Observations of two gamma-ray bursts have revealed the highest-energy photons released by these violent explosions reported to date. The findings, presented in three Nature papers published this week, provide insights into the processes that produce such energetic events.
Gamma-ray bursts are considered to be the most energetic explosions in the Universe, and are thought to arise from the formation of neutron stars or black holes. They start with bright flashes, followed by an “afterglow” period that releases emissions over a range of energies, from radio waves to gigaelectronvolt gamma rays. The origins of very-high-energy emissions (over 100 gigaelectronvolts) are poorly understood, owing to limited observations.
The three papers report the detections of emissions of over 100 gigaelectronvolts from two gamma-ray bursts named GRB 190114C and GRB 180720B. GRB 190114C, identified in January 2019, was found to emit very-high-energy photons in the range 0.2 - 1 teraelectronvolts from around one minute after the burst. In two papers, Razmik Mirzoyan and colleagues analysed data from multiple telescopes to determine the mechanism responsible for this emission. They suggest that photons are scattered by electrons, increasing their energy, a process known as inverse Compton scattering. In a third paper, Edna Ruiz-Velasco and colleagues observe photons with energies of 100 - 440 gigaelectronvolts in the afterglow, ten hours after the initial emission of GRB 180720B, which was originally detected in July 2018. They too attribute these findings to inverse Compton scattering.
In an accompanying News & Views, Bing Zhang describes the findings as triumphs both for research into gamma-ray burst observations in general - which have proved elusive for the telescopes designed to detect high-energy explosions - and also for efforts to determine the mechanisms responsible for these events. He expects that more high-energy gamma-ray burst emissions will be observed in the future, which he suggests could reveal “treasures for researchers to mine”.