We still do not know the timescale for the merging of binary black holes (BHs). This timescale has important implications for gravitational wave predictions and our understanding of BH demographics. Here we discuss efforts to constrain the fraction of BH pairs on kpc scales using observations of dual active galactic nuclei.

In this work we study the stellar-dynamical hardening of unequal mass massive black hole (MBH) binaries in the central regions of galactic nuclei. We present a comprehensive set of direct N-body simulations of the problem, varying both the total mass and the mass ratio of the MBH binary. Our initial model starts as an axisymmetric, rotating galactic nucleus, to describe the situation right after the galaxies have merged, but the black holes are still unbound to each other. We confirm that results presented in earlier works (Berczik et al. 2006; Khan et al. 2013; Wang et al. 2014) about the solution of the “last parsec problem” (sufficiently fast black hole coalescence for black hole growth in cosmological context) are robust for both for the case of unequal black hole masses and large particle numbers. The MBH binary hardening rate depends on the reduced mass ratio through a single parameter function, which quantitatively quite well agrees with standard 3 body scattering theory (see e.g., Hills 1983). Based on our results we conclude that MBH binaries at high redshifts are expected to merge with a factor of ~ 2 more efficiently, which is important to determine the possible overall gravitational wave signals. However, we have not yet fully covered all the possible parameter space, in particular with respect to the preceding of the galaxy mergers, which may lead to a wider variety of initial models, such as initially more oblate and / or even significantly triaxial galactic nuclei. Our N-body simulations were carried out on a new special supercomputers using the hardware acceleration with graphic processing units (GPUs).

A model for OJ 287 consisting of two orbiting black holes has been constructed using optical light curve data. The model has successfully predicted the occurrence of sharp optical outbursts of OJ 287 for the past 15 years. Here we test if also the variations in the radio jet position angle can be explained within the framework of this same model, which has most of its parameters fixed by the timing of the optical flares. The model applied here has only three free parameters left, the (trivial) zero point of the jet position angle, the time lag between changes in the disk and jet orientations, and the zero point of the viewing angle. Despite its simplicity and the small number of free parameters, the model appears to be able to reproduce the main properties of the observed position angle variations during the past 30 years. The best fits are obtained when the time lag is either ~4 or ~14 years. However, the jet orientation seems to be unrelated to the direction of the spin of the primary black hole. This implies, assuming that the basic model is correct, that the mean orientation of the jet is determined by the orientation of the inner accretion disk, not by the spin axis of the black hole.