Letter to the Editor

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

¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), Apartado de Correos, 3.004, 18080 Granada, Spain e-mail: jgu@iaa.es
² Astronomical Institute of St. Petersburg University, Petrodvorets, Universitetski pr. 28, 198504, Russia
³ Nikolaev State University, Nikolskaja 24, Nikolaev 54030, Ukraine
⁴ Instituto de Astrofísica de Canarias, La Laguna, Tenerife, 38200 Canary Islands, Spain
⁵ XMM-Newton Science Operations Centre, European Space Agency, Villafranca del Castillo, PO Box 50727, 28080 Madrid, Spain

Received: 13 September 2006
Accepted: 29 September 2006

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 (). 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 at days, followed by a constant polarization phase with at days. Our data suggest a decay in P, and more interestingly, show a position angle (θ) rotation of ~ 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.