VLT Spectropolarimetry Of The Fast Expanding Type Ia SN 2006X
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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.