The Ultra-long Gamma-Ray Burst 111209A: The Collapse of a Blue Supergiant?
Piro, L.; D'Elia, V.; Cutini, S.; Klotz, A.; Basa, S.; Stratta, G.; Coward, D. M.; Gendre, B.; Howell, E. J.; Atteia, J. L.; Boër, M.
Italy, Canada, France, Australia
Abstract
We present optical, X-ray and gamma-ray observations of GRB 111209A, observed at a redshift of z = 0.677. We show that this event was active in its prompt phase for about 25000 s, making it the longest burst ever observed. This rare event could have been detected up to z ~ 1.4 in gamma-rays. Compared to other long gamma-ray bursts (GRBs), GRB 111209A is a clear outlier in the energy-fluence and duration plane. The high-energy prompt emission shows no sign of a strong blackbody component, the signature of a tidal disruption event, or a supernova shock breakout. Given the extreme longevity of this event, and lack of any significant observed supernova signature, we propose that GRB 111209A resulted from the core-collapse of a low-metallicity blue supergiant star. This scenario is favored because of the necessity to supply enough mass to the central engine over a duration of thousands of seconds. Hence, we suggest that GRB 111209A could have more in common with population III stellar explosions, rather than those associated with normal long GRBs.