On 15 April 2020, a wave of gamma rays, nature’s most powerful kind of light, washed across the Solar System like a storm front. First contact came above Mars, where photons at energies comparable to the radiation from a nuclear bomb peppered a Russian particle detector on NASA’s Mars Odyssey probe. Six minutes later, the burst of light lit up a solar wind probe between the Sun and Earth. Five seconds after that, the signal splashed into specialized detectors on Earth’s surface.
Gamma ray bursts are not so unusual. Space-based observatories pick one up every day or two; roughly two-thirds of them, lasting tens or hundreds of seconds, hail from massive stars exploding in supernovae. Brief bursts of less than 2 seconds make up the rest, and are thought to arise from the cataclysmic collision of two neutron stars, the smoldering ruins left at the heart of a supernova. But when astrophysicists noticed that the 15 April event fluttered in brightness over microseconds, curiously fast variation, they began to think the mystery source was something else altogether.
The explosion was also uncommonly close. By triangulating the signal’s arrival times at the different detectors, astronomers traced it to Sculptor, a neighboring galaxy. All the evidence was pointing to a legendary but elusive type of event: a giant flare erupting from a magnetar—a neutron star with an outlandishly intense magnetic field.
The outburst, dissected in a series of studies released in January, arrived just as magnetars were becoming a go-to solution for theoretical astrophysicists looking for the engines of unexplained celestial explosions, from odd gamma ray flashes to potent eruptions of radio waves. “Originally this was a very obscure subject,” says astronomer Chryssa Kouveliotou of George Washington University. “But right now people involve magnetars in almost everything.”
Forged in supernovae, magnetars are imbued with magnetic fields a trillion times clingier than refrigerator magnets, strong enough to split x-ray photons and stretch normal atoms into oblong shapes. When those fields tangle and snap, the star can vent vast amounts of energy, enough to launch a burst of radiation across the universe.
Yet actual data on magnetar flares remain sparse. Three earlier magnetar explosions in and near the Milky Way unleashed flashes so bright they swamped detectors and even sent some spacecraft into “safe mode,” preventing astronomers from studying anything but the explosions’ aftermath in any detail. Candidates from distant galaxies were too faint to confirm.
Scientists have patiently waited for a rare convulsion to strike some unlucky magnetar in just the right place: close, but not too close. Then the Sculptor flare rolled in. The event is providing clues to long-standing questions about how common magnetars might be in the wider universe and how they might power giant flare explosions. “It’s just like earthquakes in LA, where you’re sitting there and they’re rumbling all the time at a pretty low level,” says Matthew Baring, an astrophysicist at Rice University. “Then you get the ‘big one.’ Well, the giant flare [in Sculptor] is the ‘big one.’”
The very first giant flare arrived with a bang some 4 decades ago, before anyone had even conceived of a magnetar. In the Cold War of the late 1960s, U.S. surveillance satellites had stumbled onto a surprising fact: Gamma ray flashes emanated not only from nuclear tests below, but also from deep space above. By the 1970s, after these gamma ray bursts were declassified, astrophysicists on both sides of the Iron Curtain tried to identify their cosmic sources.
Triangulating the gamma signals back to their origin required wrangling detectors not just across interplanetary distances, but also geopolitical chasms. By the late 1970s, the West had missions such as Helios around the Sun and Pioneer at Venus; the Soviets had, among others, the twin Venera probes patrolling the inner Solar System after dropping Venus landers. Kevin Hurley, a U.S. astronomer working in France, started a clearinghouse for probe data that allowed researchers at Los Alamos National Laboratory, NASA, and in Moscow to pool information using him as a middleman, forming what would come to be called the InterPlanetary Network. “There wasn’t a lot of collaboration at the time,” Hurley says, “but there was no formal interdiction not to do it.”
Ordinary gamma ray bursts kept drizzling in. Then came 5 March 1979. A split-second pulse of gamma rays 100 times brighter than any gamma ray burst yet seen blazed across the Soviet and U.S. spacecraft. The signal’s staggered arrival times indicated it came from the Large Magellanic Cloud, a satellite galaxy in the suburbs of the Milky Way. Suspiciously, the galaxy contained a known supernova remnant, which presumably held a neutron star at its heart. Aftershocks of the burst persisted for a few minutes, repeating every 8 seconds, as if the gamma rays were beaming from a specific spot on a compact, spinning object. Years afterward, a team in St. Petersburg discovered fainter x-ray bursts coming from the same part of the sky, a suggestion that the mystery source continued to simmer.
Astronomers already knew neutron stars were extreme objects, capable of extreme outbursts. When the core of a star implodes during a supernova, gravity trash compacts about one Sun’s worth of mass into a 20-kilometer-wide orb. Only the quantum repulsion between neutrons staves off a final collapse into a black hole. The implosion also concentrates the preexisting star’s magnetic field, amplifying it by up to 10 billion times. Those fields power pulsars, which sweep a radio beam past Earth at regular intervals as they spin.
But to get a gamma ray burst like the one in 1979 required an even more magnetic object. In 1992, U.S. astrophysicists Chris Thompson and Robert Duncan (and Bohdan Paczyński in Poland, nearly simultaneously) conjured up a way to do it. They considered the first 10 seconds or so of a baby neutron star’s life after its birth in a supernova. The star would be so hot its guts would be molten. For a subset of neutron stars, that fluid would churn enough to set in motion something similar to the roiling dynamos that power magnetic fields inside Earth or the Sun. That dynamo would boost and lock in magnetic fields 1000 times stronger still than on other neutron stars. “These things are so crazy,” says Oliver Roberts, an astrophysicist with the Universities Space Research Association. “If you put a magnetar halfway between the Moon and the Earth, it would strip all our credit cards and wipe all our hard disks.”