Sifting through the heavens like prospectors panning silt for a glint of gold, astronomers say they have finally pinpointed the source of so-called fast radio bursts – brief, powerful and mysterious flashes of light.
Astronomers study the universe in wavelengths of light that are higher energy than the visible spectrum (including ultraviolet, X-ray and gamma rays) and lower energy than what we can see (including infrared and radio waves). Each slice of light reveals something different about the nature of the universe.
When it comes to radio light – the longest, most low-energy wavelengths – scientists largely thought they knew what the universe looked like, said Heino Falcke, a radio astronomer and astroparticle physicist at Radboud University Nijmegen in the Netherlands, who was not involved in the research.
"You would see an impressively bright Milky Way galaxy, smoke rings from exploded stars, plumes of gas escaping from black holes and blinking radio emissions from cosmic lighthouses called pulsars," he wrote in a commentary on the Nature paper.
Fast radio bursts, first reported in 2007 by a team analyzing archived data from Australia's Parkes Observatory, changed that. Here was a new, unexplained source of radio light in the sky – and unlike any they had ever seen. If these flashes were coming from beyond the Milky Way's borders, then they must be produced by incredibly powerful sources.
Astronomers have long wondered what could cause such a blast. Was it a one-time flash triggered by a supernova explosion, for example, or a signal sent from around a supermassive black hole at the heart of a bright galaxy?
"There are literally more theories for what FRBs are than there are detected examples of FRBs," said Shami Chatterjee, an astronomer at Cornell University and leader of the Nature paper. "It's been a paradise for theorists; they've come up with all sorts of ways that you could produce these kinds of radio flashes."
To answer that question, they'd have to figure out where these fast radio bursts were coming from.
The problem was that these blasts of radio light were also infinitesimally brief, lasting only a few milliseconds – which made them extremely difficult to locate, especially given that radio telescopes can only look at a small patch of sky at a time.
"Because these flashes last only a millisecond, you can't just go back and look at that patch of sky at a different time and catch that fast radio burst," Chatterjee said. "You have to be looking at that right millisecond to be able to catch a fast radio burst."
Astronomers now believe that these fast radio bursts, once deemed a rarity, are so common that they light up the night sky roughly twice a minute, Falcke said – and yet, in the near-decade that we've known about them, they've only managed to catch a total of 18 in the act, let alone figure out where they're coming from.
"Every day, all over the sky, there are 5,000 to 10,000 of these flashes going off," Chatterjee said. "It's a huge rate. … That tells you how little of the sky we're seeing at any given time."
Then came a lucky break: In November 2012, scientists detected a burst called FRB 121102 using the Arecibo Observatory in Puerto Rico. Unlike the previous 17 fast radio bursts, this one repeated itself – which meant scientists had a chance to observe it using the Very Large Array radio telescope in New Mexico, which has 27 dish-shaped antennas that together allow it to see distant objects in very high resolution.
After watching FRB 121102 for 83 hours over six months this past year, the scientists picked up nine bursts from the same spot. Together, those flashes of light allowed them to pinpoint it – and then use the Gemini North telescope in Hawaii to study its location. This turned out to be roughly 3 billion light-years away in a small dwarf galaxy which holds just a hundredth or so of the Milky Way's mass.
This came as a surprise to the researchers. After all, many suspected that this kind of flash might come from a supermassive black hole – but those typically would sit at the hearts of large, active galaxies, not in a small, dim dwarf galaxy.
"Perhaps the authors' optical source is a dwarf galaxy that contains a supermassive black hole, or is the nucleus of a disrupted galaxy or even just an isolated black hole," Falcke mused over the mystery. "Maybe the persistent source is something completely different – for example, an exploding star 'disguised' to look like a black hole. And are these bursts made by the black hole itself, or by something else in orbit around it? After all, supermassive black holes are typically surrounded by dense star clusters. Chatterjee and colleagues, and the rest of the astrophysics community, are left scratching their heads."
Understanding the nature of fast radio bursts could help researchers probe the intergalactic medium – the nearly empty space between galaxies – and it could shed light on the physical processes that let off the burst in the first place.
But Chatterjee was quick to point out that one discovery was not enough to try to understand these fast radio bursts – astronomers need to catch more FRBs and pinpoint their location to see if FRB 121102 is typical, or if there is a diverse array of sources for these fleeting flashes of light.
"Still, even without a clear answer, the authors' finding is a real game-changer, and the hunt for FRBs is afoot," Falcke wrote.
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