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SN 2013ej is a well-studied core-collapse supernova (SN) that stemmed from adirectly identified red supergiant (RSG) progenitor in galaxy M74. The sourceexhibits signs of substantial geometric asphericity, X-rays from persistentinteraction with circumstellar material (CSM), thermal emission from warm dust,and a light curve that appears intermediate between supernovae of Types II-Pand II-L. The proximity of this source motivates a close inspection of thesephysical characteristics and their potential interconnection. We presentmulti-epoch spectropolarimetry of SN 2013ej during the first 107 days, and deepoptical spectroscopy and ultraviolet through infrared photometry past ~800days. SN 2013ej exhibits the strongest and most persistent continuum and linepolarization ever observed for a SN of its class during the recombinationphase. Modeling indicates that the data are consistent with an oblateellipsoidal photosphere, viewed nearly edge-on, and probably augmented byoptical scattering from circumstellar dust. We suggest that interaction with anequatorial distribution of CSM, perhaps the result of binary evolution, isresponsible for generating the photospheric asphericity. Relatedly, ourlate-time optical imaging and spectroscopy shows that asymmetric CSMinteraction is ongoing, and the morphology of broad H-alpha emission fromshock-excited ejecta provides additional evidence that the geometry of theinteraction region is ellipsoidal. Alternatively, a prolate ellipsoidalgeometry from an intrinsically bipolar explosion is also a plausibleinterpretation of the data, but would probably require a ballistic jet ofradioactive material capable of penetrating the hydrogen envelope early in therecombination phase (abridged).

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