SN 2013ej: a type IIL supernova with weak signs of interaction

Abstract

We present optical photometric and spectroscopic observations of SN 2013ej. It is one of the brightest Type II supernovae (SNe II) exploded in a nearby (~10 Mpc) galaxy, NGC 628. The light-curve characteristics are similar to SNe II, but with a relatively shorter (~85 days) and steeper (~1.7 mag (100 days) ⁻¹ in V) plateau phase. The SNshows a large drop of 2.4 mag in V-band brightness during the plateau-to-nebular transition. The absolute ultraviolet (UV) light curves are identical to SN 2012aw, showing a similar UV-plateau trend extending up to 85 days. The radioactive ⁵⁶Ni mass estimated from the tail luminosity is 0.02 M☉, which is significantly lower than typical SNe IIP. The characteristics of spectral features and evolution of line velocities indicate that SN 2013ej is a Type II event. However, light-curve characteristics and some spectroscopic features provide strong support in classifying it as a Type IIL event. A detailed SYNOW modeling of spectra indicates the presence of some highvelocity components in Hα and Hβ profiles, implying a possible ejecta–circumstellar medium interaction. The nebular phase spectrum shows an unusual notch in the Hα emission, which may indicate bipolar distribution of ⁵⁶Ni. Modeling of the bolometric light curve yields a progenitor mass of ~14 M☉ and a radius of ~450 R☉, with a total explosion energy of ~2.3 x 10⁵¹ erg.