Evidence for photoionization-driven variability in narrow absorption
  lines
  In this Letter, we report the discovery of a strong correlation between the
variability of narrow absorption lines (NALs) and the ionizing continuum from a
two-epoch spectra sample of 40 quasars containing 52 variable C iv
{\lambda}{\lambda}1548; 1551 absorption doublets. According to the concordance
index, this sample is classified into two subsamples. Subsample I shows an
anti-correlation between the variations of absorption lines and the continuum,
while Subsample II exhibits a positive correlation. These results imply that
these variable C iv {\lambda}{\lambda}1548; 1551 absorption doublets are
intrinsic to the corresponding quasars and that their variations are caused
primarily by the fluctuations of the ionizing continuum. Based on our analysis,
we propose that there might be two kinds of absorption gas: one that is very
sensitive to the continuum variations, the another that is not. In addition, we
suggest that in many cases the emergence or disappearance of NALs is caused by
fluctuations of the ionizing continuum.
