2020MNRAS.499L..58L

We report, for the first time, a synchronized velocity shift of Mg II and Al III broad absorption lines (BALs) in quasar SDSS J134444.33+315007.6 (hereafter, J1344+3150). We found this quasar from a sample of 134 Mg II BAL quasars with multi-epoch observations. This quasar contains three low-ionization BAL systems, the fastest of which at ∼-17 000 km s^–1 shows a kinematic shift of ∼-1101 and ∼-1170 km s^–1 in its Mg II and Al III ions, respectively, during a rest-frame time of about 3.21 yr. Meanwhile, this quasar also shows other various variation characteristics, including an obvious weakening in its continuum, a coordinated enhancement in multiple emission lines (Mg II, C III, and Al III), and a coordinated enhancement in three Al III absorption troughs. These variation characteristics convincingly indicate that the BAL outflows of J1344+3150 are under the influence from the background radiation energy. Thus, we infer that the velocity shift displayed in system A in the quasar J1344+3150 may indicate an actual line-of-sight acceleration of an outflow due to the radiation pressure from the central source.