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

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 multiepoch 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 show that asymmetric CSM interaction is ongoing, and the morphology of broad Hα 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. Finally, our latest space-based optical imaging confirms that the late interaction-powered light curve dropped below the stellar progenitor level, confirming the RSG star's association with the explosion.

The Transition of a Type IIL Supernova into a Supernova Remnant: Late-time Observations of SN 2013by

“…The spectrum of SN 2013ej on day +807 is dominated by its broad, flat-topped Hα profile that extends out to velocities of roughly -9000 and +10,000 km s −1 (Mauerhan et al 2017). Such flat-topped or 'square' emission line profiles can be produced by a spherical shell of material that becomes excited by a reverse shock.…”

The Magnetar Model of the Superluminous Supernova GAIA16apd and the Explosion Jet Feedback Mechanism

“…The possibility that jets play a role in the explosion process of massive stars has been mentioned in the literature over the years. Three examples, among many others, are the recent paper by Mauerhan et al (2017) who discuss the axisymmetrical explosion of SN 2013EJ, the polarimetric observations of SN 2015bn that indicate an elongated morphology (Inserra et al 2016), and the presence of jets in CCSN remnants (e.g., Lopez et al 2014;Milisavljevic et al 2013). As well, many studies simulated jets in CCSNe (e.g., Bromberg & Tchekhovskoy 2016).…”

The two promising scenarios to explode core collapse supernovae

“…Observations and their analysis of supernova remnants (SNRs) and of polarizations in CCSNe (e.g., Wang et al 2001;Maund et al 2007;Lopez et al 2011Lopez et al , 2013Lopez et al , 2014Milisavljevic et al 2013;González-Casanova et al 2014;Fesen & Milisavljevic 2016;Inserra et al 2016;Mauerhan et al 2017; suggest that jets play a role in at least some CCSNe. As well, several arguments based on analytical studies and numerical simulations suggest that the collapse of a pre-explosion rapidly rotating core leads to the formation of two opposite well collimated jets (e.g.…”