Spectropolarimetry of the Type Ib Supernova iPTF 13bvn

We present six epochs of spectropolarimetric observations and one epoch of spectroscopy of the Type Ib SN iPTF 13bvn. The epochs of these observations correspond to -10 to +61 d with respect to the r-band light-curve maximum. The continuum is intrinsically polarized to the 0.2-0.4 per cent level throughout the observations, implying asphericities of similar to 10 per cent in the shape of the photosphere. We observe significant line polarization associated with the spectral features of Ca II IR3, He I/Na I D, He I lambda lambda 6678, 7065, Fe II lambda 4924 and O I lambda 7774. We propose that an absorption feature at similar to 6200 angstrom, usually identified as Si II lambda 6355, is most likely to be high-velocity H alpha at -16 400 km s(-1). Two distinctly polarized components, separated in velocity, are detected for both He I/Na I D and Ca II IR3, indicating the presence of two discrete line-forming regions in the ejecta in both radial velocity space and in the plane of the sky. We use the polarization of He I lambda 5876 as a tracer of sources of non-thermal excitation in the ejecta; finding that the bulk of the radioactive nickel was constrained to lie interior to similar to 50-65 per cent of the ejecta radius. The observed polarization is also discussed in the context of the possible progenitor system of iPTF 13bvn, with our observations favouring the explosion of a star with an extended, distorted envelope rather than a compact Wolf-Rayet star.