Gamma-ray bursts. Credit: NASA/Fermi och Aurore Simonnet, Sonoma State University
The NASA satellites Swift and Fermi observe a gamma-ray bursts. When the high-energy radiation hits the Earth’s atmosphere, so-called Cherenkov light is created. These pulses of visible light can be measured by large ground-based telescopes. Picture: NASA/Fermi och Aurore Simonnet, Sonoma State University

“Even before this discovery, we sometimes had difficulties explaining how the highest-energy radiation was produced. These new results make it clear that we need to use other physical processes in our models,” says Magnus Axelsson, an astrophysicist at Stockholm University and a co-author of two of the three papers being published in the scientific journal Nature.

The new results are made by the telescopes Major Atmospheric Gamma Imaging Cherenkov (MAGIC), on the island of La Palma, and High Energy Stereoscopic System (H.E.S.S.), in the Namibian desert. Both telescopes look for the faint pulses of visible light, called Cherenkov radiation, that is produced when radiation with sufficiently high radiation reaches the Earth’s atmosphere. This is the first time such observations have been able to detect gamma-ray bursts, and provides new insight on these extreme phenomena. In the case of MAGIC, the maximum energy seen was over 1 teraelectronvolt – that’s one trillion times the energy of visible light. Just as surprising, the H.E.S.S. observation shows that this high-energy radiation can last for many hours after the initial flash of gamma-rays.

The new observations are made by two different large international collaborative projects and the articles published involve hundreds of researchers from all over the world, including Stockholm University and KTH Royal Institute of Technology in Sweden.

Gamma-ray bursts discovered almost 50 years ago

Gamma-ray bursts were first discovered almost 50 years ago. The most common type occurs when a star much more massive than the Sun runs out of fuel. Its core collapses and forms a black hole, which then blasts jets of particles outward at nearly the speed of light. These jets pierce the star and continue into space. They produce an initial pulse of gamma rays – the most energetic form of light – that typically lasts about a minute.
As the jets race outward, they interact with surrounding gas and emit light across the spectrum, from radio to gamma rays. These so-called afterglows can be detected up to months – and rarely, even years – after the burst at longer wavelengths.

Magnus Axelsson, Department of Astronomy. Photo: Stockholm University
Magnus Axelsson, astrophysicist at the Department of Astronomy. Photo: Stockholm University

“This long-lasting afterglow gives us important information about both the gamma-ray burst itself, as well as the surroundings,” says Magnus Axelsson. “The big surprise from the H.E.S.S. results is that the very high energy emission is still seen so long after the onset of the burst. It’s clear we need to rethink our theoretical models.”

In a third article, also published in the same issue of Nature, researchers have combined data from many different observatories and used them to test physical models. The best candidate, they say, is inverse Compton scattering. High-energy electrons in the jet crash into lower-energy gamma rays and boost them to much higher energies.
Magnus Axelsson highlights the importance of such multi-wavelength analyses.
“As an example, data from the Fermi satellite played a key role in the interpretation. We saw emission at lower and very high energies, but very little in the middle range covered by Fermi. This told us that the very high energy emission was a distinct afterglow component, which means some additional process must be at work.”

Further studies are needed

The observations have opened a new pathway to understand gamma-ray bursts, but further studies will be needed to clarify the physical picture behind these events.

“Gamma-ray bursts also allow us to test fundamental physics in some of the most extreme environments known, and their brightness allows us to see them across vast distances. In the future, they may become cosmic light houses whose light allows us to probe the history and structure of the Universe.”

Gamma-ray burst. Credit: NASA's Goddard Space Flight Center
Ground-based facilities have detected radiation up to a trillion times the energy of visible light from a cosmic explosion called a gamma-ray burst. This illustration shows the set-up for the most common type. Picture: NASA’s Goddard Space Flight Center

The articles in the scientific journal Nature:

Article from the observations from H.E.S.S: “A very-high-energy component deep in the γ-ray burst afterglow”

Article from the observations from MAGIC: ”Teraelectronvolt emission from the gamma-ray burst GRB 190114C”

Article from several satellites: ”Observation of inverse Compton emission from a long gamma-ray burst”

The telescopes:




The film “Overview Animation of Gamma-ray Burst” illustrates how scientists think that gamma bursts are created. Credit: NASA’s Goddard Space Flight Center.