Multi-epoch Spectropolarimetry of SN 2011fe
  We present multiple spectropolarimetric observations of the nearby Type Ia
supernova SN 2011fe in M101, obtained before, during, and after the time of
maximum apparent visual brightness. SN 2011fe exhibits time-dependent
polarization in both the continuum and strong absorption lines. At all epochs,
red wavelengths exhibit a degree of continuum polarizaiton of 0.2-0.4 per cent,
likely indicative of persistent asymmetry in the electron-scattering
photosphere. However, the degree of polarization across the Si II 6355
absorption line varies dramatically from epoch to epoch. Before maximum, Si II
6355 shows enhanced polarization at the same position angle (PA) as the
polarized continuum. During two epochs near maximum, however, Si II 6355
absorption has a lower degree of polarization, with a PA that is 90deg from the
continuum. After maximum, the absorption feature has the same degree of
polarization and PA as the adjacent continuum. Another absorption feature in
the blue (either Si II 5051 or a blend with Fe II lines) shows qualitatively
similar changes, although the changes are shifted in time to an earlier epoch.
This behavior is similar to that seen in broad absorption-line quasars, where
the polarization in absorption features has been interpreted as the line
absorbing some of the unpolarized continuum flux. This behavior, along with the
90deg shifts of the polarization PA with time, imply a time-dependent
large-scale asymmetry in the explosion.
