Detection of optical linear polarization in the SN2006aj/XRF060218 non-spherical expansion

Abstract

We have performed optical polarimetric observations of the SN2006aj associated to the X-ray flash (XRF) of February 18, 2006, XRF060218 that provide information on its expansion geometry. The data were acquired in the R-band with the 0.7m telescope of Crimea, 2.5m Nordic Optical Telescope and the 2.2m of Calar Alto. We report the detection of linear polarization between 3 and 39 days after the gamma-ray event (t-t_0). This represents the first polarization detection of a Ic supernova (SN) associated to an XRF. Our data exhibit a degree of linear polarization (P) around P~4% at t-t_0 ~ 3-5 days, followed by a constant polarization phase with P~1.4% at 13.7 < t-t_0 < 39 days. Our data suggest a decay in P, and more interestingly, show a position angle (θ) rotation of ~100 degrees comparing data taken before and after the R-band lightcurve peak. The reported polarization measurements can be explained by the evolution of an asymmetric SN expansion. We discuss on several ingredients that could account for the observed θrotation.

Submitted to A&A. Revised version, comments of the referee included. 4 pages, 3 figures

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• The Supernova -- Gamma-Ray Burst Connection
• An optical supernova associated with the X-ray flash XRF 060218
• A neutron-star-driven X-ray flash associated with supernova SN 2006aj
• Early-Time Photometry and Spectroscopy of the Fast Evolving SN 2006AJ Associated with GRB 060218
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• Supernova 2006aj and the associated X-Ray Flash 060218
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