ar X iv : 0 90 8 . 28 41 v 1 [ as tr o - ph . S R ] 2 0 A ug 2 00 9 The Early Asymmetries of Supernova 2008 D / XRF 080109 1

Spectropolarimetry of the Type Ib SN 2008D, associated with the XRF 080109, at two separate epochs, are presented. The epochs of these observations correspond to V-band light curve maximum and 15 days after light curve maximum (or 21 and 36 days after the XRF). We find SN 2008D to be significantly polarized, although the largest contribution is due to the interstellar polarization component of QIS P = 0 ± 0.1% and UIS P = −1.2 ± 0.1%. At the two epochs, the spectropolarimetry of SN 2008D is classified as being D1+L(He I)+L(Ca II). The intrinsic polarization of continuum wavelength regions is < 0.4%, at both epochs, implying an asymmetry of the photosphere of < 10%. Similar to other Type Ibc SNe, such as 2005bf, 2006aj and 2007gr, we observed significant polarization corresponding to the spectral features of Ca II, He I, Mg I, Fe II and, possibly, O I λ7774, about a close-to-sphericallysymmetric photosphere. We introduce a new plot showing the chemically distinct line forming regions in the ejecta and comment on the apparent ubiquity of highly polarized high-velocity Ca II features in Type Ibc SNe. The polarization angle of Ca II IR triplet was significantly different, at both epochs, to those of the other species, suggesting high-velocity Ca II forms in a separate part of the ejecta. The apparent structure in the outer layers of SN 2008D has implications for the interpretation of the early-time X-ray emission associated with shock break-out. We present two scenarios, within the jet-torus paradigm, which explain the lack of an apparent geometry discontinuity between the two observations: 1) a jet which punched a hole straight through the progenitor and deposited Ni outside the ejecta and 2) a jet which stalled inside the radius of the photosphere as observed at the second epoch. The lack of a peculiar polarization signature, suggesting strongly asymmetric excitation of the ejecta, and the reported properties of the shock-breakout favour the second scenario.