ASASSN-15nx: A Luminous Type II Supernova with a “Perfect” Linear Decline

Abstract

We report a luminous Type II supernova, ASASSN-15nx, with a peak luminosity of mag that is between those of typical core-collapse supernovae and super-luminous supernovae. The post-peak optical light curves show a long, linear decline with a steep slope of 2.5 mag (100 day) −1 (i.e., an exponential decline in flux) through the end of observations at phase . In contrast, the light curves of hydrogen-rich supernovae (SNe II-P/L) always show breaks in their light curves at phase ∼100 day, before settling onto 56 Co radioactive decay tails with a decline rate of about 1 mag (100 day) −1 . 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 a dense circumstellar medium (CSM). ASASSN-15nx has a number of spectroscopic peculiarities, including a relatively weak and triangular-shaped H α emission profile with no absorption component. The physical origin of these peculiarities is unclear, but the long and linear post-peak light curve without a break suggests a single dominant powering mechanism. Decay of a large amount of ( M Ni  = 1.6 ± 0.2 ) can power the light curve of ASASSN-15nx, and the steep light-curve slope requires substantial γ -ray escape from the ejecta, which is possible given a low-mass hydrogen envelope for the progenitor. Another possibility is strong CSM interactions powering the light curve, but the CSM needs to be sculpted to produce the unique light-curve shape and avoid producing SN IIn-like narrow emission lines.