Kavli Affiliate: Luis C. Ho
| First 5 Authors: Kai-Xing Lu, Yan-Rong Li, Qingwen Wu, Luis C. Ho, Zhi-Xiang Zhang
| Summary:
Changing-look active galactic nuclei (CL-AGNs), characterized by emerging or
disappearing of broad lines accompanied with extreme continuum flux
variability, have drawn much attention for their potential of revealing
physical processes underlying AGN evolution. We perform seven-season
spectroscopic monitoring on Mrk~1018, one of the earliest identified CL-AGN.
Around 2020, we detect a full-cycle changing-look transition of Mrk~1018 within
one year, associated with a nucleus outburst, which likely arise from the disk
instability in the transition region between the outer standard
rotation-dominated disk and inner advection-dominated accretion flow. Over the
past forty-five years, the accretion rate of Mrk~1018 changed 1000 times and
the maximum Eddington ratio reached 0.02. By investigating the relation between
broad-line properties and Eddington ratio ($L_{rm bol}/L_{rm Edd}$), we find
strong evidence that the full-cycle type transition is regulated by accretion.
There exists a turnover point in the Balmer decrement, which is observed for
the first time. The broad Balmer lines change from a single peak in Type
1.0-1.2 to double peaks in Type 1.5-1.8 and the double-peak separation
decreases with increasing accretion rate. We also find that the full width at
half maximum (FWHM) of the broad Balmer lines obeys FWHM$propto (L_{rm
bol}/L_{rm Edd})^{-0.27}$, as expected for a virialized BLR. The velocity
dispersion $sigma_{rm line}$ follows a similar trend in Type 1.5-1.8, but
displays a sharp increases in Type 1.0-1.2, resulting in a dramatic drop of
FWHM/$sigma_{rm line}$. These findings suggest that a virialized BLR together
with accretion-dependent turbulent motions might be responsible for the
diversity of BLR phenomena across AGN population.
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