In July 1967, in the midst of the Cold War, US satellites launched in search of Soviet nuclear weapons tests found something completely unexpected. Vela 3 and 4 satellites observed short flashes of high-energy photons or gamma rays that appeared to be coming from space. Later, in work from 1973 who have collected more than a dozen such mysterious events, astronomers would call them gamma-ray bursts. “Since then, we’ve been trying to figure out what those explosions are,” he said Andrew Taylor, a physicist from the German Electron Synchrotron (DESY) in Hamburg.
After the initial discovery, astronomers debated where these bursts of gamma radiation came from – a critical clue to what drives them. Some thought that such bright sources must be nearby, in our solar system. Others claimed to be in our galaxy, and others the cosmos wider. The theory abounded; the data are not.
Then in 1997, an Italian and Dutch satellite named BeppoSAX confirmed that gamma-ray bursts were extragalactic, in some cases resulting from many billions of light-years.
This discovery was confusing. To explain how bright these objects were — even when viewed from such a distance — 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 space,” said Sylvia Zhu, an astrophysicist from DESY.
An explosion of gamma rays will emit the same amount of energy as a supernova, formed when a star crashes and explodes, but in seconds or minutes, not weeks. Their largest lamp can be 100 billion billion times larger than the sun, and a billion times larger than even the brightest supernovae.
It turned out fortunately that they were so far away. “If there was an explosion of gamma rays in our galaxy with a jet aimed at us, the best you can hope for is rapid extinction,” Zhu said. “You would hope that the radiation broke through the ozone and immediately burned everything to death. Since it is the worst case scenario if it is further, it could cause some of the nitrogen and oxygen in the atmosphere to be converted to nitrogen dioxide. The atmosphere would turn brown. It would be a slow death. ”
Gamma rays come in two flavors, long and short. The former, which can last up to a few minutes or so, are thought to originate from the stars more than 20 times the mass of our sun collapsing into black holes and exploding like supernovae. The latter, lasting only about one second, were caused by the fusion of neutron stars (or perhaps neutron stars merging with a black hole), which was confirmed in 2017 when gravitational wave observatories discovered the fusion of neutron stars and NASA’s Fermi Gamma-ray space telescope captured an associated gamma-ray burst.
In any case, an explosion of gamma rays does not come from the explosion itself. Rather, it comes from a jet moving at a fraction below the speed of light that explodes in opposite directions. (The exact mechanism that drives the jet remains a “very fundamental issue,” Zhu said.)
“That combination of speed at high energy and focusing in the jet makes them extremely bright,” he said Nial Tanvir, an astronomer from the University of Leicester in England. “That means we can see them very far away.” On average, it is thought to exist one visible burst of gamma rays 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 as gamma rays enter our atmosphere, they collide with other particles. These particles push faster than the speed of light in the air, causing them to emit a blue glow known as Cherenkov radiation. Scientists can then look for these blue bursts of light.