An international team of astronomers, operating radio telescopes across the globe from Hawaii to the Netherlands, have pinpointed the precise position of one of the most mysterious phenomena in the cosmos: a fast radio burst (FRB). The results of this extensive study are being presented today at an , as well as in papers and the Astrophysical Journal Letters.
FRBs are incredibly energetic bursts of radio waves that last for only fractions of a second, between 1 and 5 milliseconds to be exact. These quick blasts of intense energy have perplexed astronomers since 2007, when a team of researchers sifting through the data archives of Australia's Parkes Radio Telescope discovered the first known FRB, which reached Earth in 2001.
Eighteen FRBs were discovered in the following years. Some astronomers thought they were coming from cataclysmic events in the deep universe, superluminous supernovae or supermassive black holes ejecting material. Others argued that fast radio bursts originated right in our backyard, pulsars surrounded by galactic dust or intense magnetic events in the Milky Way.
Now we have pinpointed the origin of an FRB, and it's coming from a small dwarf galaxy, some 3 billion light-years away. This proves that these fast radio bursts do in fact originate from the distant universe, meaning they must be from gargantuan events for the signal to reach us after such immense time and distance.
FRB 121102, the fast radio burst we located, originated from an event that occurred 3 billion years ago—just as the first life to use photosynthesis formed on our planet—in a dwarf galaxy that sits almost 700 million times farther away from us than the closest star system, Alpha Centauri.
"We now know that this particular burst comes from a dwarf galaxy more than 3 billion light-years from Earth," said Shami Chatterjee, of Cornell University, in a . "That simple fact is a huge advance in our understanding of these events."
FRB 121102 was first discovered in 2012 by the in Puerto Rico, the largest single-dish radio telescope at the time. Researchers working at Arecibo noticed that FRB 121102, unlike other FRBs, has recurred multiple times. So to catch it again, astronomers started searching with the (VLA), a series of 27 radio dishes in New Mexico that can be moved on railroad-style tracks to function as an —a group of telescopes that act together to produce incredibly high-resolution observations.
With the VLA, researchers saw nine radio bursts from FRB 121102 over the course of six months and 83 observing hours.
"For a long time, we came up empty, then got a string of bursts that gave us exactly what we needed," said Casey Law, the researcher at the University of California Berkeley who developed the data-acquisition system and analysis software required to search for FRBs. "The VLA data allowed us to narrow down the position very accurately," adds Sarah Burke-Spolaor of the National Radio Astronomy Observatory (NRAO) and West Virginia University.
The next step was to figure out what type of environment FRB 121102 was emitting from. We knew the coordinates in space from VLA, but discovering what sits at those coordinates other than the source of the intense radio bursts required another telescope, the in Hawaii. Using the 8.19-meter telescope to scan the area for visible light, rather than radio waves, Gemini North identified a dwarf galaxy in the location of the fast radio burst. The visual-light observations also confirmed that FRB 121102 is an incredible 3 billion light-years away.
"We are the first to show that this is a cosmological phenomenon. It's not something in our backyard. And we are the first to see where this thing is happening, in this little galaxy, which I think is a surprise," Law said. "Now our objective is to figure out why that happens."
What exactly causes these fast radio bursts is still a mystery, but we have ruled out local phenomena, and now we can start to make some educated guesses. Other galaxies similar to the one that FRB 121102 comes from, faint dwarf galaxies, are known to host incredibly active and volatile astronomical events. , which are some of the most catastrophic events in the universe, and , which are associated with rapid star formation and , have both been observed in dwarf galaxies.
Additionally, these two astronomical phenomena are both associated with the formation of . Magnetars are neutron stars—the small, dense stellar core that is left behind after a big star supernovas—except magnetars have a particularly intense magnetic field. Law thinks these could be the cause of fast radio bursts.
"All these threads point to the idea that in this environment, something generates these magnetars," he explains. "It could be created by a superluminous supernova or a long gamma-ray burst, and then later on, as it evolves and its rotation slows down a bit, it produces these fast radio bursts as well as continuous radio emission powered by that spin-down. Later on in life, it looks like the magnetars we see in our galaxy, which have extremely strong magnetic fields but rotate more like ordinary pulsars."
Additional observations from radio dishes around the world confirmed that the nine distinct bursts of FRB 121102 were coming from a relatively small area with a 100-light-year diameter. The dwarf galaxy that hosts this area was also found to be continuously emitting weak radio signals between the strong bursts, strengthening Law's hypothesis that a magnetar causes the FRB, emitting weak radio waves all the time, but releasing a stronger pulse every so often—possibly when it comes into with other material. In any case, the source of the bursts and the continuous radio waves is almost certainly the same object, or multiple objects somehow influencing each other, given the small area measured.
We need to keep in mind, however, that FRB 121102 is a unique case because it repeats, unlike any other known FRBs, and therefore it may be caused by a physical phenomenon that is not the same as the other 17 FRBs. Dwarf galaxies are also known to host supermassive black holes, and it may be that jets of material emitted from the region immediately surrounding such a black hole are the cause of these mysterious, intense bursts of radio waves that we detect on Earth.
The discovery is truly a great example of international cooperation, and the extent to which we can use telescopes around the world in conjunction to probe the cosmos for clues that would be otherwise invisible. We don't yet know what causes FRBs, or even if they are all caused by the same physical phenomena. But we do know that they originate from incredibly volatile events, billions of light-years away, in the distance and in the distant past.
What we will discover as telescope technology advances, and we refine our ability to use many telescopes across the planet as one, is anyone's guess.