Constraints on the optical polarization source in the luminous
  non-blazar quasar 3C 323.1 (PG 1545+210) from the photometric and
  polarimetric variability
  We examine the optical photometric and polarimetric variability of the
luminous type 1 non-blazar quasar 3C 323.1 (PG 1545+210). Two optical
spectro-polarimetric measurements taken during the periods 1996$-$98 and 2003
combined with a $V$-band imaging polarimetric measurement taken in 2002 reveal
that (1) as noted in the literature, the polarization of 3C 323.1 is confined
only to the continuum emission, that is, the emission from the broad line
region is unpolarized; (2) the polarized flux spectra show evidence of a
time-variable broad absorption feature in the wavelength range of the Balmer
continuum and other recombination lines; (3) weak variability in the
polarization position angle ($PA$) of $\sim$ 4 deg over a time-scale of 4$-$6
years is observed; and (4) the V-band total flux and the polarized flux show
highly correlated variability over a time-scale of one year. Taking the
above-mentioned photometric and polarimetric variability properties and the
results from previous studies into consideration, we propose a geometrical
model for the polarization source in 3C 323.1, in which an equatorial absorbing
region and an axi-asymmetric equatorial electron-scattering region are assumed
to be located between the accretion disc and the broad line region. The
scattering/absorbing regions can perhaps be attributed to the accretion disc
wind or flared disc surface, but further polarimetric monitoring observations
for 3C~323.1 and other quasars with continuum-confined polarization are needed
to probe the true physical origins of these regions.
