Spectropolarimetry of the Type Ib Supernova iPTF 13bvn: revealing the complex explosion geometry of a stripped-envelope core-collapse supernova
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2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. 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 ~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 6678, 7065, Fe II 4924 and O I 7774. We propose that an absorption feature at ~6200 , usually identified as Si II 6355, is most likely to be high-velocity Ha 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 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 ~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.