OPTICAL SPECTROPOLARIMETRY AND ASPHERICITY OF TYPE Ic SN 2007

We present optical spectropolarimetric observations of Type Ic supernova (SN) 2007gr with Subaru telescope at 21 days after the maximum brightness (∼ 37 days after the explosion). Non-zero polarization as high as ∼ 3 % is observed at the absorption feature of Ca ii IR triplet. The polarization of the continuum light is ∼ 0.5% if we assume an interstellar polarization (ISP) with which SN photosphere has a single polarization angle. It suggests that the axis ratio of the SN photosphere projected to the sky is different from unity by ∼ 10%. The polarization angle at the Ca ii absorption is almost aligned to that of the continuum light. These features may be understood by the model where a bipolar explosion with an oblate photosphere is viewed from the slightly off-axis direction and explosively synthesized Ca near the polar region obscures the light originated around the minor axis of the SN photosphere. Given the uncertainty of the ISP, however, the polarization data could also be interpreted by the model with an almost spherically symmetric photosphere and a clumpy Ca ii distribution. Subject headings: polarization — supernovae: general — supernovae: individual (SN 2007gr)