Transient LoBALs at High Velocities: A Case of Extreme Broad Absorption Line Variability in J115636.82+085628.9*

Abstract We present a multiepoch spectroscopic study of the broad absorption line (BAL) quasar J115636.82+085628.9 ( z em = 2.1077), based on five spectra spanning nearly two decades in the observer’s frame. This source exhibits remarkable variability in both low-ionization (LoBAL: Al iii and Mg ii ) and high-ionization (HiBAL: C iv and Si iv ) absorption features. For the first time, we detect the emergence and subsequent disappearance of LoBAL troughs at high velocities (∼20,000 km s −1 ), coinciding with the strengthening and weakening of the corresponding HiBAL absorption. The C iv BAL profile extends from ∼6700 km s −1 to a conservative upper limit of 30,000 km s −1 and is composed of narrow, variable absorption features embedded within a broad, smooth envelope. Both C iv and Si iv BAL troughs exhibit dramatic equivalent width (EW) changes—among the most extreme reported to date. Notably, these EW variations are strongly anticorrelated with continuum flux changes inferred from optical photometric light curves. We interpret this variability as the result of a new absorbing flow transiting into our line of sight, increasing the shielding of a more distant, preexisting outflow and giving rise to transient LoBAL absorption. This scenario supports a unified picture in which LoBAL and HiBAL features arise from similar outflow structures, with observed differences governed primarily by line-of-sight column densities consistent with previous findings.