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

Free access article

Issue		A&A Volume 459, Number 3, December I 2006


Page(s)		L33 - L36
Section		Letters
DOI		10.1051/0004-6361:20066391

A&A 459, L33-L36 (2006)
DOI: 10.1051/0004-6361:20066391

Letter

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

J. Gorosabel¹, V. Larionov², A. J. Castro-Tirado¹, S. Guziy^{1, 3}, L. Larionova², A. Del Olmo¹, M. A. Martínez¹, J. Cepa⁴, B. Cedrés⁴, A. de Ugarte Postigo¹, M. Jelínek¹, O. Bogdanov³, and A. LLorente⁵

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 (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 $P\sim4\%$ at $t-t_0 \sim 3{-}5$ days, followed by a constant polarization phase with $P \sim 1.4\%$ at $13.7 \la t-t_0 \la 39$ days. Our data suggest a decay in P, and more interestingly, show a position angle ( $\theta$ ) rotation of ~ $100^{\circ}$ 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 $\theta$ rotation.

Key words: gamma rays: bursts -- supernovae: general -- techniques: polarimetric

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