Variability of Low-ionization Broad Absorption-line Quasars Based on Multi-epoch Spectra from the Sloan Digital Sky Survey


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Yi W., Brandt W. N. , Hall P. B. , Vivek M., Grier C. J. , Ak N. , ...More

ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, vol.242, 2019 (Journal Indexed in SCI) identifier identifier

  • Publication Type: Article / Article
  • Volume: 242
  • Publication Date: 2019
  • Doi Number: 10.3847/1538-4365/ab1f90
  • Title of Journal : ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES

Abstract

We present absorption variability results for 134 bona fide Mg II broad absorption-line (BAL) quasars at 0.46 less than or similar to z less than or similar to 2.3 covering days to similar to 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. Mg II-BAL identifications and related measurements are compiled and presented in a catalog. We find a remarkable time-dependent asymmetry in the equivalent width (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 in high-ionization BALQSO samples, (ii) Mg II-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 Mg II troughs at different velocities in the same quasar, and (iv) the EW-variability correlation between Mg II and Al III BALs is significantly stronger than that between Mg II and C IV 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.