Abstract

Contributed Talk - Splinter AGN   (MW-1250)

Extreme Accretion and Multi-Wavelength Variability in the Sub-Milliparsec Binary Candidate AT 2021hdr

Jorge Cuadra
UAI, Chile / MPE, Germany

Supermassive black hole binaries (SMBHBs) at sub-parsec scales are critical tracers of galaxy evolution and future gravitational-wave sources, yet they remain extremely challenging to identify observationally. Multi-wavelength time-domain observations of Active Galactic Nuclei (AGN) offer a powerful precision tool to probe these unresolved central engines. In this talk, we present a multi-wavelength analysis of AT 2021hdr, a Seyfert 1 galaxy that has exhibited large-amplitude quasi-periodic oscillations every 60–90 days across the optical, UV, and X-ray bands. This extreme variability behaviour is inconsistent with typical AGN flares or isolated stochastic accretion. Instead, we propose a physical model in which the variability is driven by the tidal disruption and subsequent accretion of a gas cloud by an SMBHB system. Our hydrodynamical simulations broadly reproduce the observed multi-wavelength light curves, supporting an interpretation where the binary has a physical separation below 1 milliparsec and a gravitational-wave-driven merger timescale of ~70,000 years. AT 2021hdr highlights how time-domain surveys can unveil otherwise inaccessible stages of black hole growth, serving as an ideal benchmark as the community prepares for LSST and upcoming multi-wavelength facilities.