Evidence for High-Velocity Nickel from Nebular-Phase Spectropolarimetry of SN 2013ej

The mechanism that successfully ejects a star’s outer layers in a core-collapse supernova explosion remains uncertain. A powerful clue to its nature is the geometry of the ejected material, and the observational technique most capable of revealing this geometry for an unresolved source is polarimetry. Under the ansatz of simple reflection by an external scatterer, the polarized flux — an object’s observed polarization percentage multiplied by its total flux spectrum — reveals the spectrum of any light that has “taken a bounce” prior to reaching the observer. Here we present deep, nebular-phase spectropolarimetry of the Type II-P/L SN 2013ej, obtained 167 days after explosion with the European Southern Observatory’s Very Large Telescope. The polarized flux spectrum appears as a nearly perfect (92% correlation), redshifted (by ~4,000 km/sec) replica of the total flux spectrum. Such a striking correspondence has never been observed before in nebular-phase supernova spectropolarimetry, although data capable of revealing it have heretofore been only rarely obtained. Through comparison with 2D polarized radiative transfer simulations of stellar explosions, we demonstrate that localized ionization produced by the decay of a high-velocity, spatially confined clump of radioactive 56Ni — synthesized by and launched as part of the explosion with final radial velocity exceeding 4,500 km/sec — can reproduce the observations through enhanced electron scattering. Additional data taken earlier in the nebular phase (day 134) yield a similarly strong correlation (84%) and redshift, whereas photospheric-phase epochs that sample days 8 through 97, do not. This suggests that the primary polarization signatures of the high-velocity scattering source only come to dominate once the thick, initially opaque hydrogen envelope has turned sufficiently transparent. This detection in an otherwise fairly typical core-collapse supernova adds to the growing body of evidence supporting strong asymmetries across Nature’s most common types of stellar explosions, and establishes the power of polarized flux — and the specific information encoded by it in line photons at nebular epochs — as a vital tool in such investigations going forward.

This work was partially supported by NSF grants AST-1009571, AST-1210311 and AST-2010001.