SDSS J1138+3517: A quasar showing remarkably variable broad absorption
  lines
  We report on the highly variable SiIV and CIV broad absorption lines in SDSS
J113831.4+351725.2 across four observational epochs. Using the SiIV doublet
components, we find that the blue component is usually saturated and non-black,
with the ratio of optical depths between the two components rarely being 2:1.
This indicates that these absorbers do not fully cover the line-of-sight and
thus a simple apparent optical depth model is insufficient when measuring the
true opacity of the absorbers. Tests with inhomogeneous (power-law) and
pure-partial coverage (step-function) models of the absorbing SiIV optical
depth predict the most un-blended doublet's component profiles equally well.
However, when testing with Gaussian-fitted doublet components to all SiIV
absorbers and averaging the total absorption predicted in each doublet, the
upper limit of the power law index is mostly unconstrained. This leads us to
favour pure partial coverage as a more accurate measure of the true optical
depth than the inhomogeneous power law model.
  The pure-partial coverage model indicates no significant change in covering
fraction across the epochs, with changes in the incident ionizing flux on the
absorbing gas instead being favoured as the variability mechanism. This is
supported by (a) the coordinated behaviour of the absorption troughs, (b) the
behaviour of the continuum at the blue end of the spectrum and (c) the
consistency of photoionization simulations of ionic column density dependencies
on ionization parameter with the observed variations. Evidence from the
simulations together with the CIV absorption profile indicates that the
absorber lies outside the broad line region, though the precise distance and
kinetic luminosity are not well constrained.
