POLARIMETRY OF THE TYPE-IA SUPERNOVA SN 1996X

We present broadband and spectropolarimetry of the Type Ia SN 1996X ob tained on 1996 April 14 (UT), and broadband polarimetry of SN 1996X on 1996 May 22 (UT), i.e., about 1 week before and 4 weeks after optical maximum, respectively. The Stokes parameters derived from the broadba nd polarimetry are consistent with zero polarization. The spectropolar imetry, however, shows broad spectral features that are due intrinsica lly to an asymmetric supernova atmosphere. The degree of this intrinsi c component is low (similar to 0.3%). Unlike polarization in Type II S N, the features of SN 1996X are much more richly structured and show n o strong correlation with the features in the flux. Model calculations show that the polarization is produced by Thomson scattering but the spectral features are governed by line blending. Consequently, for sim ilar geometrical distortions, the residual polarization is smaller by about a factor of 2-3 compared to the less blended Type II atmosphere, making it intrinsically harder to detect asphericities in Type Ia. Th e model features are in general agreement with the observations. Taken literally, this implies an asphericity of approximate to 11% in the c hemical distribution in the region of partial burning. This may not im peril the use of Type Ia supernovae as standard candles for distance d etermination, but nonetheless 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 supernova with spectropolarimetry well prior to optical maximum.