Abstract
Abstract 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 ∼4000 km s−1) 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 4500 km s−1—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.
Highlights
The mechanism that successfully ejects a star’s outer layers in a core-collapse supernova (CC SN) explosion remains uncertain
In this Letter, we focus exclusively on high signal-to-noise ratio (S/N) spectropolarimetry of SN 2013ej obtained during the nebular phase, in an effort to uniquely constrain the scattering environment at late times
We show direct evidence from nebular-phase spectropolarimetry that a confined, asymmetrically distributed, high-velocity scattering source exists in the ejecta of the Type II-P/L SN 2013ej
Summary
The mechanism that successfully ejects a star’s outer layers in a core-collapse supernova (CC SN) explosion remains uncertain (for a recent overview, see Ono et al 2020). Under the ansatz of simple reflection by an external scatterer that does not present circular symmetry in the plane of the sky, 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 (Hough 2006) This basic framework has been most famously employed to establish the presence of hidden broad-line regions in Type II Seyfert galaxies (Antonucci & Miller 1985). SN 2013ej occurred in the nearby galaxy M74 (D = 9.6 ± 0.7 Mpc; Huang et al 2015), and has been extensively studied at X-ray, UV, optical, and infrared wavelengths (see Morozova et al 2018, and references therein) It straddles the boundary between the Type II-Plateau (II-P) and Type II-Linear (II-L) subclassifications (Dhungana et al 2016; see Figure A1 in Appendix A), and exhibits characteristics that suggest some early interaction with circumstellar material (CSM; Morozova et al 2018; Hillier & Dessart 2019). Appendices provide additional detail of the data acquisition and reductions (Appendix A), empirical analysis techniques (Appendix B), and our modeling approach and the inferences drawn from it (Appendix C; Cikota et al 2017)
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