Mysterious radio signals pack power and brilliance

Mysterious flashes of radio waves from deep space keep coming, but they are just as mysterious as ever.

Gamma rays might have accompanied one of these eruptions, researchers report in the Nov. 20 Astrophysical Journal Letters. This is the first time high-energy photons have been associated with these blasts of radio energy, known as fast radio bursts. If the gamma rays did come from the same place as the radio waves, then the underlying source could be roughly 1 billion times as energetic as thought.

Another burst, meanwhile, takes the record for brightest blast. The signal was bright enough to reveal details about the magnetic field between galaxies, astronomers report online November 17 in Science. 

Fast radio bursts, or FRBs, have intrigued astronomers since the first one was reported in 2007 (SN: 8/9/14, p. 22). Since then, astronomers have discovered 18 in total. In most cases, a blip of radio waves lasting just a few milliseconds appears in the sky and is never seen again. Only one so far is known to repeat (SN: 4/2/16, p. 12). Most seem to originate in remote galaxies, possibly billions of light-years away. Until now, no one has detected any other frequency of electromagnetic radiation besides radio waves coming from these cosmic beacons.

A flash of gamma rays appeared at about the same time and from the same direction as a radio burst detected in 2013, James DeLaunay, a physics graduate student at Penn State, and colleagues report. They pored over old data from the Swift observatory, a NASA satellite launched in 2004, to see if it recorded any surges of gamma rays that might coincide with known radio bursts.

“Gamma rays associated with an FRB would be an incredibly important thing to find,” says Sarah Burke Spolaor, an astrophysicist at the National Radio Astronomy Observatory in Socorro, N.M. But she urges caution. “We don’t have a good inkling of where a specific burst comes from.” That leaves room for other types of eruptions to occur in the vicinity just by chance. DeLaunay and collaborators calculate that the odds of that are low, about one in 800. But several researchers are taking a wait-and-see attitude before feeling more confident that the gamma rays and FRB are linked.

“It’s tantalizing, but a lot more would need to be found to be convincing,” says Jason Hessels, an astrophysicist at the Netherlands Institute for Radio Astronomy in Dwingeloo.

If the same source emits both the radio waves and gamma rays, that could rule out a couple of proposals for the causes of the eruptions. Powerful radio hiccups from pulsars, the rapidly spinning cores of dead stars, are one candidate that wouldn’t make the cut, because they aren’t known to generate gamma rays.

Collisions between two neutron stars, or between a neutron star and a black hole, look promising, says Derek Fox, an astrophysicist at Penn State and a coauthor of the study. The energy output and duration of the gamma-ray burst are a good match with what’s expected for these smashups, he says, though it’s not clear whether they happen often enough to account for the thousands of FRBs that astronomers suspect go off every day.  

No one story neatly fits all the data. “I think there are at least two populations,” says Fox. Perhaps some FRBs repeat, while others do not; some belch out gamma rays, others do not. There might be no one type of event that creates all FRBs, but rather a multitude.

That idea is tentative as well. “It’s way too early to say if there are multiple populations,” says Laura Spitler, an astrophysicist at the Max Planck Institute for Radio Astronomy in Bonn, Germany. A grab bag of cosmic calamities is plausible. But there are other astronomical events that exhibit enormous diversity, enough that all FRBs could also have just one type of trigger. “The data we have now isn’t sufficient to land on one side or the other,” Spitler says.

A more recent FRB, detected in 2015 at the Parkes radio telescope in Australia, shows off some of that diversity — and demonstrates how FRBs can be used as cosmological tools. A brief blast of radio waves from at least 1.6 billion light-years away is about four times as intense as the previous record holder. The signal’s vigor could be an intrinsic quirk of the underlying outburst, or could mean that this burst was unusually close to our galaxy — or both.

“What’s really exciting most about it is not just that it’s bright,” says Vikram Ravi, a Caltech astronomer and lead author of the study, “but really because of what we hope to use FRBs for.” This FRB was bright enough for Ravi and colleagues to deduce the magnetic field between galaxies. To do that, they measured the signal’s polarization, the alignment of radio waves imprinted by magnetized plasmas encountered en route to Earth. They found that, on average, the magnetic field is feeble, less than 21 nanogauss (or about one 10-millionth as strong as Earth’s magnetic field). That’s in line with astronomers’ theories about the strength of intergalactic magnetism.

“It’s not telling us anything that’s unexpected,” says Duncan Lorimer, an astrophysicist at West Virginia University in Morgantown who reported the first FRB in 2007. But it shows that FRBs can be used to learn more about intergalactic space, a region that is notoriously difficult to study. “It’s one thing to say we expect the magnetic field to be weak, but it’s another thing to actually measure it,” he adds. “It’s a signpost of things to come.”

This burst encountered different environments than a burst reported last year in Nature, which suggested an FRB origin in a highly magnetized environment, possibly near young stars in a remote galaxy (SN Online: 12/2/15). There’s no hint that the latest burst originated in a similar locale.

“I don’t think we contradict each other at all,” Ravi says. “Some FRBs originate in very magnetic environments and some don’t. Given that these are the only two FRBs where these measurements have been made, it’s hard to tell.”