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Astronomie - ‘Magnetic Star’ Radio Waves Could Solve the Mystery of Fast Radio Bursts

6.05.2020

The surprise detection of a radio burst from a neutron star in our galaxy might reveal the origin of a bigger cosmological phenomenon

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Artist’s impression of a magnetar, a rapidly spinning and highly magnetized neutron star. Credit: ESO and L. Calçada

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. Within the emissions, they found something surprising: a powerful blast of radio waves that lasted mere milliseconds. The blast was, in fact, the brightest outburst ever seen from this star or any of its kind—immensely magnetic neutron stars known as magnetars.

The eruption of radio waves, though originating in our own galaxy, is remarkably similar to fast radio burst (FRBs)—fleeting, intensely bright radio flashes launched by as yet unidentified objects that, until now, had only been observed coming from other galaxies. Although it may raise just as many questions as it answers, this latest observation could solve at least one riddle surrounding the cosmic origin of FRBs.

“Without overusing the word ‘breakthrough,’ this is really a breakthrough,” says Jason Hessels of the Netherlands Institute for Radio Astronomy and the University of Amsterdam. “It doesn’t quite get you all the way there, but it gets you such a huge step of the way” toward cracking the case of FRBs.

At least two radio observatories spotted the recent radio burst in late April. Teams traced the radio waves back to a highly magnetic neutron star—the remnant of a star that was maybe 40 or 50 times as massive as the sun—called SGR 1935+2154. Located deep in the disk of the Milky Way, the dense, dead celestial body had been slinging high-energy radiation into the cosmosfor a week or so, as a rare class of objects called soft gamma-ray repeaters are known to do.

It is the first time anyone has seen a blaze of radio waves alongside such a barrage of gamma rays. And because of the radio burst’s tremendous brightness and short duration, some astronomers now think it is a great local model for FRBs that come from billions of light-years away.

Even so, making that tenuous link more definitive requires a sober assessment of how this source is different from previously observed FRBs, says Emily Petroff of the University of Amsterdam. “As always with FRBs, you have to make sure that you don’t miss the forest for the trees. We can get really hung up on one source being typical. But we’ve already seen so many times—again and again over the past five years—that’s not always true.”

IN SEARCH OF EXPLANATIONS

FRBs have been among the universe’s most stubborn mysteries for more than a decade. Traveling at the speed of light, these radio blasts typically wash over Earth after traversing the cosmos for billions of years, suggesting that whatever celestial engine is heaving them into space must be extremely powerful. All the bursts observed so far have 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, massive colliding objects and—perhaps less seriously—the technobabble transmissions of smart, talkative aliens.

As the observations have piled up, the hypotheses have improved. Astronomers saw some bursts that repeated, proving that whatever their source was, producing a single FRB would not cause its self-destruction. Teams started catching bursts in real time, pointing multiple telescopes to stare at spots on the sky where one originated. It was not long before several of them had been traced back to their host galaxy. But even though astronomers had gathered data on hundreds of bursts by early 2020, their origins remained fundamentally clouded.

“Every time we find a new one, it’s different,” Petroff says. “I wish every time we found a new one, it confirmed everything we learned from all the other ones, but it’s never like that! There’s so much variety; it keeps us on our toes.”

SURPRISE LOCAL DETECTION

Astronomers first spotted the new burst using the FRB-hunting CHIME (Canadian Hydrogen Intensity Mapping Experiment) radio telescope, an instrument in southwestern Canada that resembles four skateboarding half-pipes strung together. Since fully opening its eyes in late 2018, CHIME has spotted hundreds of FRBs. This one appeared at the periphery of the telescope’s vision in the sky but was so powerful that it was still easily seen.

“It’s an extremely bright radio emission coming from a magnetar,” says the University of Toronto’s Paul Scholz, who reported the burst for the CHIME team on the real-time astronomical observations site Astronomer’s Telegram. “Is this the link between magnetars and FRBs? It might be.”

After seeing that notification, astronomers based at the California Institute of Technology performed an early scan of their own data from the time period when the burst went off. Gathered 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 coming from within the Milky Way.

Unlike CHIME, STARE2 caught the event head-on, allowing the researchers to quickly calculate the burst’s brightness. According to their estimates, if it had occurred at the distance of the nearest known extragalactic FRB—or roughly 500 million light-years away—it would still have been easily detectable from Earth. (For comparison, the nearest galaxy to our own, Andromeda, is just 2.5 million light-years away. And the Virgo group of galaxies, the nearest cluster to our own, is about 53 million light-years away.) To Caltech’s Shrinivas Kulkarni, the burst’s brightness and milliseconds-long duration make it a conclusive link with FRBs.

Based on these observations, “a plausible origin for fast radio bursts is active magnetars in other galaxies,” says Kulkarni, who is principal investigator of the STARE2 project. “If we wait long enough, maybe this [magnetar] will have [an even brighter] burst.”

A third observation, made by a team using the European Space Agency’s orbiting INTEGRAL (International Gamma-Ray Astrophysics Laboratory) observatory, pinned the radio burst on the magnetar by linking it with a simultaneous blast 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 that also points to the magnetar as the source of these outbursts. “I would bet a year’s salary on that localization,” Kulkarni says.

MAGNETAR OUTBURSTS

For several years, multiple lines of evidence have coalesced to flag magnetars as FRB culprits. These neutron stars spin extremely rapidly and possess immense magnetic fields—a combination that can create enormous eruptions of radiation. And scientists have observed some FRBs that have strong and “twisted” polarization: this arrangement suggests they originated in the vicinity of, or passed through, an intensely magnetic environment, such as those that surround these stellar corpses.

But the full picture had yet to reveal itself. “The counterargument for a long time was: ‘Yeah, but we’ve never seen magnetars in our own galaxy do anything even as close as bright, ’” Hessels says. “’So how logical is it that magnetars in other galaxies do this?’”

Now, with this new finding in hand, astronomers are taking a closer look at the connection between FRBs and magnetars. “I wouldn’t say that this seals the deal and is the missing link or something like that. It gets us one step closer to finding a link between things in our own galaxy and what’s causing FRBs,” Petroff says.

Astronomers note that although this burst is brighter than anything yet seen coming from a magnetar, it is still less powerful than most observed FRBs by several orders of magnitude. It is not surprising that researchers might have caught a fainter burst first. Such bursts are likely to be more numerous than exceedingly bright ones, just as weaker earthquakes occur more frequently than bigger ones. Stronger stellar flares might produce stronger radio bursts as well. Some magnetars produce flares so gargantuan that they alter Earth’s ionosphere across vast interstellar distances, although such superpowered flares are incredibly infrequent. “I would love to know,” Hessels says, “if we were to catch one of those 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 single events, but more than a dozen of them are now known to come repeatedly from their mysterious sources. The nearest repeating FRB, located roughly half a billion light-years away and known as R3, erupts every 16 days. Scientists suspect R3’s periodic activity is linked to some other object locked in its gravitational embrace. But the magnetar SGR 1935+2154 does not appear to have any such orbital companions.

“I hope there isn’t just one type of FRB,” Hessels says. “I hope that by scratching deeper, we discover multiple things at the same time.”

Quelle:  SCIENTIFIC AMERICAN

 
 
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