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 et al, 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
Related URLs:	http://arxiv.org/abs/1804.00025v1
Date Deposited:	01 Aug 2018 11:08
Last Modified:	04 Sep 2021 10:34
DOI or ID number:	10.3847/1538-4357/aacb35
URI:	https://researchonline.ljmu.ac.uk/id/eprint/8457

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