Spectropolarimetry of SN 1993J in NGC 3031

We have obtained low-resolution optical spectropolarimetry of SN 1993J which shows a high continuum linear polarization and a 0.5% drop in polarization across the Hα emission line accompanied by a ~15^deg^ position angle rotation. From these data we infer the presence of two distinct polarization components and conclude that at least part of the polarization is intrinsic to the supernova. We propose that the polarization observed at Hα is due to interstellar polarization within NGC 3031, while the continuum polarization is intrinsic to the object. After correcting the observed Stokes parameters for interstellar polarization, we find that the intrinsic continuum polarization is high, P = 1.6% +/- 0.1% at θ = 49^deg^ +/- 3^deg^, and is wavelength- independent. The Stokes flux displays blueshifted Hα absorption which suggests that the polarization is generated primarily in the underlying continuum source. Comparing our results to the scattering photosphere models of Shapiro & Sutherland we find that to produce the continuum polarization the ratio between the semimajor and semiminor axes of the scattering envelope must be >= 1.54 for the oblate case, and >= 2.0 for the prolate case. We also consider the point-source scattering envelope models of Brown & McLean, and find that for an oblate scattering envelope the models require that the inclination be close to edge-on unless the asymmetry is extreme. On the other hand, the prolate case can produce high polarization for a wide range of inclination angles.