ar X iv : a st ro - p h / 96 09 17 8 v 1 2 5 Se p 19 96 Polarimetry of the Type Ia Supernova SN 1996

We present broad-band and spectropolarimetry of the Type Ia SN 1996X obtained on April 14, 1996 (UT), and broad-band polarimetry of SN 1996X on May 22, 1996 (UT), when the supernova was about a week before and 4 weeks after optical maximum, respectively. The Stokes parameters derived from the broad-band polarimetry are consistent with zero polarization. The spectropolarimetry, however, shows broad spectral features which are due intrinsically to an asymmetric supernova atmosphere. The spectral features in the flux spectrum and the polarization spectrum show correlations in the wavelength range from 4900 Å up to 5500 Å. The degree of this intrinsic component is low (∼ 0.3%). Theoretical polarization spectra have been calculated. It is shown that the polarization spectra are governed by line blending. Consequently, for similar geometrical distortions, the residual polarization is smaller by about a factor of 2 to 3 compared to the less blended Type II atmosphere, making it intrinsically harder to detect asphericities in SNIa. Comparison with theoretical model polarization spectra shows a resemblance to the observations. Taken literally, this implies an asphericity of ≈ 11 % in the chemical distribution in the region of partial burning. This may not imperil the use of Type Ia supernovae as standard candles for distance determination, but nontheless poses a source of uncertainty. SN 1996X is the first Type Ia supernova for which spectropolarimetry revealed a polarized component intrinsic to the supernova and the first Type Ia with spectropolarimetry well prior to optical maximum. Subject headings: stars: individual (SN 1996X) – stars: supernovae – stars: polarimetry Also Beijing Astronomical Observatory, Beijing 100080, P. R. China email: [email protected] email: [email protected] email: [email protected] Also: Institute for Theoretical Physics, University of Basel, Klingelbergstr. 82, CH-4056 Basel, Switzerland