Brighter Than a Billion Billion Suns: Gamma-Ray Bursts Continue to Surprise
The Cherenkov Telescope Array, currently under construction, will use a network of more than 100 ground-based telescopes such as this one to monitor the long afterglows of ultrahigh-energy gamma-ray bursts.
Otger Ballester (IFAE)
Introduction
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.
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.)
“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.
Because our atmosphere has a much larger collecting area than a single telescope, this search strategy gives astrophysicists a greater chance of finding the highest-energy gamma-ray bursts, which are rare and hard to spot.
The first observation of such an ultrahigh-energy burst was made in July 2018 by an array of antennas in Namibia called the High Energy Stereoscopic System (HESS). The radiation came not from the initial gamma-ray burst itself, but from an effect called the afterglow. In this case, the gamma-ray burst’s jet collided with material thrown off from the star as it went supernova. The collision accelerated particles to high speeds, producing electromagnetic radiation that then made its way to Earth.
Now in a paper published earlier this month in the journal Science, Taylor, Zhu and colleagues have observed the longest high-energy afterglow from a gamma-ray burst yet, using HESS to study GRB 190829A — at a relatively close distance of 1 billion light-years — for 56 hours. They found that higher energies persisted more than five times longer than the result in 2018. “This is basically a breakthrough result,” said Brian Reville, a physicist at the Max Planck Institute for Nuclear Physics in Germany who was not an author of the study. “To detect very-high-energy gamma-ray photons up to three nights after the [explosion] is just really something.” The finding raises questions about our fairly simplistic model of how gamma-ray bursts are produced, suggesting more complex physics may be at play. “If that suddenly gets question marks, then it’s really exciting,” said Reville.
Gamma-ray bursts and their afterglows can play an important role in our understanding of the universe too. Supernovas and neutron star mergers are thought to produce the universe’s heavy elements, such as gold and platinum. Since bursts give a window into the wreckage following these events, scientists can use them to track how the chemical composition of the universe has changed over cosmic time.
Future instruments like the Cherenkov Telescope Array, set to come online in 2023, could study these enigmatic explosions in even greater detail. “The next [step] is to probe gamma-ray bursts over very long time scales,” said Taylor. “How [the emission] changes over time tells us the physics taking place.”
Scientists hope to clarify, too, whether the object produced at the center of a gamma-ray burst is a black hole or a neutron star. “It might be possible to find this out from the next generation of gravitational-wave detectors,” said Zhu.
Half a century after their accidental discovery, we are now beginning to study these events like never before. “We’re learning very quickly,” said Taylor, “and what we’ve learned in the last 20 years has not shown any evidence of stopping us being surprised.”
