After a decade of bewilderment, astronomers have pinpointed the source of a mysterious blast of radio waves coming from deep outside the Milky Way: a dwarf galaxy located 3 billion light years from Earth. It’s a remarkable first in the study of what has been a tremendous astronomical puzzle. Scientists still don’t know what causes these deep space pulses, but locating the galaxy that spawned one brings us closer to figuring out where they come from.

First discovered in 2007, only 18 of these phenomena have ever been detected. They’re called fast radio bursts, or FRBs, because they occur for just milliseconds; their fleeting nature makes it tough to catch one in action, and even tougher to figure out the exact spot in the sky they’re coming from.

A remarkable first in the study of what has been a tremendous astronomical puzzle

But astronomers got lucky when they found a particular burst known as FRB 121102: it is the only one known to repeat, meaning multiple radio bursts have been detected coming from the same location in the sky. That makes it easier for scientists to catch again, Shami Chatterjee, an astronomer at Cornell University who discovered the repetition, tells The Verge. That discovery gave Chatterjee the idea to continually observe FRB 121102 with a huge network of radio telescopes. And sure enough, he and his team were able to get high-resolution images of multiple bursts after many hours of observation, allowing them to track down the source of FRB 121102. Their work is detailed today in three studies published in Nature and The Astrophysical Journal Letters.

Now that researchers know the cosmic neighborhood generating the FRBs, they can study the galaxy more closely — which may help figure out the origins of these fast pulses. There may be other uses for FRBs, too. Since FRBs originate so far away, they have to pass through a lot of interstellar junk, such as gas and plasma, to reach Earth. Once scientists know which exact galaxy the signal comes from, the radio waves could help scientists determine just how much gas and plasma they had to pass through to get here. “The analogy I use is that until now, we could potentially know the country it came from,” Heino Falcke, a radio astronomer at the Radboud University Nijmegen who wrote an accompanying editorial about the discovery, tells The Verge. “Now we know the home address.”

The mystery of fast radio bursts

When FRBs were first discovered, there was debate over whether or not these signals were actually coming from space at all. Astronomers wondered if they were just bizarre interference of some kind. But after a closer look, researchers realized FRBs are unique. Typically, a burst of radio waves will have different wave frequencies occurring at once, but FRBs have frequencies that are spread out. The highest frequencies of each FRB arrive slightly earlier at Earth while the lowest frequencies arrive slightly later. It’s a sign that the these FRBs are weary travelers, having journeyed through a lot of interstellar gas and plasma that’s mucking up their signals.

And FRB signals are so mucked up that astronomers are convinced they’re coming from outside the Milky Way Galaxy. But that creates another problem: these bursts must come from a super bright source. “Like absolutely, incredibly bright,” says Chatterjee. Experts have come up with dozens of theories, such as the cataclysmic collision of neutron stars or a black hole tearing itself apart. But no one has agreed on a single explanation.

Then the discovery of FRB 121102 changed everything. Because of its repeating nature, astronomers know that its source can’t be anything explosive or an object being destroyed. “Something like that could not repeat again at the same place at the same distance,” says Chatterjee. “So that basically put the end to a huge swath of models.” Maybe more than one thing is capable of creating FRBs — and that’s why there hasn’t been a single explanation. But the only way to know for sure was to find the host galaxy.

Zeroing in on the home address

To pinpoint the source, Chatterjee and his team turned to the Karl G. Jansky Very Large Array (VLA). It’s a collection of 27 radio telescopes in New Mexico, spread out in a Y shape. Until now, FRBs have only ever been detected with large, single dish radio telescopes that observe a wide patch of sky at at time. These types of telescopes are more sensitive to picking up FRBs, but they aren’t as adept at location. The VLA, however, acts like a telescope that is miles wide, allowing astronomers to get the high-resolution images they needed of FRB 121102’s home address.

At the spot they found, the researchers discovered two enticing details. There was indeed a persistent radio source at the location — possibly the origin of the repeating FRB. And there was a faint smudge of light that turned out to be a tiny galaxy. The researchers were then able to measure how fast the galaxy was moving away from Earth, using its spectrum of light. By figuring out the movement, researchers were able to place the galaxy 3 billion light years away.

“That’s extraordinary, because that immediately tells you how much energy there must be when the pulse is emitted for it to travel for 3 billion years spreading out through the Universe and to get to us and be detectable,” says Chatterjee.

Unsolved mysteries

Chatterjee’s team still isn’t certain as to what’s causing the repetitive radio waves, though they have a few potential explanations. One idea is that the black hole at the center of the galaxy is active, spewing out jets of particles at light speed. Every now and then, a blob of plasma may vaporize one of the jets, creating a bright flash. “We don’t really like this model very much, but it’s possible,” says Chatterjee.

Another possibility is that the FRB is coming from a type of dense neutron star with an incredibly strong magnetic field, called a magnetar. Astronomers have discovered magnetars in our galaxy that produce bright radio pulses, but nothing as bright as FRB 121101. So something would have to be amplifying the pulses, like the way a magnifying glass focuses a beam of light on ants. That may mean blobs of plasma are lining up just right to focus the radio waves on Earth, making them extra bright, says Chatterjee. “This is very plausible,” he says. “We’re not invoking any radical new physics.”

The next priority is to find another fast radio burst that repeats

FRB 121101 is just one sample, though, and scientists like multiple samples. So, the next priority is to find another FRB that repeats, says Chatterjee. Astronomers want to know if all FRBs repeat — a question that hasn’t been fully answered yet. It’s possible that every FRB has multiple bursts, and we simply haven’t detected them. Or perhaps there’s a whole spectrum of FRBs. “We know from gamma rays that there are multiple classes of them; the same could be true of fast radio bursts,” Duncan Lorimer, an astrophysicist at West Virginia University who discovered the first FRB, tells The Verge. “That’s in the back of people’s minds, that this could be one end of the population.”

Finding another repeating FRB would make it easier to pinpoint another host galaxy. And once there is a big enough population of FRB sources, then scientists can really start to use them to measure the matter that’s between galaxies. Astronomers were able to probe intergalactic dust last year with a newly discovered FRB, but since they didn’t know the signal’s exact origin, their measurements aren’t ironclad. That’s not the case with today’s discovery, so it may still yield important data on the gas and plasma the signal ran through. “There’s no hand waving about this,” says Chatterjee. “We know it’s coming from this dwarf galaxy.”

The making of NASA’s most powerful space telescope