For more than five decades, gamma-ray bursts have puzzled astronomers — but new observations of their powerful afterglows are challenging long-held ideas about how these cosmic explosions work. Detected accidentally in 1967 by US military satellites searching for nuclear tests, these brief flashes of high-energy radiation were later found to originate billions of light-years away and can release extraordinary amounts of energy in seconds.
Recent observations of gamma-ray burst afterglows, including one tracked by the High Energy Stereoscopic System (HESS), have offered scientists a closer look at what happens after these explosions and raised fresh questions about the physics driving them.
The first known gamma-ray burst was discovered by accident. In July 1967, US satellites Vela 3 and Vela 4 were monitoring the atmosphere for signs of nuclear weapons tests that could violate the Nuclear Test Ban Treaty. Instead, they detected brief flashes of high-energy photons coming from space.
Scientists initially had no clear explanation for the signals. Some believed the bursts could be relatively close, perhaps originating somewhere within the Solar System or the Milky Way. Others suspected they were coming from much greater distances. The lack of an identifiable star or galaxy made the mystery even harder to solve.
The bursts also offered scientists little time to study them. They appeared suddenly and faded almost as quickly, leaving no obvious object behind for astronomers to examine.
An accidental discovery
The Vela satellites were not designed to study cosmic explosions. Their detectors were built to identify X-rays, gamma rays and neutrons that could signal a nuclear explosion.
The first gamma-ray burst was recorded on July 2, 1967, although scientists only understood the significance of the signal years later.
Researchers at Los Alamos National Laboratory examined several similar events detected by the Vela satellites. In 1973, they published their analysis of 16 bursts recorded between July 1969 and July 1972 and concluded that the events had a cosmic origin.
Further observations strengthened that conclusion. Soviet Konus satellites published data in 1974 confirming the detection of gamma-ray bursts.
In 1976, scientists established the Interplanetary Network (IPN), which brought together gamma-ray detectors aboard spacecraft studying the Sun and planets. By comparing signals detected by different spacecraft, scientists could triangulate the approximate locations of the bursts.
Even after the sources were narrowed down to a few arc minutes, however, they did not correspond to known X-ray emitters or other familiar astronomical objects. The origin of the bursts remained a mystery.
A major breakthrough
A major breakthrough came after NASA launched the Compton Gamma Ray Observatory in 1991. Its Burst and Transient Source Experiment (BATSE) was specifically designed to study gamma-ray bursts.
Over nine years, BATSE detected more than 2,700 bursts. Its observations revealed that the bursts were distributed almost uniformly across the sky rather than concentrated along the plane of the Milky Way.
That finding was crucial. If gamma-ray bursts were primarily produced by objects within the Milky Way, their distribution would be expected to follow the galaxy's structure.
Instead, the observations indicated that the sources were located far beyond the Milky Way.
This raised another question: if gamma-ray bursts were billions of light-years away and could still be detected, the explosions responsible for them had to be extraordinarily powerful.
Another major step came in 1997, when the Italian-Dutch BeppoSAX satellite detected an X-ray afterglow linked to a gamma-ray burst. The discovery allowed astronomers to study the event after the initial gamma-ray flash and helped establish that GRBs were extragalactic, with some originating billions of light-years from Earth.
Later observations showed that these explosions can release an enormous amount of energy within seconds. Their peak luminosities can be 100 billion billion times that of the Sun and about a billion times greater than even the brightest supernovas.
The search for their origins continued
The discovery of X-ray afterglows in the late 1990s opened a new era of gamma-ray burst research.
Astronomers studying some bursts detected elements including iron, silicon, sulfur and argon around the explosions. These elements are associated with supernovas and provided evidence linking at least some gamma-ray bursts to the deaths of massive stars.
In 1999, scientists detected the afterglow of GRB990123 within seconds of the original burst. Their observations suggested that the energy from the explosion was concentrated into narrow jets, meaning gamma-ray bursts could be detected when one of those jets was pointed toward Earth.
Additional observations strengthened the connection between gamma-ray bursts and supernovas. GRB990705 showed an iron-absorption feature in its afterglow that was characteristic of a supernova, while later observations of GRB991216 detected iron lines in its afterglow.
By 2003, NASA had announced strong evidence that long-duration gamma-ray bursts were produced by the deaths of massive stars accompanied by the formation of black holes.
How scientists detect gamma rays from Earth
Gamma rays cannot directly reach Earth's surface because the atmosphere blocks them. However, scientists can study very-high-energy radiation from some gamma-ray bursts using ground-based observatories.
When gamma rays interact with particles in Earth's atmosphere, they can generate particles that travel faster than the speed of light in air. These particles produce a faint blue glow known as Cherenkov radiation.
Ground-based observatories can detect this glow and use it to study very-high-energy gamma rays.
The technique also offers a major advantage: Earth's atmosphere effectively provides a much larger collecting area than a single telescope, increasing the chances of detecting rare, extremely energetic gamma rays.
The first observation of an ultrahigh-energy gamma-ray burst using this technique came in July 2018. The High Energy Stereoscopic System (HESS) in Namibia detected radiation from the afterglow of a gamma-ray burst.
The radiation did not come directly from the initial explosion. Instead, the jet produced by the burst collided with material that had previously been expelled by the star during the supernova. That interaction accelerated particles to extremely high speeds, producing electromagnetic radiation that eventually reached Earth.
A longer-lasting afterglow
Scientists later detected an even longer-lasting high-energy afterglow from GRB 190829A.
The burst occurred about 1 billion light-years from Earth, making it relatively close compared with many other gamma-ray bursts. HESS observed the event for 56 hours and found that its higher-energy radiation persisted for more than five times longer than the earlier result recorded in 2018.
“This is basically a breakthrough result,” Brian Reville, a physicist at the Max Planck Institute for Nuclear Physics in Germany, told Quanta magazine. “To detect very-high-energy gamma-ray photons up to three nights after the explosion is just really something.”
The finding has raised new questions about the relatively simple models scientists use to explain gamma-ray bursts.
The observations suggest that the processes behind these powerful cosmic explosions may be more complex than previously believed, leaving astronomers with new questions about how the universe's most energetic events unfold.
