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

Bose, S, Dong, S, Kochanek, CS, Pastorello, A, Katz, B, Bersier, D, Andrews, JE, Prieto, JL, Stanek, KZ, Shappee, BJ, Smith, N, Kollmeier, J, Benetti, S, Cappellaro, E, Chen, P, Garoffolo, A, Milne, P, Elias-Rosa, N, Tartaglia, L, Tomasella, L , Bilinski, C, Brimacombe, J, Holoien, TW-S, Kilpatrick, CD, Kiyota, S, Frank, S, Madore, BF and Rich, JA (2018) ASASSN-15nx: A luminous Type II supernova with a "perfect" linear decline. The Astrophysical Journal, 862 (2). ISSN 0004-6256

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Abstract

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.

Item Type: Article
Uncontrolled Keywords: astro-ph.HE; astro-ph.HE; astro-ph.SR
Subjects: Q Science > QB Astronomy
Q Science > QC Physics
Divisions: Astrophysics Research Institute
Publisher: American Astronomical Society IOP Publishing
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Date Deposited: 01 Aug 2018 11:08
Last Modified: 01 Aug 2018 11:08
DOI or Identification number: 10.3847/1538-4357/aacb35
URI: http://researchonline.ljmu.ac.uk/id/eprint/8457

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