Abstract
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 language | English (US) |
|---|---|
| Article number | 107 |
| Journal | Astrophysical Journal |
| Volume | 862 |
| Issue number | 2 |
| DOIs | |
| State | Published - Aug 1 2018 |
Funding
We thank the Las Cumbres Observatory and its staff for their continuing support of the ASAS-SN project. We are grateful to M. Hardesty of the OSU ASC technology group. ASAS-SN is supported by the Gordon and Betty Moore Foundation through grant GBMF5490 to the Ohio State University and NSF grant AST-1515927. Development of ASAS-SN has been supported by NSF grant AST-0908816, the Mt. Cuba Astronomical Foundation, the Center for Cosmology and AstroParticle Physics at the Ohio State University, the Chinese Academy of Sciences South America Center for Astronomy (CASSACA), the Villum Foundation, and George Skestos. This paper uses data products produced by the OIR Telescope Data Center, supported by the Smithsonian Astrophysical Observatory. This research was made possible through use of the AAVSO Photometric All-Sky Survey (APASS), funded by the Robert Martin Ayers Sciences Fund. This research uses data obtained through the Telescope Access Program (TAP), which has been funded by \u201CThe Strategic Priority Research Program: the Emergence of Cosmological Structures\u201D of the Chinese Academy of Sciences (grant No. 11 XDB09000000) and the Special Fund for Astronomy from the Ministry of Finance. We thank A. Gal-Yam and M, Fraser for helpful comments. We are grateful to I. Arcavi for providing us the spectroscopic data for PTF10iam. S.B., S.D., and P.C. acknowledge Project 11573003, supported by the NSFC. S.B. is partially supported by the China Postdoctoral Science Foundation through grant No. 2016M600848. A.P., L.T., S.B., and N.E.R. are partially supported by the PRIN-INAF 2014 project \u201CTransient Universe: Unveiling New Types of Stellar Explosions with PESSTO.\u201D C.S.K., K.Z.S., and T.A.T. are supported by the U.S. National Science Foundation (NSF) though grants AST-1515927 and AST-1515876. We acknowledge support by the Ministry of Economy, Development, and Tourism\u2019s Millennium Science Initiative through grant IC120009, awarded to The Millennium Institute of Astrophysics, MAS, Chile (J.L.P., C.R.-C.) and from CONICYT through FONDECYT grants 3150238 (C.R.-C.) and 1151445 (J.L.P.). Support for N.S. was provided by the NSF through grants AST-1312221 and AST-1515559 to the University of Arizona. A.M.G. acknowledges financial support by the University of C\u00E1diz though grant PR2017-64. The Liverpool Telescope is operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias, with financial support from the UK Science and Technology Facilities Council (STFC). This paper used data obtained with the MODS spectrographs built with funding from NSF grant AST-9987045 and the NSF Telescope System Instrumentation Program (TSIP), with additional funds from the Ohio Board of Regents and The Ohio State University Office of Research. The ModsIDL spectral data reduction reduction pipeline was developed in part with funds provided by NSF Grant AST-1108693. Partially based on observations collected at Copernico telescope (Asiago, Italy) of the INAF\u2014Osservatorio Astronomico di Padova. The Joan Or\u00F3 Telescope (TJO) of the Montsec Astronomical Observatory (OADM) is owned by the Catalan Government and operated by the Institute for Space Studies of Catalonia (IEEC).
Keywords
- supernovae: general
- supernovae: individual (ASASSN-15nx)
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science