A spectropolarimetric test of the structure of the intrinsic absorbers in the quasar HS 1603+3820

We report the results of a spectropolarimetric observation of the C vi "mini-broad" absorption line (mini-BAL) in the quasar HS 1603+3820 (z_em = 2.542). The observations were carried out with the FOCAS instrument on the Subaru Telescope and yielded an extremely high polarization sensitivity of δp ∼ 0.1%, at a resolving power of R ∼ 1500. HS 1603+3820 has been the target of a high-resolution spectroscopic monitoring campaign for more than four years, aimed at studying its highly variable Cvi mini-BAL profile. Using the monitoring observations in an earlier paper, we were able to narrow down the causes of the variability to the following two scenarios: (1) scattering material of variable optical depth redirecting photons around the absorber and (2) a variable, highly ionized screen between the continuum source and the absorber which modulates the UV continuum incident on the absorber. The observations presented here provide a crucial test of the scattering scenario and lead us to disfavor it because (1) the polarization level is very small (p ∼ 0.6%) throughout the spectrum and (2) the polarization level does not increase across the mini-BAL trough. Thus, the variable screen scenario emerges as our favored explanation of the C vi mini-BAL variability. Our conclusion is bolstered by recent X-ray observations of nearby mini-BAL quasars, which show a rapidly variable soft X-ray continuum that appears to be the result of transmission through an ionized absorber of variable ionization parameter and optical depth.