ASASSN-15nx: A Luminous Type II Supernova with a “Perfect” Linear Decline
Journal
Date Issued
2018
Author(s)
Bose, Subhash
•
Dong, Subo
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Kochanek, C. S.
•
•
Katz, Boaz
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Bersier, David
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Andrews, Jennifer E.
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Prieto, J. L.
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Stanek, K. Z.
•
Shappee, B. J.
•
Smith, Nathan
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Kollmeier, Juna
•
•
•
Chen, Ping
•
•
Milne, Peter
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Morales-Garoffolo, Antonia
•
•
•
Bilinski, Christopher
•
Brimacombe, Joseph
•
Frank, Stephan
•
Holoien, T. W. -S.
•
Kilpatrick, Charles D.
•
Kiyota, Seiichiro
•
Madore, Barry F.
•
Rich, Jeffrey A.
Abstract
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 ≈ 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 {}56{Ni} (M Ni = 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.
Volume
862
Issue
2
Start page
107
Issn Identifier
0004-637X
Ads BibCode
2018ApJ...862..107B
Rights
open.access
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