The Early Asymmetries of Supernova 2008D / XRF 080109

TL;DR

This study presents spectropolarimetric observations of Supernova 2008D, revealing asymmetries in its ejecta and insights into the geometry of the explosion, with implications for understanding early X-ray emissions.

Contribution

First spectropolarimetric analysis of SN 2008D, highlighting asymmetries and chemical structures in the ejecta related to early X-ray emission.

Findings

SN 2008D is significantly polarized, mainly due to interstellar polarization.

Intrinsic polarization of the photosphere is less than 0.4%, indicating less than 10% asymmetry.

High-velocity Ca II features are highly polarized and form in separate ejecta regions.

Abstract

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 Q_ISP=0+/-0.1% and U_ISP=-1.2+/-0.1%. At the two epochs, the spectropolarimetry of SN 2008D is classified as being D1+L(HeI)+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 7774, about a close-to-spherically-symmetric 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. (abridged)