Slowly fading super-luminous supernovae that are not pair-instability explosions

Super-luminous supernovae (1-4) that radiate more than [10.sup.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 (5,6). 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 [sup.56]Ni are synthesized; this isotope decays to [sup.56]Fe via [sup.56]Co, powering bright light curves (7,8). Such massive progenitors are expected to have formed from metal-poor gas in the early Universe (9). Recently, supernova 2007 bi 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 (1,10). 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 (2,11,12), which are not powered by radioactivity. Modelling our observations with 10-16 solar masses of magnetar-energized (13,14) 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 x [10.sup.-6] times that of the core-collapse rate.
The discovery of a luminous transient, PTF 12dam, was first reported (15) by the Palomar Transient Factory on 23 May 2012. We recovered the transient in Pan-STARRS1 (Panoramic Survey Telescope [...]