A long gamma ray burst that seems to be more like a short gamma ray burst in its origins is described in five papers published in Nature and Nature Astronomy this week. The findings challenge long standing assumptions that the duration of such events can be directly attributed to the assumed source.
Gamma ray bursts (GRBs) are pulses of gamma-ray light produced by explosions in distant galaxies. These events have been classified into two groups: short GRBs (lasting less than two seconds), which are thought to result from the merging of two neutron stars; and long GRBs (lasting for ten seconds or more), which are associated with supernovae — the collapse of a massive star. However, there have been observations of so-called ‘oddball’ GRBs, where the duration and suggested source do not fit with this classification.
Observations of a long GRB that shares all properties but its duration with short GRBs, suggesting that it arose from a merger between two compact objects (such as neutron stars), are reported in four separate research papers published in Nature, and one in Nature Astronomy. A bright burst lasting for around one minute, GRB 211211A, was observed from a galaxy 1.1 billion light years away on 11 December 2021. Eleonora Troja and colleagues suggest it is caused by a kilonova — an explosion that occurs after the collision of two compact objects. Jillian Rastinejad and colleagues come to the same conclusion, and suggest that complex light curves, as observed for GRB 211211A (not usually seen in long GRBs associated with supernovae), may be indicative of merger events. Bin-Bin Zhang and colleagues note that the burst itself is too long to be included in the short GRB population and suggested a new progenitor of this burst to account for both gamma-ray and kilonova emissions. Alessio Mei and colleagues report high energy gamma ray emissions starting around 16 minutes after the GRB and lasting for more than 5 hours, which they propose is produced by photons released by a kilonova. Benjamin Gompertz and collaborators find that the high-energy radiation from gamma-ray burst GRB 211211A is consistent with a merger-driven scenario. They used multiple instruments on the Fermi Gamma-ray Space Telescope and the Neil Gehrels Swift Observatory to observe the event over a 5-minute period, finding that the high-energy emission is generated by electrons moving close to the speed of light. According to one possible explanation, these relativistic electrons may have been accelerated into an outflow by the ‘protomagnetar’ that is created during the merger process.
Together, these findings could improve our understanding about the origin and nature of oddball GRBs.
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