A repeating fast radio burst first detected in 2012 likely formed within a potent magnetic field — opening up the ambiguity of what caused the mysterious burst.
Every day, the radio sky is pierced by 10,000 flashes of light. Although these so-called fast radio bursts are immensely bright, generating as much energy as 500 million Suns, they only last for a few milliseconds — meaning they’ve mostly gone unnoticed and their origins remain mysterious.
Then in 2016, astronomers announced that one of these bursts, named FRB 121102, had flared up again several times, making it the only fast radio burst known to repeat. In the years since, it has erupted more than 200 times, allowing astronomers to finally tie the enigma to its home galaxy: a tiny one located some 2.5 billion light-years away.
The latest observations, reported January 10th in Nature and at a meeting of the American Astronomical Society, suggest the bursts come from an environment with an unusually strong magnetic field. As such, the culprit might be a young neutron star within the vicinity of a black hole or maybe a black hole itself.
Daniele Michilli (University of Amsterdam) and his colleagues used Arecibo Observatory in Puerto Rico and the Green Bank Telescope in West Virginia to detect the new bursts at higher frequencies than before. The results show that the polarization of the radio waves had been twisted into spirals — a signature that occurs when light passes through a magnetic field.
“I could not believe my eyes when my colleagues emailed the results around,” said Victoria Kaspi (McGill University) in a press release. “This sort of enormous Faraday rotation is extremely rare. Once we digested it, we realized it was a huge clue about where this bizarre source resides.”
The light is so twisted that the scientists argue it must have passed through an exceptionally strong magnetic field, one that’s at least 100 times stronger than the average magnetic field in the Milky Way. This means that the culprit likely has an unusual neighbor: a massive black hole. In fact, the only known source that has such a strong polarization signature comes from a pulsar near Sagittarius A* — the supermassive black hole at the center of our galaxy.
In addition, the shortest burst lasted just 30 microseconds, suggesting that the source is as small as 10 km across — the typical size of a neutron star. “Now 10 kilometers might seem big,” said Vishal Gajjar (University of California, Berkeley) at the meeting. “But imagine the source is actually 3 billion light-years away from us, and we are pinning down the emission to only 10 kilometers in size. That's kind of spectacular.”
As such, the team thinks the fast radio bursts originate from a neutron star within a black hole’s vicinity. But it does pose a question: Why would this source be located so close to a massive black hole? “Presumably that would not just be a coincidence,” said Jason Hessels (University of Amsterdam). “Presumably it would be an important part of the story — but we don't know why it matters.”
Then again, that could not be the story at all. Michilli and his team members agree that other scenarios could give rise to this odd fast radio burst.
“It's likely that it would be very young and if it's very young it's quite possible that it could still be in some kind of cocoon, [like] a supernova remnant,” Hessels said. But that hypothesis isn’t perfect either. The remnant would have to be a million times brighter than the Crab Nebula, the brightest remnant in the Milky Way.
A Turbulent Home
“What's exciting about this most recent report is that I think it just reiterates the fact that we don't exactly know where these are coming from,” said Sarah Burke-Spolaor (West Virginia University), who was not involved in the study. “I think it reopens the ambiguity of whether this could be from a neutron star or a black hole.”
Though there are many different theories that explain how a fast radio burst might originate from a black hole, one idea begins with the black hole’s accretion disk — that swirl of hot matter that circles a black hole. Because the black hole can’t eat infinitely fast, some of the particles that are spiraling inward actually get pushed outward — creating these huge flows of energy shooting away from the black hole. If that ejecta hits a wall of molecules, it will ionize them and create a flash in the radio.
But more ideas will likely be forthcoming. “One of the things that excites me is that in the coming weeks we expect that very creative theorists are going to come up with explanations for our observations that we haven't thought of yet,” Hessels said.
Though the joke is that there are more theories than there are fast radio bursts (a very true observation) and Hessels does look forward to the ever-widening gap in the coming weeks, both he and Burke-Spolaor are hopeful that the gap will eventually decrease. With a number of wide-field radio telescopes coming online, it’s likely that astronomers will soon detect more fast radio bursts — helping them get a better handle on these enigmas.
“Once we start finding lots of these hopefully in the course of this year and the coming year — it's a foregone conclusion that we're going to find other repeating sources,” Hessels said. Such discoveries will only help astronomers understand how FRB 121102 formed and whether it’s an oddball compared to other bursts or not.
D. Michilli et al. “An extreme magneto-ionic environment associated with the fast radio burst source FRB 121102.” Nature.