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We present optical, X-ray and gamma-ray observations of GRB 111209A, at aredshift of z = 0.677. We show that this event was active in its prompt phasefor about 25000 seconds, making it the longest burst ever observed. This rareevent could have been detected up to z ~ 1.4. Compared to other long GRBs, GRB111209A is a clear outlier in the energy-fluence and duration plane. Thehigh-energy prompt emission shows no sign of a strong black body component, asexpected if the event was caused by a tidal disruption event or a supernovashock breakout. Given the extreme longevity of this event, and a lack of asupernova signature, we propose that GRB 111209A is a relatively rare stellarcollapse of a low metallicity blue super giant star. Only this progenitor cansupply mass to the central engine over a duration of thousands of seconds.Hence, GRB 111209A could have more in common with population III stellarexplosions, rather than normal long gamma ray bursts.

The ultra-long Gamma-Ray Burst 111209A: the collapse of a blue supergiant?