Spectropolarimetry of the Type Ib Supernova iPTF 13bvn: revealing the complex explosion geometry of a stripped-envelope core-collapse supernova

Abstract

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 days with respect to the {\it r}-band light curve maximum. The continuum is intrinsically polarised to the $0.2-0.4\%$ level throughout the observations, implying asphericities of $\sim10\%$ in the shape of the photosphere. We observe significant line polarisation 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 $\sim 6200\mathrm{\AA}$, usually identified as Si II $\lambda 6355$, is most likely to be high velocity $\mathrm{H\alpha}$ at $-16,400$ $\mathrm{km \; s^{-1}}$. Two distinctly polarised 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 polarisation 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 $\sim 50-65\%$ of the ejecta radius. The observed polarisation 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.