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# An extremely energetic supernova from a very massive star in a dense medium

## Abstract

The interaction of a supernova with a circumstellar medium (CSM) can dramatically increase the emitted luminosity by converting kinetic energy to thermal energy. In ‘superluminous’ supernovae of type IIn—named for narrow hydrogen lines1 in their spectra—the integrated emission can reach2,3,4,5,6 ~1051 erg, attainable by thermalizing most of the kinetic energy of a conventional supernova. A few transients in the centres of active galaxies have shown similar spectra and even larger energies7,8, but are difficult to distinguish from accretion onto the supermassive black hole. Here we present a new event, SN2016aps, offset from the centre of a low-mass galaxy, that radiated 5 × 1051 erg, necessitating a hyper-energetic supernova explosion. We find a total (supernova ejecta + CSM) mass likely exceeding 50−100 M, with energy 1052 erg, consistent with some models of pair-instability supernovae or pulsational pair-instability supernovae—theoretically predicted thermonuclear explosions from helium cores >50 M. Independent of the explosion mechanism, this event demonstrates the existence of extremely energetic stellar explosions, detectable at very high redshifts, and provides insight into dense CSM formation in the most massive stars.

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## Data availability

All data are publicly available via the Open Supernova Catalog61 (https://sne.space) and the Weizmann Interactive SN Repository80 (https://wiserep.weizmann.ac.il).

## Code availability

MOSFiT is publicly available at https://github.com/guillochon/MOSFiT. SuperBol is publicly available at https://github.com/mnicholl/superbol.

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## Acknowledgements

M.N. is a Royal Astronomical Society Research Fellow. The Berger Time-Domain Group acknowledge NSF grant AST-1714498 and NASA grant NNX15AE50G. R.L. acknowledges a Marie Skłodowska-Curie Individual Fellowship within the Horizon 2020 European Union Framework (H2020-MSCA-IF-2017-794467). W.-f.F. and K.P. acknowledge support from NSF grant numbers AST-1814782 and AST-1909358. The Margutti group acknowledges NSF grant number AST 1909796, NASA grants 80NSSC19K0384 and 80NSSC19K0646. A.A.M. is supported by the LSST Corporation, the Brinson Foundation, the Moore Foundation via the LSSTC Data Science Fellowship Program, and the CIERA Fellowship Program. A.V.-G. acknowledges support by the Danish National Research Foundation (DNRF132). Data were obtained via the NASA/ESA Hubble Space Telescope archive at the Space Telescope Science Institute, the Swift archive, the Smithsonian Astrophysical Observatory OIR Data Center, the MMT Observatory, the MDM Observatory, the Gemini Observatory, operated by the Association of Universities for Research in Astronomy, Inc., under agreement with the NSF, and the W.M. Keck Observatory, operated as a partnership among the California Institute of Technology, the University of California, and NASA. Operation of the Pan-STARRS1 telescope is supported by NASA under Grants NNX12AR65G and NNX14AM74G. The authors respect the very significant cultural role of Mauna Kea within the indigenous Hawaiian community.

## Author information

Authors

### Contributions

M.N. wrote the manuscript, led the analysis and obtained the HST and Gemini data. P.K.B. devised the selection algorithm and identified SN2016aps as an interesting source, and analysed the HST images. E.B. advised on the manuscript and leads the overall project. R.C. and K.B. obtained the MDM spectrum and classified SN2016aps. R.M. analysed the UVOT data. P.K.B., S.G., A.B. and P.C. obtained FLWO data. R.L. obtained the Keck spectra. A.V.-G. did the rate calculations. A.A.M., F.J.M. and R.R.L. provided PTF data. W.-f.F. and P.K.B. obtained MMT imaging. G.T., A.A.M. and K.P. obtained Keck imaging. All authors helped with the interpretation.

### Corresponding author

Correspondence to Matt Nicholl.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Supplementary information

### Extended Data Fig. 1 Bolometric light curve of SN2016aps.

Top: Comparison of the bolometric light curve to other SLSNe IIn. The integrated luminosity is greater than any previously known SN. Middle: Temperature evolution from blackbody fits (Methods). Bottom: Photospheric radius from blackbody fits. Error bars show 1σ uncertainties.

### Extended Data Fig. 2 Fit to the light curve of SN2016aps with MOSFIT using a wind-like density profile.

(a) For fixed parameters based on simple scaling relations (see Supplementary Information). The fit is reasonable overall but systematically under-predicts the UV bands. (b) Realizations of a full MCMC fit. This matches the UV but favours masses larger by a factor ~ 3. Posteriors of the model parameters are given in Extended Data Fig. 3. Error bars show 1σ uncertainties.

### Extended Data Fig. 3 Posteriors for physical parameters inferred using the MOSFIT CSM model.

Parameters are shown for fits using both shell (ρ=constant; black) and wind (ρ r−2; blue) density profiles. The additional variance (noise) parameter, σ ~ 0.1, indicates a similar quality of fit independent of the assumed density. Both models favour ejected masses 100M, with a comparable mass of CSM. Drawing from the joint Mej-vej posteriors gives a kinetic energy Ek ≈ 5 × 1052 erg in both cases.

### Extended Data Fig. 4 Simulated observer frame LSST light curves in g,r,i,z bands.

The left column shows our interaction model for SN2016aps, while the right column shows a radioactively-powered PISN model for a 130 M helium core (Methods). The rows show the same models at redshifts z=0.1, 0.5, 0.75, 1 and 1.5. The interacting model is still well detected at z=1.5, because it is bright in the UV (whereas the radioactive model is heavily absorbed by metal lines). Therefore interacting events like SN2016aps are detectable over a volume that is larger by a factor 7. Error bars show simulated 1σ uncertainties.

### Extended Data Fig. 5 Measurements of the Balmer lines.

The equivalent width of Hα increases over the first 300 days as the continuum fades, similar to other Type IIn SNe and SLSNe, before decreasing in the final epoch. The Hα/Hβ ratio is initially consistent with recombination (horizontal line), but at later times increases to >10, indicating collisional excitation. No Hβ could be measured in the final spectrum, the arrow indicates a lower limit on this ratio. Error bars show 1σ uncertainties.

### Extended Data Fig. 6 Spectroscopic comparison of SN2016aps to the energetic nuclear transients PS16dtm and PS1-10adi.

PS16dtm is thought to be a TDE [7] and PS1-10adi has been suggested to be a possible SN close to an AGN [8], these two nuclear transients closely resemble each other. The spectra shown here are at around 200 days after maximum light. SN2016aps is distinguished from these events by broader and more symmetrical Balmer lines (lacking a red shoulder), and a lack of strong, narrow Fe II emission. SN2016aps also lacks the [O III] emission seen at 5000Å (see also Supplementary Information). The apparent absorption in PS16dtm at 7000Å is a telluric feature from the Earth’s atmosphere.

## Supplementary information

### Supplementary Information

Supplementary discussion.

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Nicholl, M., Blanchard, P.K., Berger, E. et al. An extremely energetic supernova from a very massive star in a dense medium. Nat Astron 4, 893–899 (2020). https://doi.org/10.1038/s41550-020-1066-7

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• DOI: https://doi.org/10.1038/s41550-020-1066-7

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