Polarimetry of the Type I[CLC]a[/CLC] Supernova SN 1996X

We present broad-band and spectropolarimetry of the Type Ia SN 1996X obtainedon April 14, 1996 (UT), and broad-band polarimetry of SN 1996X on May 22,1996,when the supernova was about a week before and 4 weeks after optical maximum,respectively. The Stokes parameters derived from the broad-band polarimetry areconsistent with zero polarization. The spectropolarimetry, however, shows broadspectral features which are due intrinsically to an asymmetric SN atmosphere.The spectral features in the flux spectrum and the polarization spectrum showcorrelations in the wavelength range from 4900 AA up to 5500 AA. The degree ofthis intrinsic component is low (<0.3 %). Theoretical polarization spectra havebeen calculated. It is shown that the polarization spectra are governed by lineblending. Consequently, for similar geometrical distortions, the residualpolarization is smaller by about a factor of 2 to 3 compared to the lessblended Type II atmosphere, making it intrinsically harder to detectasphericities in SNIa. Comparison with theoretical model polarization spectrashows a resemblance to the observations. Taken literally, this implies anasphericity of about 11 % in the chemical distribution in the region of partialburning. This may not imperil the use of Type Ia supernovae as standard candlesfor distance determination, but nontheless poses a source of uncertainty. SN1996X is the first Type Ia supernova for which spectropolarimetry revealed apolarized component intrinsic to the supernova and the first Type Ia withspectropolarimetry well prior to optical maximum.Comment: 7 pages, 5 figures, macros 'aas2pp4.sty,psfig.tex'. LaTeX Style. Astrophysical Journal Letters, submitted September 199