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A superluminous supernova lightened by collisions with pulsational pair-instability shells

Abstract

Superluminous supernovae are among the most energetic stellar explosions in the Universe, but their energy sources remain an open question. Here we present long-term observations of one of the closest examples of the hydrogen-poor superluminous supernovae subclass SLSNe-I, supernova SN 2017egm, revealing the most complicated known luminosity evolution of SLSNe-I. Three distinct post-peak bumps were recorded in its light curve collected at about 100–350 days after maximum brightness, challenging current popular power models such as magnetar, fallback accretion, and interaction between ejecta and a circumstellar shell. However, the complex light curve can be well modelled by successive interactions with multiple circumstellar shells with a total mass of about 6.8–7.7 M. In this scenario, large energy deposition from interaction-induced reverse shocks results in ionization of neutral oxygen in the supernova ejecta and hence a much lower nebular-phase line ratio of [O i] λ6,300/([Ca ii] + [O ii]) λ7,300 (~0.2) compared with that derived for other superluminous and normal stripped-envelope supernovae. The pre-existing multiple shells indicate that the progenitor of SN 2017egm experienced pulsational mass ejections triggered by pair instability within 2 years before explosion, in robust agreement with theoretical predictions for a pre-pulsation helium-core mass of 48–51 M.

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Fig. 1: Multiband light curves of SN 2017egm.
Fig. 2: Model fits to the bolometric light curves of SN 2017egm.
Fig. 3: Spectral signs of powerful energy deposition for SN 2017egm.
Fig. 4: The nebular-phase [O i]/([Ca ii] + [O ii]) ratios of SLSNe-I and other types of envelope-stripped supernovae.
Fig. 5: Comparison with hydrogen-free PPI models.

Data availability

All data of SN 2017egm reported in this paper are publicly available via the Figshare repository (https://doi.org/10.6084/m9.figshare.22009868)86 and the Weizmann Interactive SN Repository (https://wiserep.weizmann.ac.il)87.

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Acknowledgements

We acknowledge the staff of the 50BiN, AZT, LJT, Swift, XLT, Lick, Palomar and Keck observatories for assistance with the observations. We thank R. J. Foley and S. Kulkarni for their Keck + LRIS observations, and S. Adams for the Keck + MOSFIRE observation. We also thank J. Sollerman for sharing the luminosity data of iPTF14hls, S. Saito for providing Subaru spectrum of SN 2017egm, L. Dessart and A. Jerkstrand for useful discussions, and M. Nicholl and Q. Fang for providing their results of spectral analysis for normal core-collapse and/or superluminous supernovae. The operation of XLT is supported by the Open Project Program of the Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences. Funding for the LJT has been provided by the Chinese Academy of Sciences and the People’s Government of Yunnan Province. The LJT is jointly operated and administrated by Yunnan Observatories and Center for Astronomical Mega-Science, CAS. This research has made use of the Keck Observatory Archive (KOA), which is operated by the W. M. Keck Observatory and the NASA Exoplanet Science Institute (NExScI), under contract with the National Aeronautics and Space Administration. The W. M. Keck Observatory is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA; the observatory was made possible by the generous financial support of the W. M. Keck Foundation. The Kast red CCD detector upgrade, led by B. Holden, was made possible by the Heising-Simons Foundation, William and Marina Kast, and the University of California Observatories. Research at Lick Observatory is partially funded by a generous gift from Google. The work of X.W. is supported by the National Natural Science Foundation of China (NSFC grants 12288102 and 12033003), the Scholar Program of Beijing Academy of Science and Technology (DZ:BS202002), and the Tencent Xplorer Prize. A.G.-Y.’s research is supported by the European Union via ERC grant 725161, the ISF GW excellence centre, an IMOS space infrastructure grant and BSF/Transformative and GIF grants, as well as the André Deloro Institute for Advanced Research in Space and Optics, the Schwartz/Reisman Collaborative Science Program and the Norman E. Alexander Family Foundation ULTRASAT Data Center Fund, Minerva and Yeda-Sela; A.G.-Y. is the incumbent of the The Arlyn Imberman Professorial Chair. A.V.F.’s group at UC Berkeley received financial support from the Miller Institute for Basic Research in Science (where A.V.F. was a Miller Senior Fellow), the Christopher R. Redlich Fund, and many individual donors. N.B. is funded/co-funded by the European Union (ERC, CET-3PO, 101042610). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them. L.X. acknowledges support from National Natural Science Foundation of China (grant no. 12103050), Advanced Talents Incubation Program of the Hebei University, and Midwest Universities Comprehensive Strength Promotion project. L.W. acknowledges support from the National Program on Key Research and Development Project of China (grant 2021YFA0718500).

Author information

Authors and Affiliations

Authors

Contributions

W.L. and X.W. wrote the paper. W.L. led the analysis. X.W. led the overall project. L.Y. and A.G.-Y. assisted with the paper. J.M. reduced the 50BiN, AZT and TNT data. T.G.B. reduced the Keck LRIS data. L.Y. obtained and reduced P200 telescope data. A.V.F., T.G.B. and W.Z. obtained and reduced the Lick telescope data. A.V.F. also helped with the paper. D.X. and H.L. reduced the XLT data. R.L. advised on the interpretation of narrow spectral lines. P.B. reduced the Swift data. M.K. and L.Y. provided the Keck + MOSFIRE data. R.L., C.F. and N.B. provided their Keck + LRIS observations. D.M. and S.A.E. obtained the AZT imaging. K.Z. and Jicheng Zhang obtained the XLT observations. S.Y. and Jujia Zhang reduced the LJT data. Z.C., L.D., K.W. and L.X. helped with the draft. L.W. helped check the model code.

Corresponding author

Correspondence to Xiaofeng Wang.

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The authors declare no competing interests.

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Nature Astronomy thanks Manos Chatzopoulos, Peter Blanchard and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 The bolometric light curve of SN 2017egm, as compared to some representative SLSNe-I.

Comparison SLSNe-I include objects with monotonic decay after maximum light (left panel: PTF12dam, yellow stars; SN 2010gx, yellow-green triangles; SN 2018hti, purple squares) and those with post-peak bumps (right panel: LSQ14an, orange squares; PS1-12cil, brown triangles; SN 2015bn, blue triangles; SN 2017gci, green diamonds; ZTF sample, gray lines). The bolometric luminosities of PTF12dam, SN 2017gci, SN 2018hti and the ZTF sample are retrieved from the literature18,75,90,91, while those of other comparison SLSNe-I are derived by fitting the UV-absorbed blackbody curve12 to its multiband photometry57,92,93,94,95,96. In the inset of the right panel, the luminosity evolution of a peculiar SN II iPTF14hls (red pluses) is shown for comparison19,84. The error bars show 1σ uncertainties.

Extended Data Fig. 2 Spectra of SN 2017egm and its host galaxy.

The observations were obtained with Lick+Kast (yellow-green), LJT+YFOSC (yellow), Keck+LRIS (black), Keck+MOSFIRE (light blue), P200+DBSP (green), and XLT+BFOSC (blue). All of the spectra were smoothed with a Savitzky-Golay filter. Telluric absorption that was not removed in some of the spectra is marked with an Earth symbol.

Extended Data Fig. 3 Narrow spectral features observed in the spectra of SN 2017egm and its host galaxy.

The spectra of SN and galaxy are displayed in black and red, respectively. Left panel: He I emission (yellow dot-dashed) and Na I D absorption (purple dashed). Right panel: [N II] (yellow dashed) and Hα (purple dot-dashed).

Extended Data Fig. 4 Identification of emission lines from the host galaxy of SN 2017egm.

The black line shows the Keck+LRIS spectrum obtained at the SN location on 2018 July 13, and the green line displays the residual spectrum after subtraction of the host-galaxy stellar component (red lines) obtained with a starlight-fitting tool79. The oxygen and calcium emission features from the SN are shown in gray shade. Incompletely removed telluric contamination is marked with an Earth symbol.

Extended Data Fig. 5 Near-infrared (NIR) spectrum of SN 2017egm.

The NIR spectra of superluminous SN 2017gci (yellow)75 and Type Ic SN 2013ge (blue)77 are overplotted for comparison. The prominent feature coincident with He I λ10,830 is shaded in gray. All spectra were smoothed with a Savitzky-Golay filter. See text for discussion.

Extended Data Fig. 6 Close-up view of the [O I] λλ6300, 6364 and [O II] λλ7319, 7330 + [Ca II] λλ7291, 7323 features in late-time spectra of SN 2017egm.

Upper panel: observations of [O I] (left) and [O II] + [Ca II] (right) features during the interval t = 126-374 d are shown with dashed16, dotted66 and solid (this work) lines, respectively. The rest-frame wavelengths of these spectral lines are indicated by vertical dot-dashed lines. The 6300 Å feature at t = 120-210 d appears to be asymmetric about the rest wavelengths of [O I], possibly owing to blends with other absorption or emission features. Lower panel: the photometry-calibrated profiles of [O I] (purple) and [O II] + [Ca II] (gray) at t=312 d and t=374 d can be well fitted with Gaussian functions as indicated by the dotted and dashed profiles, respectively. The spectral coverage contaminated by the incompletely removed telluric line is shaded in red and excluded from the Gaussian fit. All spectra were smoothed with the Savitzky-Golay filter.

Extended Data Fig. 7 Environmental metallicity of SN 2017egm estimated from different line-ratio diagnostics via the PYMCZ code97.

For each diagnostic, we perform 2,000 Monte Carlo sampling to obtain the median (black solid line) and interquartile range (IQR; orange box) of the metallicity distribution; the distribution spanning the range of 1.5 × IQR is represented by blue dashed lines (whisker), beyond which the sampling points are considered to be outliers; the minima and maxima values are shown by horizontal blue lines. The gray-shaded region shows the range of solar oxygen abundances reported in the literature.

Extended Data Table 1 Fitting Parameters of the MCSIRD Model for SN 2017egm

Supplementary information

Supplementary Information

Supplementary Tables 1-5 and Fig. 1.

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Lin, W., Wang, X., Yan, L. et al. A superluminous supernova lightened by collisions with pulsational pair-instability shells. Nat Astron 7, 779–789 (2023). https://doi.org/10.1038/s41550-023-01957-3

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