"The Biggest Mystery in Astrophysics Today" --Flashes from an Unknown Source Far Beyond the Milky Way (Today's "Galaxy" Stream)
Perhaps the 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,” Heino Falcke, a radio astronomer and astroparticle physicist at Radboud University Nijmegen in the Netherlands told the LA Times about mystery milliseconds-long flashes detected by researchers at Cornell University and a global team. “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."
“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,” said Shami Chatterjee, an astronomer at Cornell University and leader of the Nature paper. “You have to be looking at that right millisecond to be able to catch a fast radio burst. These radio flashes must have enormous amounts of energy to be visible from over 3 billion light-years away.”
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.”
Fast radio bursts, or FRBs, were first seen about 10 years ago. In November 2012, Cornell astronomers using the Arecibo Observatory captured its first FRB – which lasted three one-thousandths of a second, discovered as a postdoctoral researcher sifted through radio telescope data. It was called FRB 121102 located in the pentagon-shaped constellation Auriga. “There’s a patch of the sky from which we’re getting this signal – and the patch of the sky is arc minutes in diameter. In that patch are hundreds of sources. Lots of stars, lots of galaxies, lots of stuff,” said Chatterjee.
To locate the source of this sporadic flash, astronomers blended detective work with modern telescope technology, while combing through terabytes of data.
The Arecibo radio telescope has a resolution of three arc minutes or about one-tenth of the moon’s diameter, but that is not precise enough to identify uniquely the source. Needing higher resolution to find it, the astronomers sought the help of the National Radio Astronomy Observatory’s Karl G. Jansky Very Large Array, near Socorro, New Mexico, which provided more than 80 hours of observation time. The radio telescope array – a collection of dishes aimed at the cosmos – allows for better than one arc-second resolution.
After 50 fruitless hours of staring, the scientists hit the jackpot. “We caught the fast radio burst in the act,” said Chatterjee.
The astronomers used a full range of telescopes to observe that sliver of sky, including NASA’s Chandra X-ray satellite, Chile’s Atacama Large Millimeter/submillimeter Array, and the Gemini optical telescope in Mauna Kea, Hawaii. “With the Gemini telescope, this optical blob looks like a faint, faint, faint galaxy – and this faint, fuzzy blob corresponds with, smack onto, the radio source,” Chatterjee said.
Then, using a range of telescopes including NASA’s Chandra X-ray Observatory and Hawaii’s Gemini optical telescope to peer into the slice of space. According to the New York Times they were able to trace the FRB to a tiny dwarf galaxy.
“We were not sure what to expect, but I think the whole team was surprised to see that our exotic source is hosted by a very puny and faint galaxy,” co-author of the study Cees Bassa says in a press release from the Max Planck Institute for Radio Astronomy.
The next big question is the nature of the source: What powers these bursts and are there other ones that repeat? “We think it may be a magnetar – a newborn neutron star with a huge magnetic field, inside a supernova remnant or a pulsar wind nebula – somehow producing these prodigious pulses,” said Chatterjee. “Or, it may be an active galactic nucleus of a dwarf galaxy. That would be novel. Or, it may be a combination of those two ideas – explaining why what we’re seeing may be somewhat rare.”
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 Falcke. “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.
“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.
Image credit: With thanks to Danielle Futselaar, rendering of all the telescopes that helped track down FRB 121102