In July 1967, at the height of the Cold War, U.S. satellites launched in search of Soviet nuclear weapons tests discovered something completely unexpected. Vela 3 and 4 satellites observed brief flashes of high-energy photons or gamma rays that appear to come from space. Later, in a 1973 paper, more than a dozen such mysterious events were compiled, which astronomers called gamma-ray bursts. "Since then, we've been trying to understand these explosions," said Andrew Taylor, a physicist at the Electron Synchrotron (DESY) in Hamburg in Germany.
After the initial discovery, astronomers debated the source of these gamma radiation bursts — the key clues that powered them. Some people think that this bright light source must be near our solar system. Others argue that they are in our galaxy, and that there are others outside the universe. Theories abound; data doesn't.
Then in 1997, an Italian and Dutch satellite called BeppoSAX confirmed that gamma-ray bursts were beyond the river, in some cases originating billions of light-years away.
This finding is puzzling. To explain the brightness of these objects—even when viewed from such a distance—astronomers realized that the events that led them must be almost unimaginably powerful. "We don't think you could get that much energy from the explosion of any object in the universe," said DESY astrophysicist Sylvia Zhu.
When a star collapses and explodes, a gamma-ray burst will release the same energy as a supernova, but it will take seconds or minutes instead of weeks. Their peak luminosity can be 100 billion times that of our Sun, or even 1 billion times higher than the brightest supernova.
It turned out that they were lucky to be so far away. "If there's a gamma-ray burst in our galaxy and jets pointing at us, the best thing you can hope for is a rapid extinction," Zhu said. "You would want radiation to burst through the ozone layer and blow everything up immediately." Because the worst-case scenario is that if the distance is greater, it may cause some nitrogen and oxygen in the atmosphere to convert into nitrous dioxide. The atmosphere will turn brown. It will be a slow death. ”
Gamma-ray bursts come in two forms, long and short. The former, which can last up to a few minutes or so, is thought to be the result of stars with a mass of more than 20 times the mass of the Sun collapsing into a black hole and exploding in the form of a supernova. The latter, which lasts only about 1 second, is caused by the merger of two neutron stars (or possibly a neutron star merging with a black hole), which was confirmed in 2017 when the Gravitational-Wave Observatory detected neutron star mergers and NASA's Fermi when the Gamma-ray Space Telescope captured the associated gamma-ray bursts.
In each case, the gamma-ray burst did not come from the explosion itself. Instead, it comes from jets that fire from the explosion in the opposite direction at a speed a fraction of the speed of light. (The exact mechanism by which jets are powered remains a "very basic question," Zhu said.) ）
Nial Tanvir, an astronomer at the University of Leicester in the United Kingdom, said: "It is the combination of speed at high energies and focused jets that makes them very luminous. "That means we can see them from great distances." On average, it is thought that an observable gamma-ray burst occurs every day in the visible universe.
Until recently, the only way to study gamma-ray bursts was to observe them from space, as Earth's ozone layer prevented gamma-rays from reaching the surface. But when gamma rays enter our atmosphere, they hit other particles. These particles are pushed faster than the speed of light in the air, which causes them to emit blue light, called Cherenkov radiation. Scientists can then scan these blue bursts of light.
Because our atmosphere has a much larger collection area than a single telescope, this search strategy gives astrophysicists a greater chance of finding the highest-energy gamma-ray bursts that are rare and difficult to detect.
In July 2018, this ultra-high-energy burst was first observed in Namibia by a set of antenna arrays called high-energy stereoscopic systems (HESS). The radiation does not come from the original gamma-ray burst itself, but from an effect called afterglow. In this case, the jet stream of the gamma-ray burst collides with the material thrown out when the star becomes a supernova. Collisions accelerate particles to high speeds, producing electromagnetic radiation that then reaches Earth.
Now, in a paper published earlier this month in the journal Science, Taylor, Zhu and colleagues observed the longest high-energy afterglow in a gamma-ray burst, using HESS to study GRB 190829A — a relatively close distance of 1 billion light-years — 56 hours. They found that the duration of higher energies was more than five times longer than the 2018 results. "This is basically a breakthrough result," said Brian Reville, a physicist at the Max Planck Institute for Nuclear Physics in Germany, who is not the study's author. "Detecting extremely high-energy gamma-ray photons within three nights [of the explosion] is really a very important thing." This finding questions how gamma-ray bursts are produced for our fairly simple model, suggesting that there may be more complex physics at work. "If this suddenly appears with a question mark, it's really exciting," Revere said.
Gamma-ray bursts and the rest of the glow can also play an important role in our understanding of the universe. The merger of supernovae and neutron stars is thought to produce the heavy elements of the universe, such as gold and platinum. Because the eruptions provide a window into the wreckage after these events, scientists can use them to track how the chemical composition of the universe changes with cosmic time.
Future instruments such as the Cherenkov Telescope Array, which will be available in 2023, allow for more detailed study of these mysterious explosions. "The next [step] is to detect gamma-ray bursts on very long timescales," Taylor said. "How [emission] changes over time tells us what's happening in physics."
Scientists also want to clarify whether the objects produced at the center of gamma-ray bursts are black holes or neutron stars. "It's possible to discover this from the next generation of gravitational wave detectors," Zhu said.
Half a century after their unexpected discovery, we are now beginning to study these events in unprecedented ways. "We learned very quickly," Taylor said, "and what we've learned in the last 20 years doesn't show any evidence that surprises us." ”
Written by Jonathan O'Callaghan
FY：11
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