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

TL;DR

GRB 111209A is the longest gamma-ray burst observed, likely caused by the collapse of a low metallicity blue supergiant star, challenging typical progenitor models for long GRBs.

Contribution

This paper reports the discovery and analysis of the longest gamma-ray burst, proposing a novel progenitor model involving a blue supergiant star.

Findings

GRB 111209A lasted about 25,000 seconds, the longest recorded.

The event lacked a supernova signature, suggesting a different progenitor.

The burst's properties imply a collapse of a low metallicity blue supergiant star.

Abstract

We present optical, X-ray and gamma-ray observations of GRB 111209A, at a redshift of z = 0.677. We show that this event was active in its prompt phase for about 25000 seconds, making it the longest burst ever observed. This rare event could have been detected up to z ~ 1.4. Compared to other long 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 black body component, as expected if the event was caused by a tidal disruption event or a supernova shock breakout. Given the extreme longevity of this event, and a lack of a supernova signature, we propose that GRB 111209A is a relatively rare stellar collapse of a low metallicity blue super giant star. Only this progenitor can supply mass to the central engine over a duration of thousands of seconds. Hence, GRB 111209A could have more in common with population III stellar explosions, rather than normal long gamma ray bursts.