DOI

https://doi.org/10.48717/ajfk-zm59

Project

RA005

Title

e-EVN observation of a possibly imminent merger of SMBH binary

Abstract

We recently discovered a unique AGN showing quasi-periodic optical and X-ray variability. The period has decayed rapidly from about one year to one month in just three years. This damped periodic variation with rapid period decay has never been seen in any other AGN. The chirping signals can be well explained by orbital decay driven by effective gravitational wave radiation from a SMBH binary (SMBHB) with high eccentricity. The SMBHB hypothesis is also supported by a distinct Balmer emission line profile consisting of both blue-shifted and red-shifted components with velocities of up to several thousand km/s, possibly originating from the tidally disturbed BLR clouds orbiting the SMBHB. If confirmed, this would be the first SMBHB soon to coalesce and the first such phenomenon ever observed in human history. Radio emissions are critical for diagnosing the energy and environment of the black hole merger event. Trajectory evolution models predict that SMBHBs will merge in a timescale as short as a few months. A multi-band observation campaign has been initiated by the discovery team from X-ray, optical to radio bands. We propose here ToO EVN observations in L band with the immediate goal of checking the presence of compact radio sources and obtaining their precise position, providing fundamental information for future EVN observations. Moreover, the L-band EVN observation will be coordinated with the triggered observations of other telescopes to construct a quasi-simultaneous spectrum which is crucial for understanding the accretion and radiative properties of the SMBHB in the coalescence phase.