and the coordinates of the second black hole from the VLBI core. This information will be important to link the radio reference-frame system obtained from VLBI observations and the optical reference-frame system obtained from Gaia.