Billions of years ago, an unknown object sent a seriously bright burst of radio waves into space. They traveled across the universe, past galaxies and clouds of gas and who knows what else. And in 2012, the burst arrived at the Arecibo radio telescope when astronomers happened to be watching.
They kept searching that same spot in the sky. In 2015, they found 16 additional flashes. Then, in August and September 2016, nine more appeared. And this week, astronomers announced that these newest measurements helped them finally zero in on the bursts’ home: a dim dwarf galaxy three billion light-years away. Something inside this tiny galaxy was sending pulses that lasted just milliseconds but packed enormous energy, members of a still-mysterious class called “fast radio bursts.”
Learning that these particular bursts came from long ago, from some bursting object in a galaxy far, far (far) away, is an important step for this field of research. But it’s also like playing Clue and concluding that the crime was committed in the conservatory. To solve the crime, you still have to determine whether the dastardly deed was done by Mrs. Peacock with the candlestick or Colonel Mustard with the rope.
That continuing conundrum shows off the way science works, in a way we don't usually get to see. Astronomers don’t often happen upon a total mystery. Much of their work involves looking directly at objects they know exist---stars, planets, supernovae---and studying processes and properties. Fast radio bursts, though, appeared out of nowhere, unexpected and un-asked-for, coming from question-mark objects with question-mark properties because of question-mark processes. Astronomers now have the privilege of figuring out the what, where, why, and how---from total scratch---and we have the privilege of watching the discovery process take place from its start.
To forecast what will likely happen next in the ongoing case of the super-energetic fast radio bursts, history helps. Specifically, the twentieth-century discoveries of pulsars and gamma-ray bursts, which also began with on-and-off flashes from unknown entities.
The very first on-off from a fast radio burst came in 2007, when astronomer Duncan Lorimer was sifting through archived data, searching for undiscovered pulsars. But instead, he found something that flashed just once, brighter than a pulsar and seemingly much farther away. He didn’t know what he was looking at. Neither did anyone else.
This is a familiar story arc in astronomy. It’s really the best way to find something utterly new: by accident, while searching for something known. It happened to Jocelyn Bell, who was looking for the twinkling of quasars---the superbright cores of galaxies with supermassive black holes in feeding mode---when she stumbled upon a repeating radio blip. It blipped too fast to be any regular star. Was it aliens? Human technology? A planet? A mistake? It wasn’t until she found another blipper that she felt confident it was part of the natural universe at all. Then, when she and her advisor found two more, the blippers became a Thing. After they went public, people proposed more explanations, including the correct one---pulsars, the fast-spinning neutron stars left over after supernova explosions.
Gamma-ray bursts, too, are in encyclopedias because of an accident. In the 1960s, US government satellites were hanging out, watching for the high-energy indications of Soviet nuclear tests. They picked up 16 weird bursts of gamma rays that didn’t match up with nukes’ characteristics. In 1973, the government declassified the discovery and declared that the bursts must have come from space.
But after Lorimer saw his first burst, he didn’t get more of the same from the sky, as Bell and the Soviet-watchers did. No one saw any more fast radio bursts, from anywhere in the sky, for years. People doubted the astronomical origin of the original specimen, suggesting it came from Earth---and, indeed, astronomers in Australia accidentally produced a set of similar radio bursts by opening their microwave door before cooking was complete. There wasn't even a *category *for that kind of behavior.
Since then, astronomers have found 18 sources of fast radio bursts---including the only one that repeats, the one first spotted in 2012. Shami Chatterjee, the lead in this latest discovery, decided to focus efforts there. “This is a good place to go fishing because you’re more likely to see a fast radio burst at this spot,” says Chatterjee. The team began watching the area with the Very Large Array in late 2015, searching for the burster's precise location in space.
They watched for another burst for dozens of hours, in observations in November 2015 and April and May 2016, and saw nothing. “The field of transients is special in that we need to wait for the universe to provide an event for us,” says Casey Law of the University of California, Berkeley, who led the project’s software and data-taking developments. Finally, a burst appeared, in a set of observations that began in August. Then, so did eight more. That dataset allowed the team to pinpoint where the signals came from, a position they later zoomed in on even more precisely with radio telescopes around the world. And once they got images of that same spot from an optical telescope called Gemini North, they saw a faint smudge of shine, more like something you’d try to wipe off your screen than the answer to a big astronomical question. But that smudge was actually a tiny galaxy, around 3 billion light-years distant. Somewhere inside, the astronomers knew lurked the burster.
Like with pulsar signals and gamma-ray bursts, finding more instances of fast radio bursts, and perhaps repeat offenders, will allow scientists to learn about them as a population---even before they know what that population is. They can see what common characteristics the members have, characteristics that speak to their physical fundamentals. Pulsars, for instance, all have really stable spins because they are so spherical, dense, and full of angular momentum. They can watch how often the signals occur (or recur), which speaks to how common their originators are in the universe, and how they are spread across the sky.
That latter observation was initial convincing evidence, 18 years after the declaration of gamma-ray bursts’ existence, that they came from outside our galaxy. Astronomers had debated whether gamma-ray bursts were only kind of bright and nearby, or superbright and far away---as they also debated, until this week, for fast radio bursts. The Compton Gamma Ray Observatory, the first to do a true survey for such bursts, showed they came equally from all over the sky, not clustered around the Milky Way. And then, six years later, astronomers caught a gamma-ray burst in the act, nailed down its location, and calculated its distance from Earth (hint: not near our neighborhood). So by the gamma-ray metric, radio burst researchers are way ahead of schedule, having taken just 10 years to discover the same.
Cosmic mysteries do take a long time to explain. Astronomers still haven’t deciphered, for example, the details of why pulsars emit radio waves the way they do. And they don't yet know what inside that distant dwarf galaxy causes these repeating fast radio bursts, or what makes the ones that simply clap on and clap off, or if their origins are the same. Chatterjee says that inside the dwarf galaxy, a weird dude called a magnetar could be sending out giant pulses that interact with cosmic plasma. Or maybe an active black hole at the center of that little galaxy is vaporizing blobs of plasma. But right now, the number of plausible ideas about The Causes of fast radio bursts exceeds the number of bursters.
When scientists stumble upon something---a flash in the darkness that lights up the space between us and Whatever sent it---it’s going to be a while before they can enlighten the rest of us on what that Whatever is.
So while this latest burst announcement is not The Answer, it’s a step up, and sometime soon, another result will stand on its shoulders. Law likes that. “Science produces a community good in that each result contributes to the public understanding of the world,” he says. “Each result builds on those before, so when I publish a paper or my code, I feel like I get to participate in a great scientific story of human discovery.”
After all, mystery novels don’t say whodunit on page two. They present evidence piled on evidence scattered over twists and turns, letting the reader imagine several plausible scenarios, and then, in the final chapter, they reveal what’s really been going on all this time.
