SDSS J1138+3517: a quasar showing remarkably variable broad absorption lines

We report on the highly variable Si IV and C IV broad absorption lines in SDSS J113831.4+351725.2 across four observational epochs. Using the Si IV 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 Si IV optical depth predict the most un-blended doublet's component profiles equally well. However, when testing with Gaussian-fitted doublet components to all Si IV 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 C IV absorption profile indicates that the absorber lies outside the broad line region, though the precise distance and kinetic luminosity are not well constrained.