Optical Spectropolarimetry and Asphericity of the Type Ic SN 2007gr^*

We present an optical spectropolarimetric observation of the Type Ic supernova (SN) 2007gr made with the Subaru Telescope at 21 days after maximum brightness (~37 days after the explosion). Nonzero polarization as high as ~3% is observed at the absorption feature of the Ca II IR triplet. The polarization of the continuum light is ~0.5% if we estimate the interstellar polarization (ISP) component by assuming that the continuum polarization has a single polarization angle. This suggests that the axis ratio of the SN photosphere projected on the sky differs from unity by ~10%. The polarization angle at the Ca II absorption is almost aligned with that of the continuum light. These features may be understood in the context of a model in which a bipolar explosion with an oblate photosphere is being viewed from a slightly off-axis direction and explosively synthesized Ca near the polar region obscures the light that originates around the minor axis of the SN photosphere. Given the uncertainty in the ISP, however, the polarization data could also be interpreted with a model with an almost spherically symmetric photosphere and a clumpy Ca II distribution.