Detection of optical linear polarization in the SN 2006aj/XRF 060218 non-spherical expansion

Aims. We have performed optical polarimetric observations of the SN 2006aj associated to the X-ray flash (XRF) of February 18, 2006, XRF 060218 that provide information on its expansion geometry. Methods. The data were acquired in the R-band with the 0.7 m telescope of Crimea, 2.5 m Nordic Optical Telescope and the 2.2 m of Calar Alto. Results. 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 - to ≤ 39 days. Our data suggest a decay in P, and more interestingly, show a position angle (θ) rotation of ∼100° comparing data taken before and after the R-band lightcurve peak. Conclusions. 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 θ) rotation.