Narrow absorption lines with two observations of Sloan Digital Sky
  Survey
  We assemble 3524 quasars from Sloan Digital Sky Survey (SDSS) with repeated
observations to search for variations of narrow C IV1548,1551 and Mg
II2796,2803 absorption doublets in spectral regions shortward of 7000 Ang at
the observed frame, which corresponds to time-scales of about 150 ~ 2643 days
at quasar rest frame. In these quasar spectra, we detect 3580 C IV absorption
systems with z_{abs} = 1.5188 ~ 3.5212, and 1809 Mg II absorption systems with
z_{abs} = 0.3948 ~ 1.7167. In term of the absorber velocity (beta) distribution
at quasar rest frame, we find a substantial number of C IV absorbers with
beta<0.06, which might be connected to the absorptions of quasar outflows. The
outflow absorptions peak at v~2000 km/s and drop rapidly below the peak value.
Among 3580 C IV absorption systems, 52 systems (~ 1.5%) show obvious variations
in equivalent widths at the absorber rest frame (Wr): 16 enhanced, 16 emerged,
12 weaken, and 8 disappeared systems, respectively. We find that changes in
Wr548 are neither related to time-scales of the two SDSS observations, nor to
absorber velocities at the quasar rest frame. Variable absorptions in
low-ionization species are important to constraint the physical conditions of
absorbing gas. There are two variable Mg II absorption systems measured from
SDSS spectra detected by Hacker et al. However, in our Mg II$ absorption
sample, we find that neither shows variable absorption with confident levels of
>4sigma for lambda2796 lines and >3sigma for lambda2803 lines.
