The Close AGN Reference Survey (CARS): Long-term spectral variability study of the changing-look AGN Mrk 1018

The Close AGN Reference Survey (CARS): Long-term spectral variability study of the changing-look AGN Mrk 1018

Context. Changing-look active galactic nuclei (CLAGNs) are accreting supermassive black hole systems that undergo variations in optical spectral type driven by major changes in accretion rate. The CLAGN Mrk 1018 has undergone two transitions, a brightening event in the 1980s and a transition back to a faint state over the course of 2–3 years in the early 2010s.

Aims. We characterize the evolving physical properties of the source's inner accretion flow, particularly during the bright-to-faint transition, as well as the morphological properties of its parsec-scale circumnuclear gas.

Methods. We modeled archival X-ray spectra from XMM-Newton, Chandra, Suzaku, and Swift using physically motivated models to characterize X-ray spectral variations and tracked the Fe Kα line flux. We also quantified Mrk 1018's long-term multiwavelength spectral variability from optical/UV to the X-rays.

Results. Over the duration of the bright-to-faint transition, the UV and hard X-ray flux fell by differing factors, roughly 24 and 8, respectively. The soft X-ray excess faded and was not detected by 2021. In the faint state, when the Eddington ratio drops to log(Lbol/LEdd)≲−1.7, the hot X-ray corona photon index shows a “softer-when-fainter” trend that is similar to what is seen in some black hole X-ray binaries and samples of low-luminosity AGNs. Finally, the Fe Kα line flux has dropped by only half the factor of the drop in the X-ray continuum.

Conclusions. The transition from the bright state to the faint state is consistent with the inner accretion flow transitioning from a geometrically thin disk to an ADAF-dominated state, with the warm corona disintegrating or becoming energetically negligible, while the X-ray-emitting hot flow becomes energetically dominant. Meanwhile, narrow Fe Kα emission has not yet fully responded to the drop in its driving continuum, likely because its emitter extends up to roughly 10 pc.