Narrow absorption lines with two observations from the Sloan Digital Sky Survey

Abstract

We assemble 3524 quasars from the Sloan Digital Sky Survey (SDSS) with repeated observations to search for variations of the narrow <f>$\\rm C\\,\\small {IV}\\,\\lambda \\lambda 1548,1551$</f> and <f>$\\rm Mg\\,\\small {II}\\,\\lambda \\lambda 2796,2803$</f> 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 C <scp>iv</scp> absorption systems with <it>z</it><inf>abs</inf> = 1.5188–3.5212 and 1809 Mg <scp>ii</scp> absorption systems with <it>z</it><inf>abs</inf> = 0.3948–1.7167. In term of the absorber velocity (β) distribution in the quasar rest frame, we find a substantial number of C <scp>iv</scp> absorbers with β < 0.06, which might be connected to absorption of quasar outflows. The outflow absorption peaks at <f>$\\upsilon \\approx 2000\\,\\rm km\\,s^{-1}$</f> and drops rapidly below this peak value. Among 3580 C <scp>iv</scp> absorption systems, 52 systems (∼1.5 per cent) show obvious variations in equivalent widths in the absorber rest frame (<it>W</it><inf>r</inf>): 16 enhanced, 16 emerged, 12 weakened and 8 disappeared systems, respectively. We find that changes in <it>W</it><inf>r</inf>λ1548 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 Mg <scp>ii</scp> absorption systems measured from SDSS spectra detected by Hacker et al. However, in our Mg <scp>ii</scp> absorption sample, we find that neither shows variable absorption with confident levels of >4σ for λ2796 lines and >3σ for λ2803 lines.