Asphericity, Interaction, and Dust in the Type II-P/II-L Supernova 2013ej in Messier 74

TL;DR

This study of SN 2013ej combines spectropolarimetry, spectroscopy, and photometry to reveal significant asphericity, dust interaction, and circumstellar material effects, providing insights into the supernova's geometry and progenitor environment.

Contribution

It presents the first detailed multi-epoch spectropolarimetric analysis of SN 2013ej, demonstrating persistent asphericity and CSM interaction, and models the supernova's geometry as an oblate ellipsoid viewed edge-on.

Findings

SN 2013ej shows the strongest polarization observed in its class.

The supernova's geometry is consistent with an oblate ellipsoid with CSM interaction.

Asymmetrical CSM interaction influences the observed polarization and emission features.

Abstract

SN 2013ej is a well-studied core-collapse supernova (SN) that stemmed from a directly identified red supergiant (RSG) progenitor in galaxy M74. The source exhibits signs of substantial geometric asphericity, X-rays from persistent interaction with circumstellar material (CSM), thermal emission from warm dust, and a light curve that appears intermediate between supernovae of Types II-P and II-L. The proximity of this source motivates a close inspection of these physical characteristics and their potential interconnection. We present multi-epoch spectropolarimetry of SN 2013ej during the first 107 days, and deep optical spectroscopy and ultraviolet through infrared photometry past ~800 days. SN 2013ej exhibits the strongest and most persistent continuum and line polarization ever observed for a SN of its class during the recombination phase. Modeling indicates that the data are consistent with an oblate ellipsoidal photosphere, viewed nearly edge-on, and probably augmented by optical scattering from circumstellar dust. We suggest that interaction with an equatorial distribution of CSM, perhaps the result of binary evolution, is responsible for generating the photospheric asphericity. Relatedly, our late-time optical imaging and spectroscopy shows that asymmetric CSM interaction is ongoing, and the morphology of broad H-alpha emission from shock-excited ejecta provides additional evidence that the geometry of the interaction region is ellipsoidal. Alternatively, a prolate ellipsoidal geometry from an intrinsically bipolar explosion is also a plausible interpretation of the data, but would probably require a ballistic jet of radioactive material capable of penetrating the hydrogen envelope early in the recombination phase (abridged).