Nobody noticed when an Australian radio telescope captured a fleeting explosion of light coming from somewhere far beyond the Milky Way in 2001. Records of the powerful flare-up—which  produced as much energy in a few thousandths of a second as the sun does in a day—sat unseen for more than half a decade until a group of scientists sifting through archival data spotted the stupendous eruption—a so-called fast radio burst (FRB).

Such enigmatic explosions are no longer ignored. Researchers have found them happening at least 800 times per day all over the sky yet are still in the dark as to what causes them. As one of the most active topics in astrophysics, FRBs have lately seen a slew of groundbreaking and sometimes at-odds findings, with papers that reshape the field regularly appearing in the literature. Although the overall view remains murky, in just the past year, a clearer picture of these strange entities has started to emerge.

“I think we’re closer to understanding what some FRBs are,” says Ziggy Pleunis, an astrophysicist at the University of Toronto. “But as we’ve been going on this quest, new discoveries have led to new questions.”

Many astronomers feel that the subject is now at an inflection point, where some of their biggest puzzles are on the cusp of being solved. A torrent of new detections and deeper studies of these phenomena have elevated certain models of FRBs’ inner workings while eliminating others, and several upcoming projects should help further winnow down the possibilities. Even if such undertakings are unable to fully unravel the mystery, they will nonetheless be fruitful. FRBs offer to satisfy more than mere academic curiosity: We have learned that their bright light carries within it a record of the contents of the vast intergalactic depths it traversed along its way to Earth. These split-second cosmic fireworks, popping off seemingly at random across the entire sky, can thus provide information about galaxies and the material between them that no other mechanism can.

MAGNETIC MOMENTS

The biggest recent shake-up to happen to FRB enthusiasts came as a surprise. In April 2020 three separate research teams detected an enormous blast of radio energy coming from a magnetar located in the Milky Way. Magnetars are an extreme form of neutron stars, city-sized remnants with obscenely powerful magnetic fields left behind when massive stars die in supernova detonations. A magnetar’s magnetic field can be so strong that approaching within 1,000 kilometers of one would disrupt your body’s constituent atomic nuclei and electrons, causing you to effectively dissolve.

Magnetars, with their ultrastrong magnetic fields, were already a leading candidate for the source of FRBs. But the few dozen in our galaxy had never before been observed to produce eruptions that might resemble the phenomena. The discovery of a short and formidable radio burst from a galactic magnetar called SGR 1935+2154 was exactly what researchers had been missing. If the object instead existed in a neighboring galaxy such as Andromeda, its signature would have been indistinguishable from a typical FRB.

“That was a huge moment for the field,” says Kenzie Nimmo, an astronomer at the University of Amsterdam. “It alleviated all doubt that at least some FRBs come from magnetars.”

The tantalizing find fed theorists’ conjectures as to exactly how a magnetar could produce an FRB. Most ideas posit some kind of jarring starquake occurring on the object or perhaps a strong spark shooting out when its twisting magnetic field lines snap and reconnect. Such events could directly generate an FRB’s flash, or they might make a shockwave that heats up surrounding material, incinerating dust and turning gas into plasma to produce light as it travels outward.

Several telescopes saw an x-ray flash arriving just after SGR 1935+2154’s radio signal, suggesting that whatever released the radio energy also generates more complicated side effects. But what precisely that means for the explosive action is not yet clear. “Did this happen on the surface of the star or in the magnetosphere or in the material around the magnetar?” asks Emily Petroff, an astrophysicist also at the University of Amsterdam. “We still don’t really agree on that.”

COSMIC CURIOSITIES

Of course, any single FRB is unlikely to fully explain the multitudes now known. In the summer of 2021, the Canadian Hydrogen Intensity Mapping Experiment (CHIME), a dedicated FRB-hunting telescope in British Columbia, released a catalog of 536 FRBs it detected during the first year of its operation, quadrupling the number of those recorded. The bursts were already known to come in two distinct flavors—those that repeatedly flash their signals and those that are one-off events. CHIME’s data showed that nonrepeaters were far more common than repeaters and that each had different characteristics.

On average, the bursts from repeaters lasted longer than their nonrepeating counterparts and emitted their light in a narrower range of frequencies. Whether this represents an actual difference in the production mechanisms for these flashes or instead something else about their progenitors’ ages or environments remains to be seen. But the situation resembles an earlier mystery surrounding another class of gargantuan cosmic explosions: gamma-ray bursts, which were shown in the 1990s to arise from three separate types of events, some emitting energy for shorter durations, and some doing so for longer. With future detections, it is possible that scientists will be able to dive deeper into the visible properties of FRBs to find something that further distinguishes the different populations.

CHIME’s catalog includes large numbers of FRBs that have been pinpointed to a wide variety of specific galaxies, muddying the link to magnetars, which emerge almost exclusively in galaxies that are churning out stupendous numbers of massive, short-lived stars. Yet CHIME’s FRB haul includes many sources from quieter galaxies that are barely forming any new stars at all.

“Magnetars can explain some fraction of FRBs. Nobody would dispute that,” says Shami Chatterjee, an astronomer at Cornell University. “But is that all of them? Almost certainly not.”

A new paper, currently under review at Nature and initially posted on the preprint server arXiv.org in May, adds support to this assertion. Using an array of radio telescopes called the European Very Long Baseline Interferometry (VLBI) Network, a team determined the position of a repeater designated FRB 20200120E with extreme precision. The object had originally been localized to the nearby spiral galaxy M81, but VLBI allowed astronomers to zoom in further and see that it lives within an ancient hive of densely packed stars known as a globular cluster. Such collections mainly host stars around 10 billion years old—yet magnetars are thought to only endure for 10,000 years or so before lapsing into a more sedate (and presumably FRB-free) existence as a normal neutron star.

“This is a game changer,” says Mohammadtaher Safarzadeh, a theoretical astrophysicist at Harvard University. “Whatever is causing the FRB signal likely has the same age as the globular cluster and is definitely not a magnetar.”

Magnetars could perhaps occasionally arise from two neutron stars crashing into each other—a production mechanism that has never been definitively seen—which could potentially allow a young one to appear in such a timeworn place, says theoretical astrophysicist Bing Zhang of the University of Nevada, Las Vegas. But nobody knows exactly how often such events occur or how long the resulting magnetars would remain active, making it difficult to invoke such a model for any particular FRB.

Further complicating the magnetar picture is another curiosity: FRB 20180916B, also known as R3 because it was the third repeating source ever discovered. Originally pinpointed to a star-forming region toward the center of a spiral galaxy around half a billion light-years away, R3 was subsequently shown to be in the galaxy’s outskirts, suggesting that it is either an older object or one somehow kicked far out from its birthplace closer to the center. Even stranger, the entity only produces explosions during a four- to five-day window of activity that occurs every 16.35 days, making it what is known as a periodic repeater.

Researchers have since been scratching their heads as to what could be inducing such peculiar regularity. A magnetar that spins around on its axis like a top, sometimes pointing its blasts toward Earth and other times facing away, is one possibility. Another is a bursting object orbiting a second structure, such as a black hole surrounded by a disk of material, that cyclically obscures the explosive events. Even more exotic models have been invoked, such as a pair of orbiting neutron stars whose magnetospheres periodically interact, creating a cavity where eruptions can take place.

“What makes the field so fun right now is that there are so many exciting possibilities,” Chatterjee says.

APPROACHING ANSWERS

Major questions continue to dog FRB astronomers. Are nonrepeaters really one-time events, or would they be found to burst again if they were observed for long enough? The magnetar in our galaxy appears to be fairly quiet. But was it significantly more active in its younger years? Could other esoteric scenarios, such as asteroids hitting a black hole, somehow produce FRB-like signals? New observations and theories appear in preprint papers almost daily, creating opportunities and challenges for those trying to make sense of it all.

The CHIME collaboration is currently building a set of smaller add-on telescopes that will help triangulate the exact on-sky positions of huge numbers of FRBs. In a few years, researchers expect to know the precise locations of hundreds or even 1,000 events. In addition to putting further constraints on FRB models, such data will allow scientists to perform important measurements of the universe.

Astronomers only originally knew FRBs were coming from outside the Milky Way because their light was dispersed, meaning the higher frequencies were arriving a few milliseconds before the lower ones. This indicated that the radio waves were encountering enormous amounts of electrons as they traveled through the intergalactic medium. Looking at the cosmic microwave background, an afterglow from shortly after the big bang, cosmologists have estimated the amount of visible matter in the universe and come up with a number about twice as high as that seen in stars and galaxies. Researchers intend to use FRBs to shine a flashlight on intergalactic regions, where this missing matter is believed to reside. Last year a team used a handful of FRBs to estimate the amount of material their light passed through and showed it was almost exactly equivalent to the absent matter. The ultimate goal is to eventually build up a comprehensive map of matter throughout the universe. Light from some FRBs is also highly polarized—its waves have been rotated by magnetic fields during its flight—potentially giving astronomers access to information about magnetic conditions in other galaxies or the spaces between them.

In the meantime the mystery of FRBs’ origins remains. Although there is growing consensus that the phenomena requires more than one physical explanation, those in the field know that certainty can be as illusory as it is elusive. “I fully anticipate, within the next decade, we’ll get one or two more surprises, like the galactic magnetar that we didn’t even know we should be looking for, which will push our understanding forward in a massive way,” Petroff says. One widespread suspicion is that at least some nonrepeating FRBs arise from cataclysmic events such as neutron stars crashing together, which would also send out gravitational waves. Were a radio telescope to see a blast at the same time as the Laser Interferometer Gravitational-Wave Observatory (LIGO) or its counterparts around the world, it would greatly swing minds toward that possibility. And if such a collision produced a magnetar, could it be that the initial cataclysmic one-off FRB would give rise to a distinct, repeating FRB source? As of yet, no one can say.

As topics in astronomy go, FRBs are still young and bustling. Given recent history, one of their original discoverers, astrophysicist Duncan Lorimer of West Virginia University, does not foresee research on FRBs quieting any time soon. “Just when you think things are settling down, you have a year with all these remarkable discoveries,” he says.

Quelle: SCIENTIFIC AMERICAN