Editor’s Note (4/11/20): This story is republished to match the peer-reviewed publication of the research it discusses.
In recent weeks, astronomers have been monitoring strange high-energy emissions from the corpse of a long-dead star some 30,000 light-years away. In the broadcasts, they found something startling: a powerful burst of radio waves that only lasted a few milliseconds. The explosion was, in fact, the brightest explosion ever seen from this star or any of its species – immensely magnetic neutron stars known as magnetars.
The radio wave burst, although originating in our own galaxy, is remarkably similar to fast radio bursts (FRBs) – fleeting, intensely bright radio flashes launched by as-yet-unidentified objects that, until now, have not had been observed only from other galaxies. While it may raise as many questions as it answers, this latest observation could solve at least one enigma surrounding the cosmic origin of FRBs.
“Without overusing the word ‘breakthrough’, it really is a breakthrough,” says Jason Hessels of the Netherlands Institute for Radio Astronomy and the University of Amsterdam. “It doesn’t get you quite there, but it does take you a huge step forward” toward solving the FRB case.
At least two radio observatories spotted the recent radio burst in late April. The teams traced the radio waves to a highly magnetic neutron star – the remnant of a star that was perhaps 40 or 50 times more massive than the sun – called SGR 1935+2154. Located deep in the disk of The Milky Way, the dense, dead celestial body had been beaming high-energy radiation out into the cosmos for about a week, as do a rare class of objects called soft gamma-ray repeaters.
This is the first time anyone has seen a flare of radio waves alongside such a barrage of gamma rays. And because of the enormous brightness and short duration of the radio burst, some astronomers now believe it to be an excellent local model for FRBs that originate billions of light-years away.
Even so, making this tenuous link more definitive requires a sober assessment of how different this source is from previously observed FRBs, says Emily Petroff of the University of Amsterdam. “As always with FRBs, you have to make sure you don’t miss the forest for the trees. We can be really hooked to a typical source. But we’ve seen so many times – again and again over the past five years – that it’s not always true.
In search of explanations
FRBs have been among the universe’s most enduring mysteries for more than a decade. Traveling at the speed of light, these radio bursts typically sweep Earth after traversing the cosmos for billions of years, suggesting that whatever celestial engine is propelling them through space must be extremely powerful. . All the bursts observed so far come from distant galaxies. Over the years, astronomers have amassed dozens of hypothetical origins for the phenomenon. Among them are evaporating black holes, explosively dying stars, colliding massive objects and, perhaps less seriously, technobabble transmissions from intelligent, talkative aliens.
As the observations accumulated, the hypotheses improved. Astronomers have seen repeated bursts, proving that whatever their source, the production of a single FRB would not cause it to self-destruct. Teams began capturing bursts in real time, pointing multiple telescopes to look at points in the sky where one of them originated. It wasn’t long before several of them were traced back to their host galaxy. But even though astronomers had collected data on hundreds of bursts by early 2020, their origins remained fundamentally obscure.
“Every time we find a new one, it’s different,” says Petroff. “I would like that each time we find a new one, it would confirm everything we learned from all the others, but it’s never like that! There is so much variety; it keeps us on our toes.
Surprise local detection
Astronomers first spotted the new burst using the FRB-chunting CHIME (Canadian Hydrogen Intensity Mapping Experiment) radio telescope, an instrument in southwestern Canada that looks like four chained skateboard half-moons. Since fully opening its eyes in late 2018, CHIME has spotted hundreds of FRBs. This appeared on the periphery of the telescope’s view in the sky but was so powerful it was still easily visible.
“It’s an extremely bright radio emission from a magnetar,” says Paul Scholz of the University of Toronto, who reported the burst for the CHIME team at the Astronomer’s Real-Time Astronomical Observations site. Telegram. “Is this the link between magnetars and FRBs? It could be.”
After seeing this notification, astronomers based at the California Institute of Technology carried out an initial analysis of their own data from the period when the burst started. Collected by three radio antennas in California and Utah as part of the STARE2 (Survey for Transient Astronomical Radio Emission 2) project, the Caltech team’s observations are specifically designed to detect fast radio bursts originating from within the Lane. milky.
Unlike CHIME, STARE2 captured the event head-on, allowing researchers to quickly calculate the brightness of the burst. According to their estimates, if it had occurred at the distance of the the closest known extragalactic FRB – about 500 million light-years away – would still have been easily detectable from Earth. (For comparison, the closest galaxy to ours, Andromeda, is just 2.5 million light-years away. And the Virgo Group of galaxies, the closest cluster to ours, is about 53 million light-years away.) At Caltech’s Shrinivas Kulkarni, the burst’s brightness and millisecond duration make it a conclusive link to FRBs.
Based on these observations, “a plausible origin of fast radio bursts is active magnetars in other galaxies,” says Kulkarni, principal investigator of the STARE2 project. “If we wait long enough, maybe [magnetar] will have [an even brighter] to burst.”
A third observation, made by a team using the European Space Agency’s International Gamma-Ray Astrophysics Laboratory (INTEGRAL) orbital observatory, pinned the radio burst to the magnetar by linking it to a simultaneous burst of X-rays from the same object. And China’s Five Hundred Meter Aperture Spherical Radio Telescope (FAST) has since detected another radio burst from SGR 1935+2154 which also points to the magnetar as the source of these outbursts. “I’d bet a year’s salary on that location,” Kulkarni says.
For several years, several sources of evidence have come together to point to magnetars as the culprits of the FRB. These neutron stars spin extremely fast and have immense magnetic fields, a combination that can create huge flares of radiation. And scientists have observed some FRBs that have a strong, “twisted” polarization: this arrangement suggests that they originated in the vicinity or passed through an intensely magnetic environment, like those surrounding these stellar corpses.
But the full image had yet to be revealed. “The counter-argument for a long time was, ‘Yeah, but we’ve never seen magnetars in our own galaxy do anything close to bright,'” Hessels says. “‘So how logical is it that magnetars from other galaxies would do this?'”
Now, with this new discovery in hand, astronomers are taking a closer look at the connection between FRBs and magnetars. “I wouldn’t say that seals the deal and is the missing link or anything like that. This brings us one step closer to finding a link between elements in our own galaxy and what causes FRBs,” says Petroff.
Astronomers note that although this burst is brighter than anything seen so far from a magnetar, it is still orders of magnitude less powerful than most observed FRBs. It’s no surprise the searchers caught a weaker burst first. Such bursts are likely to outnumber extremely bright ones, just as weaker earthquakes occur more frequently than larger ones. Stronger stellar flares could also produce stronger radio bursts. Some magnetars produce flares so gargantuan that they alter Earth’s ionosphere over vast interstellar distances, although these superpowered flares are incredibly infrequent. “I’d like to know,” Hessels said, “if we were to catch one of these giant flares, would we see an even brighter burst that’s easily comparable to an FRB?”
Another lingering question is whether FRBs can come from different sources. Most of those observed to date have been isolated events, but more than a dozen are now known to have repeatedly originated from their mysterious sources. The nearest repeating FRB, located about half a billion light-years away and known as R3, erupts every 16 days. Scientists suspect that the periodic activity of R3 is linked to another object locked in its gravitational embrace. But the SGR 1935+2154 magnetar does not seem to have such orbital companions.
“I hope there’s not just one type of FRB,” says Hessels. “I hope that by scratching deeper, we discover several things at the same time.”