A very luminous magnetar-powered supernova associated with an ultra-long gamma-ray burst

A new class of ultra-long duration (>10,000 s) gamma-ray bursts has recently been suggested 1,2,3 . They may originate in the explosion of stars with much larger radii than normal long gamma-ray bursts 3,4 or in the tidal disruptions of a star 3 . No clear supernova had yet been associated with an ultra-long gamma-ray burst. Here we report that a supernova (2011kl) was associated with the ultra-long duration burst 111209A, at z =0.677. This supernova is more than 3 times more luminous than type Ic supernovae associated with long gamma-ray bursts 5,6,7 , and its spectrum is distinctly different. The continuum slope resembles those of super-luminous supernovae 8,9 , but extends farther down into the rest-frame ultra-violet implying a low metal content. The light curve evolves much more rapidly than super-luminous supernovae. The combination of high luminosity and low metal-line opacity cannot be reconciled with typical type Ic supernovae, but can be reproduced by a model where extra energy is injected by a strongly magnetized neutron star (a magnetar), which has also been proposed as the explanation for super-luminous supernovae 20,20a .