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We present optical spectropolarimetric observations of Type Ic supernova (SN)2007gr with Subaru telescope at 21 days after the maximum brightness (~37 daysafter the explosion). Non-zero polarization as high as ~3% is observed at theabsorption feature of Ca II IR triplet. The polarization of the continuum lightis ~0.5% if we estimate the interstellar polarization (ISP) component assumingthat the continuum polarization has a single polarization angle. This suggeststhat the axis ratio of the SN photosphere projected to the sky is differentfrom unity by ~10%. The polarization angle at the Ca II absorption is almostaligned to that of the continuum light. These features may be understood by themodel where a bipolar explosion with an oblate photosphere is viewed from theslightly off-axis direction and explosively synthesized Ca near the polarregion obscures the light originated around the minor axis of the SNphotosphere. Given the uncertainty of the ISP, however, the polarization datacould also be interpreted by the model with an almost spherically symmetricphotosphere and a clumpy Ca II distribution.

Optical Spectropolarimetry and Asphericity of Type Ic SN 2007gr