Spectropolarimetry of the changing-look active galactic nucleus Mrk 1018

TL;DR

This study presents spectropolarimetric observations of the changing-look AGN Mrk 1018, revealing insights into its emission line structure, polarization characteristics, and the mechanisms behind its variability, challenging some existing models.

Contribution

First spectropolarimetric analysis of Mrk 1018, providing new insights into its emission lines, polarization, and variability mechanisms, and testing models of supermassive binary black holes.

Findings

Single broad emission line component fits observed spectra.

Continuum polarization remains low across states.

Polarization profile suggests line formation in a polar outflow.

Abstract

We have obtained new spectropolarimetric observations at visible wavelengths of the changing-look active galactic nucleus (AGN) Mrk 1018. The AGN direct spectrum shows an extremely weak continuum with faint broad H$\beta$ and H$\alpha$ emission lines. Both lines can be fit with a single very broad emission line component of full width at half maximum FWHM $\simeq$ 7200 km s$^{-1}$, with no evidence of the additional 3000 km s$^{-1}$-wide component that was previously detected. While this is in agreement with line formation in a Keplerian disk, the line profile variability suggests that the broad emission line region is likely more complex. The continuum polarization of Mrk 1018 is low; it is not higher in the current faint state compared to the past bright state, confirming that dust obscuration is not the mechanism at the origin of the change of look. The polarization profile of the H$\alpha$ line is asymmetric with no rotation of the polarization angle, which possibly reveals line formation in a polar outflow. Alternatively, the polarization profile may be the consequence of a time delay between the direct and the polarized light. Interestingly, the polarization signatures predicted for broad lines emitted around supermassive binary black holes are not observed.