2018MNRAS.481.5570V

We report the discovery of rapid variations of a high-velocity Mg II broad absorption line (BAL) trough in the quasar SDSS J133356.02 + 001229.1 (z_em ∼ 0.9197). Vivek et al. revealed the emergence and subsequent near disappearance of a BAL component in this source having an ejection velocity of ∼28 000 km s^–1. Our further follow-up studies with South African Large Telescope (SALT) reveal the dramatic nature of the absorption line variability in this source. The absorption line emerged again at the same velocity and nearly disappeared within the SALT observations. Our observations allow us to probe variability over time-scales of the order of few days to 4.2 yr in the QSO rest frame. The observed velocity stability of BAL absorption does not point to any line-of-sight acceleration/deceleration of BAL clouds. The ionization parameter of the absorbing cloud is constrained from the column density ratio of Mg II to Fe II ground state absorption. In the absence of strong optical continuum variability, we suggest that photoionization-driven BAL variability due to changes in the shielding, multiple streaming clouds across our line of sight in a corotating wind, or a combination of both as possible explanations for the observed strong equivalent width variations.