AIMS: The main goal of this work is to study possible signs of asymmetry in
the bright optical transient in NGC300, with the aim of getting independent
information on the explosion mechanism, the progenitor star and its
circumstellar environment.
METHODS: Using VLT-FORS1 we have obtained low-resolution optical linear
spectropolarimetry of NGC300 OT2008-1 on two epochs, 48 and 55 days after the
discovery, covering the spectral range 3600--9330A.
RESULTS: The data show a continuum polarization at a very significant level.
At least two separate components are identified. The first is characterized by
a strong wavelength dependency and a constant position angle (68.6+/-0.3
degrees), which is parallel to the local spiral arm of the host galaxy. The
second shows a completely different position angle (151.3+/-0.4) and displays a
mild but statistically significant evolution between the two epochs. While the
former is identified as arising in the interstellar dust associated with
NGC300, the latter is most likely due to continuum polarization by dust
scattering in the circumstellar environment. No line depolarization is detected
in correspondence of the most intense emission lines, disfavoring electron
scattering as the source of intrinsic polarization. This implies a very small
deviation from symmetry in the continuum-forming region. Given the observed
level of intrinsic polarization, the transient must be surrounded by a
significant amount of dust (>4x10^-5 Msun), asymmetrically distributed within a
few thousand AU. This most likely implies that one or more asymmetric outflow
episodes took place during the past history of the progenitor.Comment: Accepted for publication in Astronomy and Astrophysics. 16 pages, 16
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