Super-luminous supernovae that radiate more than 10^44 ergs per second attheir peak luminosity have recently been discovered in faint galaxies atredshifts of 0.1-4. Some evolve slowly, resembling models of 'pair-instability'supernovae. Such models involve stars with original masses 140-260 times thatof the Sun that now have carbon-oxygen cores of 65-30 solar masses. In thesestars, the photons that prevent gravitational collapse are converted toelectron-positron pairs, causing rapid contraction and thermonuclearexplosions. Many solar masses of 56Ni are synthesized; this isotope decays to56Fe via 56Co, powering bright light curves. Such massive progenitors areexpected 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 afterthe Big Bang) with a metallicity one-third that of the Sun was observed to looklike a fading pair-instability supernova. Here we report observations of twoslow-to-fade super-luminous supernovae that show relatively fast rise times andblue colours, which are incompatible with pair-instability models. Theirlate-time light-curve and spectral similarities to supernova 2007bi call thenature of that event into question. Our early spectra closely resemble typicalfast-declining super-luminous supernovae, which are not powered byradioactivity. Modelling our observations with 10-16 solar masses ofmagnetar-energized ejecta demonstrates the possibility of a common explosionmechanism. The lack of unambiguous nearby pair-instability events suggests thattheir local rate of occurrence is less than 6x10^-6 times that of thecore-collapse rate.
展开▼
