Multiepoch Spectroscopy of Mg ii Broad Absorption Line Transitions

Built upon a sample of 134 quasars that was dedicated to a systematic study of Mg ii–BAL variability from Yi et al., we investigate these quasars, showing Mg ii–BAL disappearance or emergence with the aid of at least three epoch optical spectra sampled more than 15 yr in the observed frame. We identified 3/3 quasars undergoing pristine/tentative BAL transformations. The incidence of pristine BAL transformations in the sample is therefore derived to be ${2.2}_{-1.2}^{+2.2}$%, consistent with that of high-ionization BAL transformations from the literature. Adopting an average Mg ii–BAL disappearance timescale of rest frame 6.89 yr among the six quasars, the average characteristic lifetime of Mg ii BALs in the sample is constrained to be >160 yr along our line of sight. There is a diversity of BAL profile variability observed in the six quasars, probably reflecting a variety of mechanisms at work. Our investigations of Mg ii–BAL transitions, combined with observational studies of BAL transitions from the literature, imply an overall FeLoBAL/LoBAL → HiBAL/non-BAL transformation sequence, along with a decrease in reddening. This sequence is consistent with the evacuation models for the origin of commonly seen blue quasars, in which LoBAL quasars are in a short-lived blowout phase.