Fun with Fast Radio Bursts

[rhoenix]

From nowhere, they appear as a sudden surge of power in the radio spectrum. Then, a few milliseconds later, they're gone—and as far as we could tell, they never come back. They've picked up the name "fast radio bursts," but nobody's entirely sure of what produces them. Follow-up observations have generally failed to find anything interesting in their direction, and the bursts didn't seem to repeat, leaving everyone who cares about these sorts of things a bit mystified.

One possible explanation for their one-time-only appearance would be that they're the product of a process that destroys the object that creates them. Thus, if they were produced by the collapse of a neutron star into a black hole (to give just one example), there'd be no way for that to happen twice.

But a new study suggests that at least one of them has repeated, which would take cataclysmic explanations off the table. There are enough differences between this burst and previously observed ones, however, to raise the question of whether there might be several processes producing similar surges in radio emissions.

The burst in question (which goes by FRB 121102) was identified using the Arecibo Observatory in Puerto Rico. There was nothing especially unusual about it, but the authors scheduled it for follow-up observations. Some of these turned up nothing. But others turned up several bursts, sometimes within hours of each other. All told, they saw 10 additional bursts (FRB 121102 goes to 11!) over the course of two months. These had different brightnesses, and different intensity peaks showed up at different wavelengths, suggesting a somewhat chaotic process.

Looking through the timing data, the authors found no indication of a periodic process, which makes some sort of orbital behavior unlikely; the quick repetition in some cases also makes an orbital phenomenon unlikely. And, as the authors put it, "Repeat bursts rule out models involving cataclysmic events."

So what is still on the table? Neutron stars with intense magnetic fields, called magnetars, can create giant flares. But none of the ones we know about have produced more than a single flare. The authors suspect it might be a pulsar—a rapidly rotating neutron star—where sporadically, one of the normal pulses is greatly amplified. This sort of behavior has been seen in at least some neutron stars in our galaxy.

Confusing matters further is the question of whether FRB 121102 is a typical fast radio burster. The other ones we know about are more intense by about a factor of 10, and they appear to originate outside our galaxy. Based on the direction it's coming from, however, the authors suggest there's a chance that FRB 121102 comes from an object inside our galaxy. Finally, there's the issue of instrumentation. Most fast radio bursts have been spotted by Australia's Parkes telescope, which isn't as sensitive as Arecibo and wouldn't have identified most of the bursts emitted by FRB 121102.

So, rather than definitively ruling out a destructive cause for fast radio bursts, the best that this paper can do is indicate that at least some of them leave their source intact. Which, given how little we knew about these previously, represents progress.

[rhoenix]

Astronomers for the first time have detected repeating short bursts of radio waves from an enigmatic source that is likely located well beyond the edge of our Milky Way galaxy. The findings indicate that these "fast radio bursts" come from an extremely powerful object which occasionally produces multiple bursts in under a minute.

Prior to this discovery, reported in Nature, all previously detected fast radio bursts (FRBs) have appeared to be one-off events. Because of that, most theories about the origin of these mysterious pulses have involved cataclysmic incidents that destroy their source -- a star exploding in a supernova, for example, or a neutron star collapsing into a black hole. The new finding, however, shows that at least some FRBs have other origins.

FRBs, which last just a few thousandths of a second, have puzzled scientists since they were first reported nearly a decade ago. Despite extensive follow-up efforts, astronomers until now have searched in vain for repeat bursts.

That changed last November 5th, when McGill University PhD student Paul Scholz was sifting through results from observations performed with the Arecibo radio telescope in Puerto Rico -- the world's largest radio telescope. The new data, gathered in May and June and run through a supercomputer at the McGill High Performance Computing Centre, showed several bursts with properties consistent with those of an FRB detected in 2012.

The repeat signals were surprising -- and "very exciting," Scholz says. "I knew immediately that the discovery would be extremely important in the study of FRBs." As his office mates gathered around his computer screen, Scholz pored over the remaining output from specialized software used to search for pulsars and radio bursts. He found that there were a total of 10 new bursts.

The finding suggests that these bursts must have come from a very exotic object, such as a rotating neutron star having unprecedented power that enables the emission of extremely bright pulses, the researchers say. It is also possible that the finding represents the first discovery of a sub-class of the cosmic fast-radio-burst population.

"Not only did these bursts repeat, but their brightness and spectra also differ from those of other FRBs," notes Laura Spitler, first author of the new paper and a postdoctoral researcher at the Max Planck Institute for Radio Astronomy in Bonn, Germany.

Scientists believe that these and other radio bursts originate from distant galaxies, based on the measurement of an effect known as plasma dispersion. Pulses that travel through the cosmos are distinguished from human-made interference by the influence of interstellar electrons, which cause radio waves to travel more slowly at lower radio frequencies. The 10 newly discovered bursts, like the one detected in 2012, have three times the maximum dispersion measure that would be expected from a source within the Milky Way.

Intriguingly, the most likely implication of the new Arecibo finding -- that the repeating FRB originates from a very young extragalactic neutron star -- is at odds with the results of a study published last week in Nature by another research team. That paper suggested FRBs are related to cataclysmic events, such as short gamma-ray bursts, which can not generate repeat events. "However, the apparent conflict between the studies could be resolved, if it turns out that there are at least two kinds of FRB sources," notes McGill physics professor Victoria Kaspi, a senior member of the international team that conducted the Arecibo study.

In future research, the team hopes to identify the galaxy where the radio bursts originated. To do so, they will need to detect bursts using radio telescopes with far more resolving power than Arecibo, a National Science Foundation-sponsored facility with a dish that spans 305 metres and covers about 20 acres. Using a technique called interferometry, performed with radio telescope arrays spread over large geographical distances, the astronomers may be able to achieve the needed resolution.

"Once we have precisely localized the repeater's position on the sky, we will be able to compare observations from optical and X-ray telescopes and see if there is a galaxy there," says Jason Hessels, associate professor at the University of Amsterdam and the Netherlands Institute for Radio Astronomy as well as corresponding author of the Nature paper. "Finding the host galaxy of this source is critical to understanding its properties," he adds.

Canada's CHIME telescope could help unravel the puzzle, adds Kaspi, who is Director of the McGill Space Institute. Thanks to the novel design of the soon-to-be completed apparatus, it is expected to be able to detect dozens of fast radio bursts per day, she says. "CHIME will further our quest to understand the origin of this mysterious phenomenon, which has the potential to provide a valuable new probe of the Universe."

[rhoenix]

Fast radio bursts are millisecond-duration astronomical radio pulses of unknown physical origin that appear to come from extragalactic distances. Previous follow-up observations have failed to find additional bursts at the same dispersion measure (that is, the integrated column density of free electrons between source and telescope) and sky position as the original detections.

The apparent non-repeating nature of these bursts has led to the suggestion that they originate in cataclysmic events. Here we report observations of ten additional bursts from the direction of the fast radio burst FRB 121102. These bursts have dispersion measures and sky positions consistent with the original burst. This unambiguously identifies FRB 121102 as repeating and demonstrates that its source survives the energetic events that cause the bursts.

Additionally, the bursts from FRB 121102 show a wide range of spectral shapes that appear to be predominantly intrinsic to the source and which vary on timescales of minutes or less. Although there may be multiple physical origins for the population of fast radio bursts, these repeat bursts with high dispersion measure and variable spectra specifically seen from the direction of FRB 121102 support an origin in a young, highly magnetized, extragalactic neutron star.

[rhoenix]

Last week, scientists reported a huge step in the hunt for mysterious cosmic phenomena known as Fast Radio Bursts (FRBs). But now their results are being called into question by others in the field. Amid that controversy, a new paper has just hit the presses: For the first time ever, they've found an FRB that keeps repeating. Published Wednesday in Nature, the study provides evidence that FRBs come from a source other than the massive star collision suggested by last week's researchers.

FRBs are bright radio flashes that last just a few milliseconds, and until now have never been known to repeat. Scientists believe they might occur thousands of times a day, but to date less than 20 events have been detected. Last week's study – also published in Nature – claimed to have pinpointed the exact origin point of an FRB for the first time ever.

Now some scientists are questioning whether the signal used to track down the galaxy associated with the FRB was actually related to the radio burst at all. And this new study adds another possible point of contention: Based on the apparent age of the galaxy pinpointed in the first study and the strength of the radio burst, researchers had suggested a collision of massive stars as the cause of the mysterious signal.

But massive collisions don't repeat – and now it seems apparent that FRBs can and do.

"I don’t think the final nail is in the coffin on that," Jason Hessels, corresponding author of the latest study, told The Post in reference to the other team's research."There are more observations that need to be done, but it seems less convincing than it did last week." It is possible, he and other experts said, that there is more than one kind of FRB out there – some sent out by massive crashes in space, and others coming from different, more sustainable sources.

Harvard University’s Edo Berger, who is a co-author on an as-yet-unpublished paper that sets out to refute the supposed FRB origin, was more blunt: "Essentially I would say that the whole rationale behind the paper has gone away within about two or three days of when it was published," he told The Post, explaining that he believes the signal described in that paper has lasted too long to be associated with an FRB, and is likely some other space phenomenon they stumbled upon by accident.

Evan Keane, an astronomer with the Square Kilometer Array Organization who led the first study, told The Post that he won't comment on the particulars of Berger's takedown until it's reviewed by other scientists and published in a journal. But Keane and Berger agree on one thing: The latest paper is the real deal.

"My initial thoughts on the paper are that it is quite exciting, and as it is repeating it is clearly not due to a 'one off’ event, like (say) a supernova, or merger of two objects; they would only happen once," Keane told The Post in an email. He wasn't surprised or concerned about implications for his own findings, since scientists have floated the idea of multiple FRB origins before, "but it is clearly excellent to see the repetition, and so clearly," he said.

The findings stem from a discovery made in November, when McGill University Ph.D. student Paul Scholz was working with FRB data from the Arecibo radio telescope in Puerto Rico, which is the largest of its kind. After months of gathering and analyzing more data from the same spot in the sky, Scholz, Hessels and their colleagues showed several bursts with properties consistent with those of an FRB first detected in 2012.

"This one I think is a really great result," Berger said. "It's a great clue for what might be causing this. It’s not a one off thing, it’s a stable system that can be active again and again and again."

What might that stable system be? The scientists suggest a super-powerful neutron star. These stellar zombies are the remnants of huge stars gone supernova. They're just on the cusp of becoming black holes, with just barely enough pressure in their cores to keep them from collapsing into oblivion. That makes them the densest stars in the universe, and scientists have a lot to learn about their behavior.

Such an origin would bring the FRB mystery full circle, Hessels explained: Scientists first found FRBs just over a decade ago when searching for pulsing neutron stars (or pulsars) inside the Milky Way.

"They came across this signal that didn't quite fit with what we knew," Hessels said. The distance implied by the properties of these signals placed them outside the realm of neutron stars inside the galaxy.

"So these new findings could suggest some kind of very extreme pulsating neutron star, spinning very fast, with a strong magnetic field. . . beyond what you'd find with one in our own galaxy. It would have to be a very rare type of source," Hessels said.

As with last week's paper, more work is needed to confirm these findings – and it will take even more detections to determine whether FRBs have one source or many. Pinpointing the exact location of the sources of multiple FRBs is vital in puzzling out their true origins. Three highly sensitive FRB-detecting instruments are set to open this year, so it's possible that FRBs won't be mysterious for too much longer.

"That could be a month from now or it could be five years from now," Berger said.

[Josh]