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

Abstract: We present a thorough study of the Changing-Look Active Galactic Nucleus (CL-AGN) Mrk 1018, utilizing an extensive dataset spanning optical, UV, and X-ray spectro-photometric data from 2005 to 2019. We 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. We found that over the 14 years in analysis, significant changes in X-ray spectra occurred, as the hardness ratio increases by a factor of ~2. We validated also the broad-band dimming, with optical, UV, and X-ray luminosities decreasing by factors of >7, >24 and ~9, respectively. These dims are attributed to the declining UV emission. We described the X-ray spectra with a two-Comptonization model, revealing 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 UV dimming. We 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=$0.06>0.02) in the bright state and sub-critical ($\mu=$0.01<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. We 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.