Variability of low-ionization broad absorption-line quasars based on multi-epoch spectra from the sloan digital sky survey

W. Yi, W. N. Brandt, P. B. Hall, M. Vivek, C. J. Grier, N. Filiz Ak, D. P. Schneider, S. M. McGraw

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Abstract

We present absorption variability results for 134 bona fide Mg II broad absorption-line (BAL) quasars at 0.46 z 2.3 covering days 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.

Original languageEnglish (US)
Article number28
JournalAstrophysical Journal, Supplement Series
Volume242
Issue number2
DOIs
StatePublished - 2019

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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