December 2, 2021

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What Causes Gamma-Ray Bursts? Their Ultrabright Flashes Hold Clues

In July 1967, at the height of the Cold War, American satellites that had been launched to look for Soviet nuclear weapons tests found something wholly unexpected. The Vela 3 and 4 satellites observed brief flashes of high-energy photons, or gamma rays, that appeared to be coming from space. Later, in a 1973 paper that compiled more than a dozen such mysterious events, astronomers would dub them gamma-ray bursts. “Since then, we’ve been trying to understand what these explosions are,” said Andrew Taylor, a physicist at the German Electron Synchrotron (DESY) in Hamburg.

After the initial discovery, astronomers debated where these bursts of gamma radiation were coming from—a critical clue for what’s powering them. Some thought that such bright sources must be nearby, in our solar system. Others argued that they’re in our galaxy, still others the cosmos beyond. Theories abounded; data did not.

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

This discovery was baffling. In order to account for how bright these objects were—even when observing them from such distances—astronomers realized that the events that caused them must be almost unimaginably powerful. “We thought there was no way you could get that amount of energy in an explosion from any object in the universe,” said Sylvia Zhu, an 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 peak luminosities can be 100 billion billion times that of our sun, and a billion times more than even the brightest supernovas.

It turned out to be fortunate that 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 could hope for is a quick extinction,” said Zhu. “You would hope that the radiation smashes through the ozone and immediately fries everything to death. Because the worst scenario is if it’s farther away, it could cause some of the nitrogen and oxygen in the atmosphere to turn into nitrous dioxide. The atmosphere would turn brown. It would be a slow death.”

Gamma-ray bursts come in two flavors, long and short. The former, which can last up to several minutes or so, are thought to result from stars more than 20 times the mass of our sun collapsing into black holes and exploding as supernovas. The latter, which last only up to about a second, are caused by two merging neutron stars (or perhaps a neutron star merging with a black hole), which was confirmed in 2017 when gravitational-wave observatories detected a neutron star merger and NASA’s Fermi Gamma-ray Space Telescope caught the associated gamma-ray burst.

In each instance, the gamma-ray burst does not come from the explosion itself. Rather it comes from a jet moving at a fraction below the speed of light that gets fired out from the explosion in opposite directions. (The exact mechanism that powers the jet remains a “very fundamental question,” said Zhu.)

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

 

“It is that combination of the speed at high energy and the focusing into a jet that makes them extremely luminous,” said Nial Tanvir, an astronomer at the University of Leicester in England. “That means we can see them very far away.” On average, there is thought to be one 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, as Earth’s ozone layer blocks gamma rays from reaching the surface. But as gamma rays enter our atmosphere, they bump into other particles. These particles get pushed faster than the speed of light in air, which leads them to emit a blue glow known as Cherenkov radiation. Scientists can then scan for these blue bursts of light.