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

Abstract

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 - t0). 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 - t0 ∼ 3-5 days, followed by a constant polarization phase with P ∼ 1.4% at 13.7 ≲ t - t0 ≲ 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. © ESO 2006.