[Results] We report positive linear polarization detections at several epochs for XRF 080109/SN 2008 at a level of ∼1%. For SN2007uy the measured polarization is around ∼ 1.5%. In both cases the observed linear polarization seems dominated by the host galaxy interstellar polarization (HGIP), especially for the case of SN2007uy. Dust emission at 1.2 mm detected at and around the XRF position supports this scenario. Despite the dominant HGIP, a statistical analysis of the distribution of the XRF 080109/SN 2008D Stokes parametres suggests that it could show a possible intrinsic variable polarization component. Moreover we show that the temporal evolution of the intrinsic XRF 080109/SN 2008D polarization could be explained by an aspherical axisymmetric expansion with variable eccentricity, although other more complex geometric scenarios are also compatible. In contrast, the SN2007uy polarization could be described by the HGIP plus a constant eccentricity expansion on the plane of the sky. We suggest that at least the projected, if not the intrinsic, geometry of XRF 080109/SN 2008D and SN2007uy could be different.

The research of JG, AJCT, MJ and IA is supported by the Spanish programmes ESP2005-07714-C03-03, AYA2004-01515, AYA2007- 67627-C03-03, AYA2008-03467/ESP and AYA2009-14000-C03-01. AdUP acknowledges support from an ESO fellowship. AR and SK acknowledge support by DFG Kl 766/11-3, PF and DAK by the Thüringer Landessternwarte. IA acknowledges support by an I3P contract with the Consejo Superior de Investigaciones Científicas (CSIC). The Dark Cosmology Centre is funded by the DNRF. The German-Spanish Astronomical Center, Calar Alto, is jointly operated by the MPIA Heidelberg and IAA (CSIC). IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). The VLT data were acquired with target of opportunity programme 080.D-0008(A).

[Methods] We present a V-band linear polarization monitoring campaign carried out for the X-ray flash XRF 080109/SN 2008D and SN2007uy, which shone for weeks contemporaneously in NGC2770. This fortunate coincidence brought us the opportunity to observe both objects simultaneously, and most importantly, with identical instrumental setups. The observations span 74.9 days, starting 3.6 days after the X-ray flash and are distributed in 11 visits. In addition we performed observations in the millimetre (mm) range in order to identify the dominant origin of the observed polarization.

[Context] There is a general consensus that Long Gamma-Ray Bursts (GRBs), including X-ray Flashes (XRFs), are created by the explosion of massive stars. However little is still known about the geometry of such stellar explosions. In this paper we study the optical polarimetric properties of an XRF dominated by the associated hypernova, in other words, an XRF with no contaminating afterglow. The final scope of this study is to shed light on the still uncertain picture of the GRBs’ expansion geometry.

[Aims] The main aim is to investigate the evolution of the linear optical polarization of the X-ray flash XRF 080109/SN 2008D. As a secondary product, we also report the polarization evolution of SN2007uy, and discuss the properties of the host galaxy interstellar medium (ISM) towards the XRF.

9 pages, 3 tables, 2 figures.

Peer reviewed