Velocity Shifts in Quasar Outflows: Evidence for Acceleration Driven by Central Source Radiation

Previously mentioned studies indicate that velocity shifts of BALs are rare in quasar outflows. However, recent research on BALs that can be separated into NALs has revealed several cases of velocity shifts (Lu & Lin 2019; Yao et al. 2020; Lu & Lin 2020a,b; Lin & Lu 2020). For instance, in 2019, we identified an accelerated CIV BAL in the quasar SDSS J120819.29+035559.4, which experienced a velocity shift of 1166 km s-1 within 3 years (rest frame). Another example is from 2020 when we reported coordinated acceleration of MgII and AlIII BALs in the quasar SDSS J134444.33+315007.6, which experienced velocity shifts of 1101 and 1170 km s-1, respectively, over 3.21 years (rest frame). We attempted to explore the reasons for the observed velocity shifts and found that, in addition to the shifts, these BALs also shared similar characteristics: (1) increased equivalent widths (EW), (2) a noticeable reduction in the continuum, and (3) coordinated enhancement of emission lines. These characteristics suggest that the outflows in these sources are influenced by radiation from the central source, leading us to speculate that the velocity shifts of these BALs may be an actual acceleration caused by central source radiation.