Mysterious radio signals may come from a super massive black hole
After five years, scientists might have figured out what’s going on with blasts of mysterious radio waves coming from outside the Milky Way: they’re coming from a zombie star in an extreme environment. In a new study, astronomers suggest this might explain these bizarre intergalactic radio waves, known as fast radio bursts. And it may be the best explanation we have yet for what’s causing them.
Fast radio bursts, or FRBs, have been one of the biggest enigmas for astronomers since 2007. These intense blasts of radio waves come from beyond our galaxy, lasting for only milliseconds at a time. No one knows exactly what’s causing them, and studying them is incredibly difficult since they’re so brief. It’s thought that one FRB is being produced in the Universe every second, but only 20 have been detected from Earth over the last decade.
Fortunately, one of those FRBs, called FRB 121102, is different from the rest: it’s the only one known to repeat. After it was found in 2012, astronomers have been able to observe this event as it burps up waves over and over again. And they found that the waves coming from this FRB are actually twisted — a sign that they’ve passed through some highly magnetized material before reaching our planet. A good place to find material like that? The nucleus of a galaxy. “If you think about the type of regions that have properties like this in our galaxy, the only region is around the center of the galaxy where there’s a supermassive black hole,” Jason Hessels, an astronomer at the University of Amsterdam and lead author of a Nature study on this discovery, told.
Of course, there are other ways to pass through some other highly magnetized material, and Hessels and his team are open to others’ interpretations. Figuring out what the environment is like around the place where this FRB originated will get scientists closer to understanding what these radio waves are in the first place.
Scientists have floated a number of ideas for what might be causing FRBs. Perhaps these waves are produced during cataclysmic events, like when two dense black holes slam into one another. Or perhaps they’re caused when something collapses into a black hole and gets ripped apart. But these scenarios don’t quite explain FRB 121102; whatever is producing the waves can’t be destroyed. “If the source is repeating it needs to continue producing such bursts,” says Hessels.
That’s why astronomers think the waves from FRB 121102 might be coming from a stellar corpse known as a neutron star — the dense leftover core of star after it’s collapsed. Special kinds of neutron stars can periodically send out flashes of radiation, which may explain the repeating waves. But the waves we’ve seen from FRB 121102 are incredibly bright and more powerful than a neutron star could produce from so far away. Astronomers think the waves are coming from a galaxy 3 billion light-years away, which means they have to be super intense to fit what we’ve seen.
To learn more about the source, Hessels and his team used the Arecibo Observatory in Puerto Rico and the Green Bank Telescope in West Virginia to observe the radio blasts coming from this galaxy, ultimately measuring 16 bursts in 2016 and 2017. When analyzing their data, they found a distortion in the radio waves. Normally, a natural burst of radio waves will have wavelengths moving in multiple directions. But the waves coming from FRB 121102 all seemed to move in one similar direction, an effect known as polarization. “It’s like how sunglasses reduce glare from reflections off the snow. They’re only sensitive to a certain direction of light,” says Hessels. “And this light has a preferred direction.”
When polarized light moves through a strong magnetic field, it can actually get twisted. And Hessels found that the radio wave signals had been twisted so much, they must have passed through incredibly hot, super magnetized material. The area around a supermassive black hole fits that bill. Enormous discs of gas and dust surround black holes, which gets super heated and magnetized as it spirals inward toward the hole. That could explain the twisting, as well as why the signal from FRB 121102 is so bright. It’s possible that the material is acting like a magnifying glass, amplifying the signal when the radio waves pass through.
Hessels and his team suggest other explanations for the twist, too. Maybe the waves are passing through a strongly magnetized nebula of gas. Or the neutron star producing the signal just recently exploded in a supernova, and the waves are passing through the outer shell of the star that was blown off during the violent death. However, the researchers think the supermassive black hole is the best explanation for now, and others agree. “Really the only environment that we’ve seen with that type of [twisting] is the galactic center in the parts of our own galaxy,” Duncan Lorimer, an astronomer at West Virginia University who discovered the first FRB and who was not involved in this study, tells The Verge.
It’s a big step in the ongoing quest to figure out FRBs. But today’s finding may be limited to explaining only the repeating signals of FRB 121102. The other FRBs we’ve seen may be coming from completely different types of sources in other types of environments. “This is the only one known to repeat, so we could be looking at a very specific subclass of FRB and something not representative of the whole population,” says Lorimer.
Hopefully, more and more FRBs will be discovered in the years ahead to help scientists unravel the mystery. Powerful new radio telescopes are about to come online, which should be able to pick up FRBs more frequently. And as we find more of these radio bursts, we might be able to learn more about them — especially if we find another one that repeats. “We expect to find many dozens, if not hundreds, of these sources over the next few years,” says Hessels. “And it may not be long before we find the next repeating source.”
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