Narrow absorption line variability in repeat quasar observations from
  the Sloan Digital Sky Survey
  We present the results from a time domain study of absorption lines detected
in quasar spectra with repeat observations from the Sloan Digital Sky Survey
Data Release 7 (SDSS DR7). Beginning with over 4500 unique time separation
baselines of various absorption line species identified in the SDSS DR7 quasar
spectra, we create a catalogue of 2522 quasar absorption line systems with two
to eight repeat observations, representing the largest collection of unbiased
and homogeneous multi-epoch absorption systems ever published. To investigate
these systems for time variability of narrow absorption lines, we refine this
sample based on the reliability of the system detection, the proximity of
pixels with bright sky contamination to individual absorption lines, and the
quality of the continuum fit. Variability measurements of this sub-sample based
on the absorption line equivalent widths yield a total of 33 systems with
indications of significantly variable absorption strengths on time-scales
ranging from one day to several years in the rest frame of the absorption
system. Of these, at least 10 are from a class known as intervening absorption
systems caused by foreground galaxies along the line of sight to the background
quasar. This is the first evidence of possible absorption line variability
detected in intervening systems, and their short time-scale variations suggest
that small-scale structures (~10-100 au) are likely to exist in their host
foreground galaxies.
