Evidence for Photoionization Driven Broad Absorption Line Variability
  We present a qualitative analysis of the variability of quasar broad
absorption lines using the large multi-epoch spectroscopic dataset of the Sloan
Digital Sky Survey Data Release 10. We confirm that variations of absorption
lines are highly coordinated among different components of the same ion or the
same absorption component of different ions for C IV, Si IV and N V.
Furthermore, we show that the equivalent widths of the lines decrease or
increase statistically when the continuum brightens or dims. This is further
supported by the synchronized variations of emission and absorption line
equivalent width, when the well established intrinsic Baldwin effect for
emission lines is taken into account. We find that the emergence of an
absorption component is usually accompanying with dimming of the continuum
while the disappearance of an absorption line component with brightening of the
continuum. This suggests that the emergence or disappearance of a C IV
absorption component is only the extreme case, when the ionic column density is
very sensitive to continuum variations or the continuum variability amplitude
is larger. These results support the idea that absorption line variability is
driven mainly by changes in the gas ionization in response to continuum
variations, that the line-absorbing gas is highly ionized, and in some extreme
cases, too highly ionized to be detected in UV absorption lines. Due to
uncertainties in the spectroscopic flux calibration, we cannot quantify the
fraction of quasars with asynchronized continuum and absorption line
variations.
