The Most Energetic And Longest Gamma-Ray Burst Afterglow Ever Has Been Detected

Artist's impression of a relativistic jet of a collapsing star creating a gamma-ray burst. Image Credit: DESY, Science Communication Lab

Astronomers using the High Energy Stereoscopic System (H.E.S.S.) in Namibia have recorded the most energetic and longest gamma-ray afterglow from one of the brightest explosions in the universe, a gamma-ray burst.

On August 29, 2019, a long gamma-ray burst was produced by a jet of material moving at almost the speed of light released by the collapse of a massive star. This event, known as GBR 190829 A, was detected by the Fermi and Swift satellites, swiftly followed by H.E.S.S. as soon as the afterglow became visible.

This quick response and the fact that the event was relatively close (1 billion light-years away) has meant some incredible insights into such an event. The afterglow was tracked for three days after the gamma-ray burst and it recorded gamma-rays up up to a trillion times more energetic than visible light. As reported in Science, the observations have revealed that the energetic emission of the afterglow, X-rays and gamma-rays, fade in sync, hinting at a common production process.

“We were really sitting in the front row when this gamma-ray burst happened," co-author Dr Andrew Taylor from the Deutsches Elektronen-Synchrotron (DESY) said in a statement. “We could observe the afterglow for several days and to unprecedented gamma-ray energies.”

“Our observations revealed curious similarities between the X-ray and very-high energy gamma-ray emission of the burst's afterglow,” added Dr Sylvia Zhu, also from DESY.

X-rays from the gamma-ray burst were detected by NASA's Swift satellite in Earth's orbit. Very-high-energy gamma rays entered the atmosphere and initiated air showers that were detected by the H.E.S.S. telescopes from the ground (artist's impression). Image credit: DESY, Science Communication Lab

This is a particularly exciting observation because leading theories suggest that X-rays and gamma-rays are produced by two different mechanisms, but this doesn’t fit with what the telescopes have seen here.

“It is rather unexpected to observe such remarkably similar spectral and temporal characteristics in the X-ray and very-high-energy gamma-ray energy bands, if the emission in these two energy ranges had different origins,” noted co-author Dmitry Khangulyan from Rikkyo University in Tokyo.


GBR 190829 A is only the fourth gamma-ray burst detected from the ground. The previous three were much more distant, their afterglow was observable for a shorter period and they appeared to be a lot weaker, so such a detailed analysis was not possible. But clearly, H.E.S.S. and other instruments can study these events in more detail.

“Looking to the future, the prospects for the detection of gamma-ray bursts by next-generation instruments like the Cherenkov Telescope Array that is currently being built in the Chilean Andes and on the Canary Island of La Palma look promising,” said H.E.S.S. spokesperson Stefan Wagner from Landessternwarte Heidelberg. “The general abundance of gamma-ray bursts leads us to expect that regular detections in the very-high energy band will become rather common, helping us to fully understand their physics.”  


 This Week in IFLScience

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