Spectral Optical Monitoring of the Narrow Line Seyfert 1 galaxy Ark 564

TL;DR

This study presents a long-term spectral optical monitoring of the NLS1 galaxy Ark 564, analyzing emission line variability and correlations to understand the ionization sources in its broad line region.

Contribution

It provides the first detailed analysis of optical Fe II emission and its correlation with continuum flux in Ark 564 over a 11-year period, revealing complex ionization mechanisms.

Findings

Fe II lines vary with a factor of 1.8 during monitoring.

Strong correlation between Fe II and Hb flux variations.

Permitted-line profiles remain Lorentzian and stable.

Abstract

We present the results of a long-term (1999--2010) spectral optical monitoring campaign of the active galactic nucleus (AGN) Ark 564, which shows a strong Fe II line emission in the optical. This AGN is a narrow line Seyfert 1 (NLS1) galaxies, a group of AGNs with specific spectral characteristics. We analyze the light curves of the permitted Ha, Hb, optical Fe II line fluxes, and the continuum flux in order to search for a time lag between them. Additionally, in order to estimate the contribution of iron lines from different multiplets, we fit the Hb and Fe II lines with a sum of Gaussian components. We found that during the monitoring period the spectral variation (F_max/F_min) of Ark 564 was between 1.5 for Ha to 1.8 for the Fe II lines. The correlation between the Fe II and Hb flux variations is of higher significance than that of Ha and Hb (whose correlation is almost absent). The permitted-line profiles are Lorentzian-like, and did not change shape during the monitoring period. We investigated, in detail, the optical Fe II emission and found different degrees of correlation between the Fe II emission arising from different spectral multiplets and the continuum flux. The relatively weak and different degrees of correlations between permitted lines and continuum fluxes indicate a rather complex source of ionization of the broad line emission region.