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

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

来源：Astrophysical Journal, 2018, 862(2): 107.

DOI：10.3847/1538-4357/aacb35

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

We report a luminous Type II supernova, ASASSN-15nx, with a peak luminosity of M-v = -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 approximate to 260 day. In contrast, the light curves of hydrogen-rich supernovae (SNe II-P/L) always show breaks in their light curves at phase similar to 100 day, before settling onto Co-56 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 alpha 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 Ni-56 (M-Ni = 1.6 +/- 0.2 M-circle dot) can power the light curve of ASASSN-15nx, and the steep light-curve slope requires substantial gamma-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.

• 出版日期2018-8-1
• 单位北京大学