Velocity Shift in Quasar SDSS J1344+3150: Evidence for Radiation Pressure

Secondly, we suggest that the observed velocity shift may be related to the radiation pressure from the central source. As mentioned in the introduction, at present, the physical mechanism of the velocity shift of absorption line is still difficult to determine, and there is even no observational case of velocity shift that can be perfectly explained by existing models. For J1344+3150 in this letter, only relying on the data of two observations from the SDSS, we have not been able to determine the physical mechanism of its line velocity shift. However, between the two observations of this source, we detected that the coordinated BAL variations cover a large velocity range, and significant fluctuations in the continuum and BELs. These variation characteristics convincingly indicate that the BAL outflow of J1344+3150 is under the influence from the background radiation energy. Thus, we infer that the velocity shift displayed in system A in quasar J1344+3150 may indicate an actual line-of-sight acceleration of an outflow due to radiation pressure from the central source. If the actual acceleration of an outflow is indeed the reason for the BAL velocity shift displayed in J1344+3150, then its BAL measurements may be comparable with those of the J1042+1646 in Xu et al. (2020), whose BAL velocity shift is attributed to the same reason. First, both these two sources show a synchronized velocity shift of more than one ion, but the ion ionization states of J1042+1646 (Ne VIII, O V, and Mg X) are higher than those of J1344+3150 (Mg II and Al III); secondly, the outflow velocity of J1042+1646 (∼ - 20 000 km s-1) is faster than that of J1344+3150 (∼ - 17 000 km s-1); thirdly, the velocity shift (∼ - 1550 km s-1) and average acceleration rate (1.52 cm s-2) of J1042+1646 are both larger than those of J1344+3150. These comparisons may infer that more dramatic acceleration can be seen at higher ionization states and/or higher velocities. In conclusion, we have reported a velocity shift of a lowionization broad absorption system (including the Mg II and Al III ions) in quasar SDSS J1344+3150. According to the variation characteristics displayed in its two-epoch SDSS spectra, 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, we infer that the velocity shift of the absorption lines shown in quasar J1344+3150 may be dominated by radiation pressure.