Something has breathed new life into a faraway cosmic mystery machine and caused it to repeatedly hurl tremendous amounts of energy into the void.
It’s not clear exactly what that object is, but scientists refer to the observable phenomenon as a fast radio burst: a fleeting but extremely powerful blast of radio waves. In this case, astronomers caught a rapid stream of radio bursts coming from a galaxy about three billion light-years away.
Scientists with the Breakthrough Listen project made the discovery because, fortunately, they had a pretty good idea where to look. The team had tuned the giant Green Bank Telescope in West Virginia to a spot on the sky where a fast radio burst known as FRB 121102 had previously been singing to the stars.
Of the roughly two dozen bursts discovered so far, FRB 121102 is the only one known to play on repeat. Because of that, it’s the only burst with a known home galaxy, which was identified in late 2016 after scientists used several telescopes to pinpoint its origin.
After that discovery, FRB 121102 fell silent for a good stretch of 2017. Though it had previously offered periods of quiescence and activity before, its silence was alarming.
“We feared that our opportunity to study it has passed,” says UC Berkeley’s Casey Law. “This new detection suggests that FRB 121102 is coming back into an active state and will be easier to study how and what is producing these powerful bursts.”
First identified in 2007, fast radio bursts are still fundamentally enigmatic. Scientists don’t know what powers them, but they do know the blasts come from very, very far away and occasionally from sites with fantastically strong magnetic fields.
Designed to search the skies for signs of intelligent radio transmissions, the Breakthrough Listen team took aim at FRB 121102 to help them test equipment for that search. They also wanted to understand, among other things, how narrow, pulsed signals are modified as they travel through interstellar plasmas and other intergalactic junk.
A composite image of the Messier 81 (M81) galaxy shows what astronomers call a "grand design" spiral galaxy, where each of its arms curls all the way down into its center. Located about 12 million light-years away in the Ursa Major constellation, M81 is among the brightest of the galaxies visible by telescope from Earth.
After all, those types of signals are among the main emissions the team expects from extraterrestrial civilizations, says Andrew Siemion, director of the Berkeley SETI Research Center.
For a while, though, FRB 121102 seemed to have run out of steam. Then, when the Breakthrough team took a look on August 26, they saw 15 bursts within an hour of observing—the most FRB 121102 had produced since this time last year. They also caught the bursts coming in at the highest frequencies yet seen, which could suggest a large amount of variability in its emission.
No one knows why FRB 121102 has turned on again, or if this new round of bursts is a clue about the astrophysical engine driving the phenomenon. Some scientists suspect it doesn’t have much to do with what powers the object and instead depends on the various cosmic lenses that periodically magnify its songs.
With FRB 121102, “we’ve seen ‘burst storms’ before, including last August through September when the Very Large Array came up empty, empty, empty, and then boom, there it was at every session,” says Cornell University’s Shami Chatterjee.
“The month coincidence has also raised eyebrows,” he says. ”Are we more likely to see bursts during particular seasons, when the lensing geometry lines up just so? This is as yet wild, wild speculation—people apologizing for bringing it up—but who knows.”
Even wilder speculation swirls around the source of fast radio bursts, which refuses to reveal itself. Early theories include primordial black holes, colliding neutron stars, and even some idle talk about extraterrestrial civilizations, though aliens are not an explanation that’s generally taken seriously.
“We agree with the majority of the astronomical community that the likelihood of ET being involved in this source is low, but the source is nevertheless very mysterious,” Siemion says. “We know, without a doubt, that the universe is capable of giving rise to technologically capable life. We would be remiss as scientists to exclude this possibility a priori.”
Now, more serious investigations are focused on magnetars, the extremely magnetic, fantastically dense corpses of exploded stars. These rotating neutron stars launch humongous flares into the cosmos, but whether those flares are somehow linked to the barrage of radio waves is still a mystery.
It’s also possible that FRB 121102 is so active it could have exhausted the total magnetic energy in a magnetar, Law says. “That is a stupendous amount of energy being emitted.”