# Variability of Low-ionization Broad Absorption Line Quasars Based on Multi-epoch Spectra from The Sloan Digital Sky Survey

## TL;DR

This study analyzes multi-epoch spectra of 134 low-ionization MgII broad absorption line quasars from SDSS over days to years, revealing a time-dependent asymmetry in absorption strength changes and insights into the variability mechanisms.

## Contribution

It provides the largest BAL variability sample to date, identifies a novel asymmetry in EW variations, and offers new understanding of the physical processes driving BAL variability.

## Key findings

- Weakening troughs outnumber strengthening ones, a first in BAL variability.
- Lower variability frequency compared to high-ionization BAL quasars.
- MgII BALs tend to have high optical depths and small covering factors.

## Abstract

We present absorption variability results for 134 bona fide \mgii\ broad absorption line (BAL) quasars at 0.46~$\lesssim z \lesssim$~2.3 covering days to $\sim$ 10 yr in the rest frame. We use multiple-epoch spectra from the Sloan Digital Sky Survey, which has delivered the largest such BAL-variability sample ever studied. \mgii-BAL identifications and related measurements are compiled and presented in a catalog. We find a remarkable time-dependent asymmetry in EW variation from the sample, such that weakening troughs outnumber strengthening troughs, the first report of such a phenomenon in BAL variability. Our investigations of the sample further reveal that (i) the frequency of BAL variability is significantly lower (typically by a factor of 2) than that from high-ionization BALQSO samples; (ii) \mgii\ BAL absorbers tend to have relatively high optical depths and small covering factors along our line of sight; (iii) there is no significant EW-variability correlation between \mgii\ troughs at different velocities in the same quasar; and (iv) the EW-variability correlation between \mgii\ and \aliii\ BALs is significantly stronger than that between \mgii\ and \civ\ BALs at the same velocities. These observational results can be explained by a combined transverse-motion/ionization-change scenario, where transverse motions likely dominate the strengthening BALs while ionization changes and/or other mechanisms dominate the weakening BALs.