SIMBAD references

We assemble 3524 quasars from the Sloan Digital Sky Survey (SDSS) with repeated observations to search for variations of the narrow C IV λλ1548,1551 and Mg II λλ2796,2803 absorption doublets in spectral regions shortward of 7000 Å in the observed frame, which corresponds to time-scales of about 150-2643 d in the quasar rest frame. In these quasar spectra, we detect 3580 Civ absorption systems with zabs = 1.5188-3.5212 and 1809 Mgii absorption systems with zabs = 0.3948-1.7167. In term of the absorber velocity (β) distribution in the quasar rest frame, we find a substantial number of Civ absorbers with β < 0.06, which might be connected to absorption of quasar outflows. The outflow absorption peaks at υ~2000 km s^-1 and drops rapidly below this peak value. Among 3580 Civ absorption systems, 52 systems ( ∼ 1.5 percent) show obvious variations in equivalent widths in the absorber rest frame (Wr): 16 enhanced, 16 emerged, 12 weakened and 8 disappeared systems, respectively. We find that changes in Wrλ1548 are related neither to the time-scales of the two SDSS observations nor to absorber velocities in the quasar rest frame. Variable absorption in low-ionization species is important to constrain the physical conditions of the absorbing gas. There are two variable Mgii absorption systems measured from SDSS spectra detected by Hacker et al. However, in our Mgii absorption sample, we find that neither shows variable absorption with confident levels of >4σ for λ2796 lines and >3σ for λ2803 lines.