The long-term broad-line responsivity in MKN 110

TL;DR

This study analyzes 30 years of optical spectra of AGN MKN 110, revealing complex, long-term BLR responses to continuum changes, including line saturation, structural evolution, and multiple emission components.

Contribution

It provides the first long-term, multi-epoch analysis of BLR responsivity and structural evolution in MKN 110, highlighting non-linear line responses and multiple emission contributions.

Findings

He II λ4686 varies fortyfold, more than the optical continuum.

Line responses saturate at high FUV fluxes depending on excitation energy.

Evidence of structural evolution and multiple emission components in the BLR.

Abstract

We examine the long-term history of the optical spectrum of the extremely variable Active Galactic Nucleus (AGN) MKN 110. By combining various archival data with new data, we cover an unprecedented long period of $\sim$30 years (1987 - 2019). We find that the He II $\lambda 4686$ emission line changes by a factor of forty and varies more strongly than the optical continuum. Following Ferland et al. (2020), we take He II $\lambda 4686$ as a proxy for the FUV continuum and compare the flux of several other line species against it. This comparison reveals a clear pattern, whereby lines respond close to linearly at low FUV fluxes, and saturate at high FUV fluxes. The saturation level of the response appears to depend on the excitation energy of the line species. In addition to this global pattern, we note changes among observational epochs, indicating a structural evolution in the broad line region (BLR). The line profiles in our spectra show an offset between the narrow and broad components of the He II $\lambda 4686$ and H$\beta$ lines. This offset shows a significant negative correlation with the FUV flux and a positive correlation with the line velocity width. Our analysis reveals a complex BLR response to a changing continuum. The clear presence of a non-responsive component of the broad lines indicates the existence of multiple contributions to the line emission. We find there are several kinematic models of the BLR and inner regions of the AGN that match our data.