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We report on the discovery of broad Balmer absorption lines variability inthe QSO SDSS J125942.80+121312.6, based on the optical and near-infraredspectra taken from the SDSS-I, SDSS-III Baryon Oscillation Spectroscopic Survey(BOSS), and TripleSpec observations over a timescale of 5.8 years in the QSO'srest-frame. The blueshifted absorption profile of H$\beta$ shows a variation ofmore than 5$\sigma$ at a high velocity portion ($>3000\ \mathrm{km\ s}^{-1}$)of the trough. We perform a detailed analysis for the physical conditions ofthe absorber using Balmer lines as well as metastable He I and optical Fe IIabsorptions ($\lambda 4233$ from b$^4$P$_{5/2}$ level and $\lambda 5169$ froma$^6$S$_{5/2}$) at the same velocity. These Fe II lines are identified in theQSO spectra for the first time. According to the photoionization simulations,we estimate a gas density of $n(\mathrm{H})\approx 10^{9.1}\ \mathrm{cm}^{-3}$and a column density of $N_{\mathrm{col}}(\mathrm{H})\approx 10^{23}\\mathrm{cm}^{-2}$ for the BOSS data, but the model fails to predict thevariations of ionic column densities between the SDSS and BOSS observations ifchanges in ionizing flux are assumed. We thus propose transverse motion of theabsorbing gas being the cause of the observed broad Balmer absorption linevariability. In fact, we find that the changes in covering factors of theabsorber can well-reproduce all of the observed variations. The absorber isestimated $\sim 0.94$ pc away from the central engine, which is where theoutflow likely experiences deceleration due to the collision with thesurrounding medium. This scheme is consistent with the argument that LoBAL QSOsmay represent the transition from obscured star-forming galaxies to classicQSOs.

Broad Balmer Absorption Line Variability: Evidence of Gas Transverse Motion in the QSO SDSS J125942.80+121312.6