Hydrogen-poor superluminous supernovae (SLSN-I) are a class of rare and energetic explosions that have been discovered in untargeted transient surveys in the past decade1,2. The progenitor stars and the physical mechanism behind their large radiated energies (about 1051 erg or 1044 J) are both debated, with one class of models primarily requiring a large rotational energy3,4 and the other requiring very massive progenitors that either convert kinetic energy into radiation through interaction with circumstellar material (CSM)5,6,7,8 or engender an explosion caused by pair-instability (loss of photon pressure due to particle–antiparticle production)9,10. Observing the structure of the CSM around SLSN-I offers a powerful test of some scenarios, although direct observations are scarce11,12. Here, we present a series of spectroscopic observations of the SLSN-I iPTF16eh, which reveal both absorption and time- and frequency-variable emission in the Mg ii resonance doublet. We show that these observations are naturally explained as a resonance scattering light echo from a circumstellar shell. Modelling the evolution of the emission, we infer a shell radius of 0.1 pc and velocity of 3,300 km s−1, implying that the shell was ejected three decades before the supernova explosion. These properties match theoretical predictions of shell ejections occurring because of pulsational pair-instability and imply that the progenitor had a helium core mass of about 50–55 M⊙, corresponding to an initial mass of about 115 M⊙.
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The photometry of iPTF16eh is available in Supplementary Table 1, and the spectra are available from WISeREP35 (http://wiserep.weizmann.ac.il/). In general, the data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.
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We are grateful for discussions with C.-I. Björnsson, S. Blinnikov, E. Sorokina, E. Ramirez-Ruiz and J. Fuller. The iPTF project is a scientific collaboration among the California Institute of Technology, Los Alamos National Laboratory, the University of Wisconsin, Milwaukee, the Oskar Klein Centre, the Weizmann Institute of Science, the TANGO Program of the University System of Taiwan, and the Kavli Institute for the Physics and Mathematics of the Universe. This work was supported by the GROWTH project funded by the National Science Foundation (NSF grant no. 1545949). This research was supported by the Swedish Research Council, the Swedish National Space Board, and the Knut and Alice Wallenberg Foundation. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (NASA). A.G.-Y. is supported by the European Union through ERC grant no. 725161, the Quantum Universe I-Core programme, the Israel Science Foundation, the BSF Transformative programme and a Kimmel award. P.E.N. acknowledges support from the US Department of Energy (DOE) through DE-FOA-0001088, Analytical Modeling for Extreme-Scale Computing Environments. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the US DOE under contract no. DE-AC02-05CH11231. IRAF is distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy under cooperative agreement with the NSF. Some of the data were obtained at the W.M. Keck Observatory, which 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 financial support of the W.M. Keck Foundation. The authors wish to recognize and acknowledge the cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are fortunate to have the opportunity to conduct observations from this mountain.