Behaviour of the MgII 2798AA Line Over the Full Range of AGN Variability

TL;DR

This study examines how the Mg II 2798AA emission line in AGN responds to variability over several years, revealing diverse behaviors and correlations with other lines and properties, and providing new insights into AGN emission mechanisms.

Contribution

It provides the first comprehensive analysis of Mg II line variability across a large AGN sample, highlighting its responsiveness and relation to other spectral features and accretion properties.

Findings

Mg II responds to continuum variability in some AGN.

In 74% of highly variable AGN, Mg II behavior aligns with Hβ.

Highly variable quasars tend to have lower Eddington ratios.

Abstract

We investigate the responsiveness of the 2798AA Mg II broad emission line in AGN on timescales of several years. Our study is based on a sample of extremely variable AGN as well as a broad population sample. The observed response of the line in previous studies has been mixed. By focussing on extreme variability ($|\Delta g|>$ 1) we find that Mg II clearly does respond to the continuum. However, the degree of responsiveness varies strikingly from one object to another; we see cases of Mg II changing by as much as the continuum, more than the continuum, or very little at all. In 74% of the highly variable sample the behaviour of Mg II corresponds with that of H$\beta$, with 30% of the objects showing large variations in both lines. We do not detect any change in the line width that would correspond to Broad Line Region `breathing', in accordance with results from literature. Some of the objects in our highly variable sample show a clear asymmetry in the Mg II profile. This skewness can be both to the blue and the red of the line centre. Results from our broad population sample show that highly variable quasars have lower Eddington ratios. This result holds for the variability of the continuum, but the correlation is significantly reduced for the variability of the Mg II line. For the first time, we present an overview of the value of the intrinsic Baldwin Effect for Mg II in a large sample.