Astronomy & Astrophysics Seminar: Multi-messenger astrophysics: astronomy at the threshold of the third revolution

Zoom: https://tau-ac-il.zoom.us/j/87185167411?pwd=FpTUyIgXaOturzOFSetA6pPVYQilen.1

Abstract:

By the turn of the millennium, astronomy completed covering almost all domains of the electromagnetic spectrum as the field of research. This brought to the end the second revolution in astronomy. The next challenge – gravitational wave astronomy. Together, electromagnetic and gravitational wave branches of astronomy form multi-messenger astrophysics. 

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. Besides several directly detected dual active galactic nuclei (AGN) with separations between black holes measured in hundreds kiloparsecs, thus belonging to early stages of the SMBHB evolution, closer binary systems in more advanced stages of inspiralling are mostly suspected on the basis of temporal variability or morphological patterns of milliarcsecond-scale structures. Direct detections of the components of SMBHB at the sub-parsec scales, which correspond to the late stages of inspiralling, 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. Using the available observational data as starting points we reconstruct physical models of these potential SMBHBs. At the next step we analyse the evolution of these binary systems leading to coalescence and associated with this GW outburst. The work offers an example of VLBI study of a potential GW event precursor.

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.

Seminar Organizer: Dr. Jonathan Stern