ASASSN-15nx: A luminous Type II supernova with a "perfect" linear decline

We present optical observations of an exceptional Type II supernova, ASASSN-15nx. Its peak luminosity at M_V = -20 mag is between those of typical core-collapse supernovae (CCSNe) and super-luminous supernovae (SLSNe). The post-peak multi-band optical light curves show a remarkably long, linear decline with a steep slope of 2.5 mag / 100 d (i.e., an exponential decline in flux), through to the end of our observations at phase ~ 260d. In contrast, the light curves of the two major subclasses of SNe II, Type II-P ("plateau") and Type II-L ("linear"), always show breaks in slopes at phase ~100 d, before settling onto Co56 radioactive decay tails with a decline rate typically of about 1 mag / 100 d. The spectra of ASASSN-15nx do not exhibit the narrow emission-line features characteristic of Type IIn SNe, which can have a wide variety of light-curve shapes usually attributed to strong interactions with circumstellar medium (CSM). ASASSN-15nx also exhibits a number of peculiar spectroscopic properties, including a relatively weak H-alpha emission line that has an unusually triangularly-shaped profile with no absorption component. The physical origin giving rise to these peculiarities is unclear, while the long and linear post-peak light curve without a break likely suggests a single dominant powering mechanism. The light curve is consistent with being predominantly powered by the decay of a large amount of Ni56 (M_Ni = 1.6 +/- 0.2 M_sun), and substantial gamma-ray escape from the ejecta is needed to explain the steep light-curve slope, which is possibly due to a low-mass hydrogen envelope of the progenitor. Another possibility is strong CSM interactions mostly powering the light curve, and the CSM needs to be sculpted to produce the unique light-curve shape as well as avoid producing SN IIn-like narrow emission lines. © 2018. The American Astronomical Society. All rights reserved