VLT spectropolarimetry of the fast expanding type Ia SN 2006X

Aims. The main goal of this study is to probe the ejecta geometry and to get otherwise unobtainable information about the explosion mechanism of type Ia Supernovae (SNe). Methods. Using VLT-FORS1 we performed optical spectropolarimetric observations of the type Ia SN 2006X on 7 pre-maximum epochs (day -10 to day -1) and one post-maximum epoch (+39 days). Results. The SN shows strong continuum interstellar polarization reaching about 8% at 4000 angstrom, characterized by a wavelength dependency that is substantially different from that of the Milky-Way dust mixture. Several SN features, like Si II 6355 angstrom and the Ca II IR triplet, present a marked evolution. The Ca II near-IR triplet shows a pronounced polarization (similar to 1.4%) already on day -10 in correspondence with a strong high-velocity feature (HVF). The Si II 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 Si II line, while the Ca II 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 O I 7774 angstrom absorption. Conclusions. The fast-expanding SN 2006X 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 Ca II 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(-1)) 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(-1)) implies a substantial amount of mixing in these deeper regions.