# Spectral Variability of a Sample of Extreme Variability Quasars and Implications for the MgII Broad-line Region

## TL;DR

This study investigates the spectral variability of extreme variability quasars over several years, focusing on the Mg II broad-line region, revealing that Mg II flux correlates with continuum changes but its width does not, affecting black hole mass estimates.

## Contribution

It provides the first detailed analysis of Mg II line variability in EVQs, highlighting the limitations of using Mg II width for black hole mass estimation due to luminosity-dependent biases.

## Key findings

- Mg II flux varies with continuum, but line width remains mostly unchanged.
- Mg II variability indicates reverberation to continuum changes.
- Using Mg II width for black hole mass estimates introduces bias.

## Abstract

We present new Gemini/GMOS optical spectroscopy of 16 extreme variability quasars (EVQs) that dimmed by more than 1.5 mag in the $g$ band between the Sloan Digital Sky Survey (SDSS) and the Dark Energy Survey (DES) epochs (separated by a few years in the quasar rest frame). The quasar sample covers a redshift range of $0.5 < z < 2.1$. Nearly half of these EVQs brightened significantly (by more than 0.5 mag in the $g$ band) in a few years after reaching their previous faintest state, and some EVQs showed rapid (non-blazar) variations of greater than 1-2 mag on timescales of only months. Leveraging on the large dynamic range in continuum variability between the earlier SDSS and the new GMOS spectra, we explore the associated variations in the broad Mg II,$\lambda2798$ line, whose variability properties have not been well studied before. The broad Mg II flux varies in the same direction as the continuum flux, albeit with a smaller amplitude, which indicates at least some portion of Mg II is reverberating to continuum changes. However, the width (FWHM) of Mg II does not vary accordingly as continuum changes for most objects in the sample, in contrast to the case of the broad Balmer lines. Using the width of broad Mg II to estimate the black hole mass therefore introduces a luminosity-dependent bias.