Description
Formation of super-massive black hole binaries (SMBHB) is deemed to be inevitable in various cosmological models. Their search poses one of the most challenging problems of modern observational astrophysics. Dissipation of kinetic energy in SMBHB controls the evolution of these objects and leads to coalescing into a single black hole. This process (often called “inspiralling”) is accompanied by increasingly intensive emission of gravitational waves (GW) and ends with the final GW burst (a “chirp”). While the first direct detection of GW made by the LIGO and Virgo collaboration in 2015 dealt with coalescence of stellar-mass black holes, recent results by multiple Pulsar Timing Arrays (PTA) increased attention to the SMBHB population as a likely source of the GW background.
SMBHB objects remain rather elusive: at present, there are only several dozens of candidates of which just a handful can be treated as certain cases. Direct detections of the components of SMBHB at the sub-parsec scales remain beyond reach for today’s observing techniques at all domains of the electromagnetic spectrum.
Recently several AGNs distinguished by oscillating astrometric positions at the milliarcsecond angular scale with periods of several years attracted our attention as SMBHB candidates. Our study does not allow us to “see” directly the components of possible SMBHBs. But we see a “smoking gun” of orbital motion in these potential SMBHBs. Several examples of such the oscillating behaviour are detected with VLBI astrometry. We analyse the evolution of these binary systems leading to coalescence and associated with this GW outburst.
The estimates presented in this work provide inputs into design studies of future mm/sub-mm VLBI systems with spaceborne radio telescopes. Such the systems will allow us to resolve images of binary SMBHBs at the microarcsecond angular scales, principally unachievable with the Earth-based observational facilities.