GRB 111209A is the second longest lasting gamma-ray burst (GRB) detected by the Swift Gamma-Ray Burst Mission, observed on December 9, 2011. Its duration is longer than 7 hours, implying this event has a different kind of progenitor than normal long GRBs. It was first proposed that the progenitor of this event was a blue supergiant star with low metallicity. Later, it was also proposed that this event is the prototype of a new class of GRBs, ultra-long GRBs.
The GRB was associated with the magnetar-powered supernova SN 2011kl, an object of intermediate luminosity between conventional GRB supernovae and superluminous supernovae.
See also List of gamma-ray bursts Supermassive black hole Tidal force GRB 250702B, the longest-duration GRB ever detected
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