What are the causes of gamma-ray bursts? Their super-bright flashes contain clues


In July 1967, at the height of the Cold War, American satellites that had been launched to search for Soviet nuclear weapons tests found something totally unexpected. The Vela 3 and 4 satellites observed brief flashes of high-energy photons, or gamma rays, which appeared to be coming from space. Later in an article from 1973 who compiled more than a dozen such mysterious events, astronomers would call them gamma-ray bursts. “Since then we have tried to figure out what these explosions are,” said Andrew Taylor, physicist at the German Electronic Synchrotron (DESY) in Hamburg.

After the initial discovery, astronomers wondered where these bursts of gamma radiation came from, a vital clue as to what is fueling them. Some thought that such light sources must be nearby, in our solar system. Others have argued that they are in our galaxy, still others in the cosmos beyond. Theories abounded; the data did not.

Then in 1997, an Italian and Dutch satellite called BeppoSAX confirmed that gamma-ray bursts were extragalactic, in some cases originating several billion light years away.

This discovery was disconcerting. In order to account for the brightness of these objects, even when observing them from such distances, astronomers realized that the events that brought about them must be of almost unimaginable power. “We thought there was no way to get this amount of energy in an explosion from any object in the universe,” said Sylvia Zhu, astrophysicist at DESY.

A gamma ray burst will emit the same amount of energy as a supernova, caused when a star collapses and explodes, but in seconds or minutes rather than weeks. Their maximum luminosities can be 100 trillion times that of our sun, and a billion times more than even the brightest supernovas.

Fortunately they were so far away. “If there was a gamma ray burst in our galaxy with a jet pointed at us, the best thing you can hope for is rapid extinction,” Zhu said. “You hope the radiation breaks the ozone and immediately burns everything to death. Because the worst-case scenario is if it is further away, it could turn some of the nitrogen and oxygen in the atmosphere into nitrogen dioxide. The atmosphere would turn brown. It would be a slow death.

Gamma-ray bursts are available in two flavors, long and short. The former, which can last up to about several minutes, are said to be the result of stars more than 20 times the mass of our sun collapse into black holes and explode into a supernova. The latter, which only last about a second, are caused by the fusion of two neutron stars (or perhaps a neutron star merging with a black hole), which has been confirmed in 2017 when gravitational wave observatories detected a fusion of neutron stars and NASA’s Fermi Gamma Space Telescope picked up the associated gamma-ray burst.

In each case, the gamma-ray burst does not come from the explosion itself. Rather, it is a jet moving at a fraction of the speed of light that is projected by the explosion in opposite directions. (The exact mechanism that powers the jet remains a “very basic question,” Zhu said.)

This artist’s impression shows the moments before and the nine days after a kilonova. Two neutron stars spiral inward, creating gravitational waves (pale arcs). After fusion, a jet produces gamma rays (magenta), while expanding radioactive debris produces ultraviolet (violet), optical (blue-white) and infrared (red) light.

“It’s this combination of high energy speed and focus in a jet that makes them extremely bright,” said Nial Tanvir, astronomer at the University of Leicester in England. “It means we can see them very far away.” On average, we think that there are an observable gamma-ray burst in the visible universe every day.

Until recently, the only way to study gamma-ray bursts was to observe them from space, because the Earth’s ozone layer prevents gamma rays from reaching the surface. But when gamma rays enter our atmosphere, they collide with other particles. These particles are pushed faster than the speed of light through air, causing them to emit a blue glow known as Cherenkov radiation. Scientists can then search for these bursts of blue light.



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