2025AJ....170...80L

We report the discovery of a velocity shift signature in the C IV and Si IV broad absorption lines (BALs) of the quasar SDSS J140058.79+260619.4 (z_em = 2.351). From its two-epoch spectra, we find two BAL systems (systems A and B) in C IV and/or Si IV ions. Firstly, we find obvious variations of equivalent widths across all BAL troughs, which are most plausibly explained by changes in ionization states. Secondly, system A, characterized by a higher velocity and smoother profile shape (than those of System B), shows significant velocity shifts in C IV and Si IV BALs between two epochs, indicating potential outflow accelerations. Over 1.83 yr in the rest frame, we measure velocity differences of -481 ± 64 km s^–1 for C IV and -550 ± 95 km s^–1 for Si IV, corresponding to acceleration rates of -0.83 ± 0.11 cm s^–2 and -0.95 ± 0.17 cm s^–2, respectively. Given the limited observational data, the physical origin of this velocity shift signature remains an open question. We considered several plausible mechanisms, favoring either variations in the gas's ionization state or actual line-of-sight acceleration driven by radiation pressure from the central source.