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.