simone veronese GMT20240521 120141 Recording 1920x1080

Interpreting the long-term variability of the changing-look AGN Mrk 1018

Simone Veronese (Netherlands Institute for Radio Astronomy (ASTRON) - Kapteyn Astronomical Institute)

Tuesday 21/05/2024 @ 14:00, Sala Antonio Sollima (IV piano Battiferro)

I present a thorough study of the Changing-Look Active Galactic Nucleus (CL-AGN) Mrk 1018. I analysed X-ray spectra and broad-band photometry, and performed optical-to-X-ray spectral energy distribution (SED) fitting to comprehend the observed changing-look behaviour of this source from 2005 to 2019. By describing the X-ray spectra with a two-Comptonization model, I validate the changes in X-ray spectral shape and the optical-to-X-ray photometric dimming reported in the literature, and I also reveal a consistent hot comptonizing medium but a cooling warm component. This cooling, linked to the weakening of the magnetic fields in the accretion disk, explains the broad-band spectro-photometric variability and possibly the disappearing of the broad optical lines. I propose that the weakening is caused by the formation of a jet, in turn originated from the change of state of the inner accretion flow. Our optical-to-X-ray SED fitting supports this conclusion, as the normalised accretion rate is super-critical ($\mu=$0.06 less than 0.02) in the bright state and sub-critical ($\mu=$0.01less than 0.02) in the faint state. Instabilities arising at the interface of the state-transition are able to reduce the viscous timescale to the observed ~10 years of Mrk 1018 variability. I explored a possible triggering mechanism for this state transition, involving gaseous clouds pushed onto the AGN sub-pc regions by a recent merging event or by cold chaotic accretion. This scenario, if validated by future simulations, could enhance our understanding of CL-AGN and raises questions about an accretion rate of ~0.02, coupled with minor disturbances in the accretion disk, being the primary factor in the changing-look phenomenon.