Optical Spectropolarimetry and Asphericity of Type Ic SN 2007gr

TL;DR

This study uses optical spectropolarimetry to investigate the asphericity of SN 2007gr, revealing polarization features indicative of a slightly oblate, bipolar explosion viewed off-axis, with implications for supernova explosion geometry.

Contribution

First spectropolarimetric analysis of SN 2007gr showing detailed polarization features and models of explosion geometry and ejecta distribution.

Findings

Polarization of ~3% at Ca II IR triplet absorption.

Continuum polarization estimated at ~0.5%, indicating ~10% asphericity.

Polarization angle alignment suggests bipolar explosion or clumpy ejecta.

Abstract

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 estimate the interstellar polarization (ISP) component assuming that the continuum polarization has a single polarization angle. This 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.