TRUE2: Establishing a detectability limit on hidden broad line regions

True Seyfert 2 candidates are those Seyferts galaxies whose optical spectral
do not show broad lines, nevertheless in the X-ray domain, they exhibit some
characteristic behavior of Seyferts 1 such as lack of X-ray obscuration and/or
short timescale variability. A true 2 candidate will be confirmed as a true
Seyfert 2 if the lack of its broad line region (BLR) is not only observational
but physical. These kind of objects are thought to accrete at low Eddington
rates, in agreement with theoretical models that predict that the BLR disappears
below a certain critical value of accretion rate and/or luminosity. In the
last decade, a significant number of true Seyfert 2s with low accretion rates
has been claimed in the literature. However, some exceptions as GNS 069 or
2XMM J1231+1106 show high accretion rates, which seem to contradict the
generally accepted explanation.
A limit on the detection of hidden broad line regions (HBLRs) must be established
in order to make sure that BLRs are not present intrinsically. Since true
Seyfert 2 candidates are selected by the absence of X-ray obscuration, the
most plausible explanation to cause the non-detection of a physically present
HBLR would be the absence of an adequate scattering medium. Polarimetry
can play a key role to answer this question. The presence of an efficient scattering
region would imply a high continuum of polarization. We propose to
assess what degrees of polarization are high enough to indicate the presence
of a scattering medium able to act as a mirror and thus providing us
with the indirect view of the HBLRs.
We got new imaging polarimetry data from ISIS@WHT of 10 true 2 candidates
which had not been checked in polarized light. If scattering regions are
present, undeniable degrees of polarization around 1−3% should be measured.
Comparing the measured continuum of polarization with simulations
we will be able to estimate a decidability limit on HBLRs. Specifically, we will
apply STOKES, a Monte Carlo radiative transfer code which can be used to
model, predict, fit and interpret the polarization of AGN