Gamma-ray burst
A gamma-ray burst, or GRB (Gamma-Ray Burst), is an extremely intense flash of gamma rays that lasts from a few milliseconds to several minutes. These are the most energetic explosions ever observed in the universe: a typical GRB releases in a few seconds more &energy than the Sun will release over its entire 10-billion-year lifetime.
Accidental discovery
Gamma-ray bursts were discovered in 1967 by the American military Vela satellites, which were monitoring Soviet nuclear tests. Instead of detecting terrestrial explosions, they recorded mysterious gamma-ray flashes coming from the sky. The cosmic nature of these bursts was not confirmed until 1997, when scientists succeeded in identifying the host galaxyhost galaxy of a GRB—located billions of light-years away.
Two Main Types
GRBs are divided into two families based on their duration. Long bursts (lasting more than 2 seconds, often several minutes) are associated with the death of very massive stars that collapse into black holes— these are referred to as hypernovae or collapsars. Short bursts (less than 2 seconds) result from the merger of two neutron stars ora neutron star merging with a black hole, as confirmed by the multi-messenger event GW170817 in 2017.
The mechanism of the relativistic jet
The colossal energy of a GRB does not radiate in all directions, but rather in two narrow, opposing jets, emitted at speeds close to the speed of light. This concentration explains the apparent intensity: if the jet is pointed toward Earth, the event appears extremely bright. This geometry also means that we detect only a fraction of the GRBs that actually occur.
Importance in Astronomy
GRBs are the most powerful light beacons in the universe, visible throughout the entire observable universe. They allow us to explore the earliest galaxies and probe the history of the universe. Their association with the mergers ofneutron stars also makes them key sources of heavy elements: gold, platinum, and uranium are formed during these explosions.
Concrete example: GRB 221009A
Detected in October 2022, GRB 221009A is one of the brightest gamma-ray bursts ever observed, nicknamed BOAT (Brightest Of All Time). Located 2.4 billion light-years away, it was so intense that it disrupted Earth’s ionosphere. Its statistical brightness suggests that such events occur only once every 10,000 years or so.
Did you know?
If a gamma-ray burst occurred in the Milky Way within 6,000 light-years of Earth and was directed toward Earth, it could cause a mass extinction by destroying the ozone layer and exposing the biosphere to solar UV radiation. Some paleontologists have suggested that a GRB may have contributed to the Ordovician extinction 450 million years ago.
Frequently Asked Questions
How often do gamma-ray bursts occur?
Satellites detect about one per day, but these are only the ones whose jets are pointed toward us. The actual rate in the observable universe is estimated to be several hundred per day. In a galaxy like the Milky Way, it is estimated that a GRB occurs about once every 100,000 to 1 million years.
Can a gamma-ray burst be predicted?
No. GRBs are unpredictable events. When a satellite detects one, it immediately alerts a network of ground-based telescopes, which reorient themselves within minutes to observe the afterglow in visible and infrared light.
What is the difference between a gamma-ray burst and a supernova?
An ordinary supernova emits light in all directions and can shine for weeks. A GRB is an even more extreme explosion whose energy is concentrated in two relativistic jets, shining in gamma rays for a few seconds. All long gamma-ray bursts are supernovae, but not all supernovae produce a GRB.