Seriously, What’s Making All These Mysterious Space Signals?
In astronomy, the study of fast radio bursts can sometimes feel like a game of Clue.
Astronomy can be, in some ways, a bit like the classic board game Clue. Scientists explore a sprawling but ultimately contained world, collecting pieces of information and testing out theories about a big mystery. You can’t cover every corner, but with the right combination of strategy and luck, you can gather enough clues to make a reasonable guess at the tidy answer—who, where, and how—enclosed in a little yellow envelope at the center of it all.
Only, instead of a fictional killer, astronomers are trying to track down the source of strange flashes of radio signals that reach Earth from the depths of space.
Scientists have discovered hundreds of such flashes, known as fast radio bursts (FRBs), over the past 15 years. The signals are intense and fleeting things. They come from all directions in the night sky and sneak up on our telescopes. Most are one-offs, never to be seen again. A few “repeating FRBs” have shown up more than once. Astronomers have gathered as much evidence as they can and have traced the approximate origins of FRBs in the enormous mansion that is our universe. Nearly all of them spring from distant galaxies, while just one so far arose from somewhere in our own Milky Way. But astronomers still haven’t figured out whodunit, or how; they don’t know for sure what kind of astrophysical objects produce these powerful emissions of radio waves.
But astronomers have found a new, tantalizing clue.
A team of researchers has detected a new FRB from a galaxy several billion light-years from Earth, and this one is weirder than all the rest. Most bursts last for just a few milliseconds, pulsing with such intensity that they shine as brightly as galaxies before vanishing. But this emission of radio waves lasted about 1,000 times longer: three whole seconds. And there was something unusual about the signal itself. Astronomers detected little pulses, peaking about every 0.2 seconds, within the three-second burst. Researchers had previously detected an FRB source that followed a discernible pattern, producing millisecond-long flickers for several days before quieting down and then starting back up again. But the flashes themselves were random. This was the first time that the signal itself exhibited such a precise rhythm.
“In FRB world, this is certainly big news,” Sarah Burke-Spolaor, an astronomer at West Virginia University who studies FRBs and was not involved in the new detection, told me. “The main question we are still after with FRB is: What is making them? A strict periodicity like this would be major.”
The existence of such a pattern supports the growing evidence that suggests the culprit behind FRBs is a neutron star, the leftover core of a once-giant star that has burned through its fuel. Professor Plum could be a pulsar, a type of neutron star that rotates fast and spits beams of radiation from its poles. Or Miss Scarlet could be a magnetar, another kind of neutron star, known for its powerful magnetic fields. “It is very difficult to contrive a natural clock like that, but pulsars are the only known emitting objects with enough momentum to behave that way,” Burke-Spolaor said.
The researchers behind the detection didn’t have enough to definitively pin the FRB on a pulsar, Shami Chatterjee, an astrophysicist at Cornell University and a co-author on the new research, told me. Nor do they have a good explanation for why this signal was so intense. Perhaps invisible gravitational forces magnified a pulsar’s emissions as they headed our way, making them appear brighter to radio telescopes. Or maybe a magnetar is undergoing a giant flare. The latest detection bears some similarities to the radio emissions of pulsars and magnetars found in our own Milky Way galaxy, but the weird new signal seems, well, weirder. “The whole thing is just very peculiar,” Chatterjee said.
Around this time, you might be thinking, Okay, so astronomers have their suspicions about what’s responsible for FRBs, but they haven’t solved the case. Add in the discovery of a surprisingly clear-cut pattern, and you might wonder: Could it be aliens? Sorry, no. “Periodic signals are very, very common from normal astronomical sources,” Sofia Sheikh, an astronomer at the SETI Institute who works on the search for signs of advanced technology beyond Earth, told me. Such sources include pulsars and magnetars. “If the source was pulsing out the digits of pi or the Fibonacci sequence or something, then it would be a SETI story,” Sheikh said.
If pulsars can indeed produce FRBs, astronomers can study these flashes to help them solve other cosmic mysteries. Scientists have already used the rhythms of less mysterious pulsars in the Milky Way as a kind of astrophysical clock, allowing them to do such various tasks as measure the mass of Jupiter, study the properties of the space between stars, and even discover an exoplanet made of diamond, Burke-Spolaor said. In the case of the diamond planet, which also began with an unusual signal, the clues quickly added up: When astronomers noticed some intriguing variation in the radio emissions of a pulsar 4,000 light-years away, they realized that the best explanation was the presence of a nearby planet. The planet, according to their analysis, was mostly made of carbon and oxygen, and dense enough to crystallize into a diamond world.
Astronomers hope they’ll stumble across more FRBs like this one in their search of our cosmic grounds. The Canadian telescope that detected this burst is constantly looking out for more, and future observatories may discover thousands of them every month. “Every step of the way with FRBs, every answer we have gotten comes with so many more questions,” Burke-Spolaor said. “This detection does the same.” Astronomers have so far looked only for FRBs that last a few milliseconds because they didn’t think the flashes could last much longer, and it’s possible that “we could be missing a heap of FRBs that are seconds long,” Vikram Ravi, an astronomer at Caltech who wasn’t involved in the new research but who studies FRBs, told me. The story of FRBs is a long game, and scientists now know to expect sudden twists. The secret envelope remains unopened, but astronomers still have plenty of cosmic rooms to search, and every turn promises to reveal a new clue.