MgII-BAL quasar variability in SDSS spectra

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 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) 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.