|The dishes of the Karl G. Jansky Very Large Array are seen making the first-ever precision localization of a fast radio burst, and thereby pointing the way to the host galaxy. (Photo: Danielle Futselaar)|
For the first time, astronomers have pinpointed the home galaxy of "fast radio bursts," which are very short-lived pulses of radio waves that come from outer space. The discovery pushes scientists closer to understanding what causes these powerful but fleeting pulses of radio waves, which some people speculate could be signals from alien life.
Fast radio bursts, which are highly energetic but last just a few thousandths of a second, have puzzled astrophysicists since their discovery a decade ago. Since 2007, 18 of these bursts have been recorded by telescopes around the world. But only one, discovered in November 2012 at the Arecibo Observatory in Puerto Rico, has repeated numerous times.
By using telescopes from around the world, "we now know that this particular burst comes from a dwarf galaxy more than 3 billion light-years from Earth," said astronomer Shami Chatterjee of Cornell University.
Prior to this discovery, astronomers had lacked the definitive proof that the bursts come from far outside our Milky Way galaxy.
The dwarf galaxy itself, which is less than 1% of the mass of our Milky Way galaxy, is rather unremarkable. “It is surprising that such an exotic source is hosted by such an unimpressive galaxy,” said Joan Schmelz of the Arecibo Observatory.
Though where the burst comes from may have been solved, exactly what it is remains uncertain. The top candidates, the astronomers suggested, are a potpourri of outer-space objects, anything from a neutron star to a supermassive black hole.
“There is still a lot of work to do to unravel the mystery surrounding fast radio bursts,” said McGill physics professor Victoria Kaspi, a senior member of the international team that conducted the research. “But identifying the host galaxy for this repeating fast radio burst marks a big step toward solving the puzzle.”
Scientists presented their findings at the American Astronomical Society's meeting in Grapevine, Texas, in the peer-reviewed British journal Nature, and in companion papers in the Astrophysical Journal Letters.
Radio Bursts Traced to Faraway Galaxy, but Caller Is Probably ‘Ordinary Physics’
Astronomers have traced a series of brief, enigmatic bursts of radio waves to a galaxy far, far away and indeed a long time ago — some three billion years or so.
But as much as you might be hoping or dreading it to be true, this is probably not E.T.
“We’ve joked about spaceship battles and death stars blowing up, but we think we can explain it with ordinary physics,” said Shami Chatterjee, a Cornell astronomer.
Dr. Chatterjee is the lead author of a paper published in Nature on Wednesday that details the search for the source of the radio waves known as “fast radio bursts,” intense pulses of radiation from the sky lasting only a few milliseconds. He also spoke at a news conference sponsored by the American Astronomical Society in Grapevine, Tex.
These have been disappointing times for those yearning for some alien direction from Out There. Last summer, Russian astronomers reported that they had recorded a promising-sounding signal from a star in the Hercules constellation, but they dismissed it when it became public as a freak bit of random radio noise, the astrophysical equivalent of a cosmic butt dial.
There are problems with both explanations, however, he added.
Fast radio bursts have led astronomers on a merry chase ever since they were discovered in 2007 in data recorded earlier by the Parkes radio telescope in Australia.
Because they are so short and until recently have never been seen to repeat, these phantoms have been hard for astronomers to study. Usually, astronomers notice them after the fact. Moreover, radio telescopes have poor angular resolution, making it impossible to determine exactly what star or distant galaxy they came from.
The radio emissions themselves, Dr. Chatterjee said, resemble the blasts from pulsars — the spinning neutron stars that emit clocklike pulses of radiation and whose discovery in 1968 did indeed elicit speculation about little green men. But the radio waves arrive on Earth dispersed or spread out in time by wavelength, which implies that they have traveled from far outside our galaxy.
That great distance also implies that they are enormously more powerful than pulsars in our galaxy, adding to the mystery of what they are and raising the question of why they are not seen within our own galaxy, the Milky Way.
In all, 18 of the fast bursters have been spotted since they were first recognized in 2007 — a small number. If extrapolated to the whole sky, that means 5,000 to 10,000 of these flashes should happen every day. Where are they?
Lacking much evidence to the contrary, astronomers theorized that the bursts resulted from apocalyptic events like collisions of neutron stars. At one point, Dr. Chatterjee said, there were more theoretical models of the bursts than observed bursts. “Many things go bang,” he explained.
The big break came in 2012 when the burst known as 121102 repeated itself. Subsequent observing campaigns with the Very Large Array of telescopes in New Mexico and the Arecibo radio telescope in Puerto Rico recorded a total of nine bursts over 83 hours of observing time and a terabyte of data during a six-month period in 2016.
More recently, searches for radio signals from a set of stars with anomalous spectral features and another star known as Tabby’s Star that has shown suspicious variations in its light seem to have come up empty.
So at least for now, the skies appear to be bereft of intelligence. But the new results from the fast radio burster, known as 121102 — after Nov. 2, 2012, the date it was first observed — need not discourage any aficionados of cosmic mystery.
Most likely, Dr. Chatterjee said in a telephone interview, the bursts could be caused by weird reactions between a neutron star — the dense spinning magnet left behind by a supernova explosion — and the debris from that explosion. Or perhaps from some unexpected quirk of a supermassive black hole in the center of the galaxy, a dwarf assemblage of stars some three billion light-years away in the constellation Auriga.
That meant that whatever was causing 121102, at least, was not destroying it. “We definitely know, for this one case anyway, the radio burst is not cataclysmic,” Dr. Chatterjee said.
In addition, papers also have been submitted to the Astrophysical Journal Letters by two other groups led by Shriharsh Tendulkar of McGill University and by Benito Marcote of the Joint Institute for VLBI in Europe, in Dwingeloo, the Netherlands.
Following up on the Very Large Array observations, Dr. Marcote’s team on the European VLBI network was able to pinpoint the location of the burst to a faint dwarf galaxy in the Auriga constellation. Dr. Tendulkar and his colleagues then used the eight-meter Gemini North telescope on Mauna Kea in Hawaii to observe the galaxy and measure its distance.
That distance, three billion light-years, confirmed the original supposition that the fast bursts come from far, far away. “The host galaxy is puny,” Dr. Tendulkar said during the news conference in Grapevine. That dwarf galaxy is only a hundredth of the mass of the Milky Way.
If this burster, 121102, is indeed typical of the bunch, the astronomers said, this might be a clue.
Such galaxies are typically home to some of the most violent events in the universe, Dr. Chatterjee said, things that go seriously boom in the night like certain kinds of gamma-ray bursts and superluminous supernova explosions that result in extremely magnetic pulsars known as magnetars. These are the signatures of massive stars, of the deaths of massive stars, he said.
But this only raises more questions.
“The only one that repeats is from three billion light-years,” Dr. Chatterjee mused.
“Where are all the nearby ones?” he asked, noting that they should be even brighter, saturating our radio receivers.
“It’s very curious,” he said.
In a first, scientists detect 'fast radio bursts' from beyond the Milky Way galaxy
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.
The discovery, described in a paper in Nature and two others in Astrophysical Journal Letters, could help scientists understand the origins of these strange phenomena, which have eluded them for nearly a decade.
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.