Supernova 2013by: a Type IIL supernova with a IIP-like light-curve drop★

We present multiband ultraviolet and optical light curves, as well as visual-wavelength and near-infrared spectroscopy of the Type II linear (IIL) supernova (SN) 2013by. We show that SN 2013by and other SNe IIL in the literature, after their linear decline phase that start after maximum, have a sharp light-curve decline similar to that seen in SNe IIP. This light-curve feature has rarely been observed in other SNe IIL due to their relative rarity and the intrinsic faintness of this particular phase of the light curve. We suggest that the presence of this drop could be used as a physical parameter to distinguish between subclasses of SNe II, rather than their light-curve decline rate shortly after peak. Close inspection of the spectra of SN 2013by indicate asymmetric line profiles and signatures of high-velocity hydrogen. Late (∼90 d after explosion) near-infrared spectra of SN 2013by exhibit oxygen lines, indicating significant mixing within the ejecta. From the late-time light curve, we estimate that 0.029 M_⊙ of ⁵⁶Ni was synthesized during the explosion. It is also shown that the V-band light-curve slope is responsible for part of the scatter in the luminosity (V magnitude 50 d after explosion) versus ⁵⁶Ni relation. Our observations of SN 2013by and other SNe IIL through the onset of the nebular phase indicate that their progenitors are similar to those of SNe IIP.