VLT Spectropolarimetry of the fast expanding Type Ia SN2006X
  Using VLT-FORS1 we performed optical spectropolarimetric observations of the
Type Ia SN2006X on 7 pre-maximum epochs (day -10 to day -1) and one
post-maximum epoch (+39 days). The SN shows strong continuum interstellar
polarization reaching about 8% at 4000A, characterized by a wavelength
dependency that is substantially different from that of the Milky-Way dust
mixture. Several SN features, like SiII 6355A and the CaII IR triplet, present
a marked evolution. The CaII near-IR triplet shows a pronounced polarization
(~1.4%) already on day -10 in correspondence with a strong high-velocity
feature (HVF). The SiII polarization peaks on day -6 at about 1.1% and
decreases to 0.8% on day -1. By day +39 no polarization signal is detected for
the SiII line, while the CaII IR triplet shows a marked re-polarization at the
level of 1.2%. As in the case of another strongly polarized SN (2004dt), no
polarization was detected across the OI 7774A absorption. The fast-expanding
SN2006X lies on the upper edge of the relation between peak polarization and
decline rate, and it confirms previous speculations about a correlation between
degree of polarization, expansion velocity, and HVF strength. The polarization
of CaII detected in our last epoch, the most advanced ever obtained for a Type
Ia SN, coincides in velocity with the outer boundary of the Ca synthesized
during the explosion (15,000-17,000 km/s) in delayed-detonation models. This
suggests a large scale chemical inhomogeneity as produced by off-center
detonations, a rather small amount of mixing, or a combination of both effects.
In contrast, the absence of polarization at the inner edge of the Ca-rich layer
(8000-10,000 km/s) implies a substantial amount of mixing in these deeper
regions.
