Binary Black Holes
We know that mergers play an important role in the evolution of galaxies and potentially are instrumental in triggering accretion onto supermassive black holes under certain conditions. We also know that almost every galaxy hosts a supermassive black holes at its center. It is therefore to be expected that during the course of a galactic merger a supermassive binary black hole is formed. These systems, through hard to observe, have been long hypothesized to exist. They are the holy grail of both theoretical and observational astrophysicists as they would consistute an ideal gravity laboratory and an opportunity to study extreme astrophysics at play.
Astronomers have been hunting down these exotic systems with a variety of methods, including but not limited to optical spectroscopy, X-ray photometry, radio VLBI observations, and others. The problem with the majority of these methods is that they require both black holes to be simultaneously accreting material and therefore producing the typical observational signatures of AGN. However, this is not at all a trivial assumption, particularly since the accretion disk around an infalling black hole to the center of a merging galaxy would most probably be disrupted by tidal forces and dynamical friction.
Mezcua et al. (2014)
Roland et al. (2008)
Going the indirect way!
There are however more indirect ways of pinpointing these elusive systems, which I have utilized in my Ph.D. research and later on as a postdoc. These mainly rely on morphological evidence and radio observations to give us hints about the presence of a secondary massive object at the center of a system.
During my Ph.D. I used a sample of flat-spectrum radio-AGN and looked at their morphologies and multi-wavelength variability to identify possible binary black hole candidates. Systems in the last stages of a merger (based on their morphologies) are good candidates of supermassive binary black holes. Alternatively, AGN showing correlated variability at different wavelengths may indicate the presence of a periodical phenomenon happening at their centers, i.e., the orbital motion of a binary black hole.
Radio jets can also be used to identify binary black holes systems. In particular, it is expected that a jet would precess in the presence of a secondary black hole, due to effects like the Lense-Thering effect and gravitational torques excerted by the secondary black hole on the accretion disk of the primary one. This precession is then imprinted on the trajectories of bright components moving along the radio jet. By studying these precession imprints, we are able to deduce the orbital period, separation, and even mass of these binary black hole system candidates.
Related papers: Roland et al 2008, Karouzos et al. 2010, Kudryavtseva et al. 2011, Mezcua et al. 2014, Kun et al. 2014