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

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 lines in their spectra—the integrated emission can reach ~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 energies, 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.