  We report the results of a spectropolarimetric observation of the C IV
mini-BAL in the quasar HS1603+3820. The observations were carried out with the
FOCAS instrument on the Subaru telescope and yielded an extremely high
polarization sensitivity of 0.1%, at a resolving power of 1500. HS1603+3820 has
been the target of a high-resolution spectroscopic monitoring campaign for more
than four years, aimed at studying its highly variable C IV 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 (a) the polarization
level is very small (p~0.6%) throughout the spectrum, and (b) the polarization
level does not increase across the mini-BAL trough. Thus, the variable screen
scenario emerges as our favored explanation of the C IV 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.
