Super-luminous supernovae that radiate more than 10 44 ergs per second at their peak luminosity have recently been discovered in faint galaxies at redshifts of 0.1-4. Some evolve slowly, resembling models of 'pair-instability' supernovae. Such models involve stars with original masses 140-260 times that of the Sun that now have carbon-oxygen cores of 65-130 solar masses. In these stars, the photons that prevent gravitational collapse are converted to electron-positron pairs, causing rapid contraction and thermonuclear explosions. Many solar masses of 56 Ni are synthesized; this isotope decays to 56 Fe via 56 Co, powering bright light curves. Such massive progenitors are expected to have formed from metal-poor gas in the early Universe. Recently, supernova 2007bi in a galaxy at redshift 0.127 (about 12 billion years after the Big Bang) with a metallicity one-third that of the Sun was observed to look like a fading pair-instability supernova. Here we report observations of two slow-to-fade super-luminous supernovae that show relatively fast rise times and blue colours, which are incompatible with pair-instability models. Their late-time light-curve and spectral similarities to supernova 2007bi call the nature of that event into question. Our early spectra closely resemble typical fast-declining super-luminous supernovae, which are not powered by radioactivity. Modelling our observations with 10-16 solar masses of magnetar-energized ejecta demonstrates the possibility of a common explosion mechanism. The lack of unambiguous nearby pair-instability events suggests that their local rate of occurrence is less than 6 × 10 -6 times that of the core-collapse rate.
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Acknowledgements We thank D. Kasen and L. Dessart for sending us their model data. The Pan-STARRS1 Surveys (PS1) have been made possible through contributions of the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max Planck Society (and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg, and the Max Planck Institute for Extraterrestrial Physics, Garching), The Johns Hopkins University, Durham University, the University of Edinburgh, Queen’s University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, NASA grant no. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, National Science Foundation grant no. AST-1238877, and the University of Maryland. S.J.S. acknowledges FP7/2007-2013/ERC Grant agreement no. 291222; J.L.T. and R. P. Kirshner acknowledge NSF grants AST-1009749, AST-121196; G.L. acknowledges Swedish Research Council grant no. 623-2011-7117; A.P., L.T., E.C., S.B. and M.T.B. acknowledge PRIN-INAF 2011. Work is based on observations made with the following telescopes: the William Herschel Telescope, Gran Telescopio Canarias, the Nordic Optical Telescope, Telescopio Nazionale Galileo, the Liverpool Telescope, the Gemini Observatory, the Faulkes North Telescope, the Asiago Copernico Telescope and the United Kingdom Infrared Telescope.
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