The Variable Mg ii Narrow Absorption Line Systems

Abstract

Abstract Compared to high ionization C iv absorption lines, variable Mg ii absorption lines are rare. Using spectra from the Sloan Digital Sky Survey, we investigate the variations in Mg ii narrow absorption lines (NALs) for quasars with multi-epoch observations. We have compiled 8958 Mg ii NALs in the spectral regions from the red wings of C iv emission lines to the red wings of Mg ii emission lines. Among these 8958 Mg ii NALs, 22 variable NALs are detected with and with velocity offsets ranging from to 145,513 . We find that: (1) the detected frequency of Mg ii NALs with is significantly larger than the uniformly random value expected for the Mg ii NALs with , (2) the incidence rates of the variable Mg ii NALs with are much larger than those with , (3) the velocity offsets of variable Mg ii NALs with are much smaller than the maximum velocities expected from radiation-driven outflows, and (4) the variations of variable Mg ii NALs with are obviously correlated with the changes in the quasar radiative output. Therefore, the 16 variable Mg ii NALs, whose velocities are smaller than the maximum velocities expected from radiation-driven outflows, are likely related to quasar outflows, while the 6 variable Mg ii NALs, whose velocities are much larger than the maximum velocities expected from radiation-driven outflows, possibly originated in intervening gas. We also find that both the variations and fractional variations in absorption strengths are not related to the velocity offsets of Mg ii NALs and the time intervals between the two epochs of observations. Also, the fractional variations in absorption strengths are inversely correlated with absorption strengths. In addition, both the associated and intervening Mg ii NALs can significantly vary on a timescale of days.