We present spectropolarimetric observations of the Type IIb SN 2001ig in NGC 7424 ; conducted with the ESO VLT FORS1 on 2001 Dec 16 , 2002 Jan 3 and 2002 Aug 16 or 13 , 31 and 256 days post-explosion .
These observations are at three different stages of the SN evolution : ( 1 ) The hydrogen-rich photospheric phase , ( 2 ) the Type II to Type Ib transitional phase and ( 3 ) the nebular phase .
At each of these stages , the observations show remarkably different polarization properties as a function of wavelength .
We show that the degree of interstellar polarization is 0.17 % .
The low intrinsic polarization ( \sim 0.2 \% ) at the first epoch is consistent with an almost spherical ( < 10 \% deviation from spherical symmetry ) hydrogen dominated ejecta .
Similar to SN 1987A and to Type IIP SNe , a sharp increase in the degree of the polarization ( \sim 1 \% ) is observed when the outer hydrogen layer becomes optically thin by day 31 ; only at this epoch is the polarization well described by a “ dominant axis. ” The polarization angle of the data shows a rotation through \sim 40 \arcdeg between the first and second epochs , indicating that the asymmetries of the first epoch were not directly coupled with those observed at the second epoch .
For the most polarized lines , we observe wavelength-dependent loop structures in addition to the dominant axis on the Q - U plane .
We show that the polarization properties of Type IIb SNe are roughly similar to one another , but with significant differences arising due to line blending effects especially with the high velocities observed for SN 2001ig .
This suggests that the geometry of SN 2001ig is related to SN 1993J and that these events may have arisen from a similar binary progenitor system .