ASASSN-15nx: A Luminous Type II Supernova with a "perfect" Linear Decline

Subhash Bose, Subo Dong, C. S. Kochanek, Andrea Pastorello, Boaz Katz, David Bersier, Jennifer E. Andrews, J. L. Prieto, K. Z. Stanek, B. J. Shappee, Nathan Smith, Juna Kollmeier, Stefano Benetti, E. Cappellaro, Ping Chen, N. Elias-Rosa, Peter Milne, Antonia Morales-Garoffolo, Leonardo Tartaglia, L. TomasellaChristopher Bilinski, Joseph Brimacombe, Stephan Frank, T. W.S. Holoien, Charles D. Kilpatrick, Seiichiro Kiyota, Barry F. Madore, Jeffrey A. Rich

Research output: Contribution to journalArticlepeer-review

21 Scopus citations


We report a luminous Type II supernova, ASASSN-15nx, with a peak luminosity of MV = -20 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 260 day. 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 56Co 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 Hα 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 56Ni (MNi=1.6±0.2 M) 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.

Original languageEnglish (US)
Article number107
JournalAstrophysical Journal
Issue number2
StatePublished - Aug 1 2018


  • supernovae: general
  • supernovae: individual (ASASSN-15nx)

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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