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.”
At the time, magnetars were still hypothetical. One way to show they were real was to test a prediction based on the laws of electromagnetism. Magnetars’ intense fields should act as a powerful brake on their spin, so that magnetars born rotating every few milliseconds would, in just a few thousand years, slow down to once every few seconds—like the apparent 8-second spin of the 1979 source. Kouveliotou, then at NASA, set out to document this slowing in real time by training an x-ray telescope on a suspected Milky Way magnetar for 3 years. In 1998, she found it did, in fact, slow, by about one-hundredth of 1 second—proof that Duncan and Thompson’s theoretical beastie existed in the wild. She included the two theorists on the discovery paper. “They were more than exhilarated,” she says. “They were ionized.”
From the start, Duncan and Thompson also realized giant fields could crank out giant flares, a scenario theorists continued to elaborate on after Milky Way magnetars sent two more searingly bright gamma ray bursts crashing into Earth in 1998 and 2004. After its turbulent start, the top few meters of a magnetar would cool enough—still millions of degrees kelvin—to freeze into a crystal lattice of neutrons, electrons, and atomic nuclei. Crackling with electric currents and threaded by magnetic field lines, the crust would shiver and sometimes develop small cracks that would vent puffs of plasma into the magnetized atmosphere around the star itself. That plasma would emit pulsing x-ray storms like those the St. Petersburg team observed.
But beneath the crust, greater stresses might build up. Onionlike layers at different depths are thought to rotate at different rates, which would cause magnetic fields to grind against each other at the layer boundaries, exerting massive forces on the crust. And occasionally, maybe once per century or maybe only once ever, that strained surface might undergo a sudden snap: a starquake that would rearrange as big a fraction of a neutron star’s surface, Thompson says, as an earthquake swapping California and New York.
By tearing open large swaths of crust near the magnetar’s poles, where its magnetic field lines splay out into space, a large quake could instantaneously disgorge vast volumes of plasma, sending electrons and positrons rocketing into space at relativistic speeds, approaching that of light. A tight, short-lived beam of photons would emerge from that plasma like a headlight, boosted to higher energies and focused into a beam by the plasma’s motion. Electrons in the plasma would collide with the photons, nudging them up to gamma ray strength. If beams like that ever swept across Earth, well—they might just explain these misfit gamma ray bursts.
When the April 2020 giant flare hit after a 16-year drought, observers could finally check at least some of this scenario with today’s cutting-edge instruments. The InterPlanetary Network, still cycling space probes in and out, clinched the burst’s origin in the Sculptor galaxy. The distance allowed Hurley and his collaborators to calculate the intrinsic strength of the giant flare, which packed 100,000 years of the Sun’s shine into a few milliseconds.
As the burst flickered, a detector on NASA’s Fermi probe caught gamma ray photons several times more energetic than any measured in a giant flare before, and showed they arrived at the brightest moments, as expected from a plasma moving at speeds high enough to both beam light and boost photon energies.
That, in turn, confirmed the picture of an explosion near the magnetar poles—the only place where magnetic field lines would allow plasma to escape so quickly. Yet another instrument, also on Fermi, caught a few gamma rays at even higher energies starting 19 seconds after the main event. They suggest that some of the ejected plasma, after generating its initial beam of gamma rays, continued on and collided with a distant layer of gas surrounding the star, provoking a powerful afterburst (see graphic, above).
For Eric Burns, an astronomer at Louisiana State University, Baton Rouge, the flare offered a chance to search for similar events in the gamma ray back catalog. Astronomers had suggested a handful of other bursts might have been giant flares from nearby galaxies like Andromeda. But when the coordinates of a burst overlap with a nearby galaxy, it’s never clear whether the event actually happened in that galaxy or somewhere more distant along the same line of sight. Many astronomers presumed these bursts were standard-issue short bursts, likely from the collision of neutron stars, seen at greater distances.
Burns searched for very short bursts that overlapped with recent maps of nearby star-forming galaxies—the ones most likely to make magnetars. He found three more old bursts that really did seem to come from Milky Way neighbors. That brings the total count of likely magnetar giant flares in the neighborhood of the Milky Way to seven. Based on that rate, Burns calculated, giant flares should occur across the universe several times more often than all types of supernovae combined and maybe 1000 times more often than other exotic transients like neutron star mergers. But giant flares in the distant universe are too faint to see, and they concentrate most of their energy into a small number of very high-energy photons. “These things are incredibly common,” he says. “It turns out they are just harder to detect.”
If Burns’s rates hold up, they suggest supernovae often do give birth to neutron stars with supercharged magnetic fields. Furthermore, the rates imply that at least several percent of previously observed short gamma ray bursts are actually masquerading giant flares, too. And finally, they suggest there are enough erupting magnetars to power the mysterious fast radio bursts observed from more distant parts of space, a case bolstered by a second watershed magnetar event, also in April 2020: the first detection of a faint fast radiolike signal from a known Milky Way magnetar undergoing a small outburst.
“It’s sort of a running joke, that you can explain everything with a magnetar,” says Brian Metzger, an astrophysicist at the Flatiron Institute. “But what’s nice is it does seem like these different things are coming together now.”
With the Sculptor giant flare as a template, astronomers want to collect a larger statistical sample of flaring magnetars from other galaxies. Burns is digging into more archival data to do that, and hopes gamma observatories like StarBurst, an upcoming smallsat mission, will be sensitive enough to higher energy gamma rays to distinguish giant flares from the more common bursts. Another Milky Way flash, whenever it comes, might help, too; its initial rise would still swamp sensitive gamma detectors, but today’s gravitational wave detectors might be able to pick up ripples in spacetime from the starquake itself, offering more clues to what actually happens on the surface of the star.
For now, though, astronomers are stuck with some basic questions—like whether a magnetar that has launched one giant flare has enough magnetic energy left to muster another—which could take centuries to answer. “Not in my lifetime,” Hurley says. “I’ve become a lot more patient since I started working on the InterPlanetary Network.”
Hurley is still wrangling gamma ray sensors across the Solar System—and space agencies. NASA committees are less enthused about funding the project, he says, now that the world has higher tech gamma observatories like Fermi and Swift, leaving its future in doubt. But Europe’s BepiColombo probe, en route to Mercury, will soon be the 33rd mission to participate, and NASA’s Psyche mission to an asteroid, slated to launch next year, will follow. Their gamma detectors will be ready, attuned to the off chance of another big, rare shot in the dark.
*Correction, 14 April, 3 p.m.: An earlier version of the graphic in this story misstated the power of a magnetar’s gamma ray burst.