For the past few years, radio astronomers have been haunted by a phantom called "fast radio bursts." Radio observatories detect a fast, high energy signal coming from seemingly out of nowhere, then just as quickly disappearing before they've had any chance to figure out where they came from or what caused them. The radio signal is spread out enough to indicate it came from a far off galaxy, but beyond that, not much is known.
But finally, as relayed today in , radio astronomers at the NRAO Green Bank Observatory have taken one very small step toward it. It all happened by accident.
Most of the fast radio bursts fleetingly discovered so far have happened during pulsar surveys. Pulsars are the collapsed remnants of stars that let out radio waves or pulses at regular intervals, and are recorded in precisions like 100 microseconds. But a recent sweep for neutral hydrogen in other galaxies yielded a fast radio burst.
It was only caught at resolution of 1 millisecond, usually not enough to discern anything about the signal. Except, in this case, the survey was searching for neutral hydrogen, meaning it had to be attuned to both negative and positive particles as well. This meant that, for the first time, the Green Bank team caught richer data on polarization of the signal.
"It tells us about the transmission mechanism, the source of the burst," Maura McLaughlin, a professor at the University of West Virginia, pulsar expert, and paper co-author, said in a phone interview. "We know that pulsars are linearly polarized, so maybe the source is a pulsar."
Let's unpack linear polarization for a second. The waves move straight forward, along a single plane. You can see a little bit about how those light waves move in this video:
The movement of the light in linear polarization rules out main sequence stars, which emit light in more circular patterns. The "dispersive region," the size of the material emitted, also seemed inconsistent with a normal star. The astronomers believe the size may be as much as 10 AU, an expanse roughly the distance of the Sun to Saturn, the type of materials that aren't in the region of most stars. The burst appears to move through a dense cloud of ionized material before exiting whatever generated it.
Something like a magnetar may be at play, another type of neutron star and cousin to the pulsar that emits intense magnetic fields. But there are a couple scenarios that are what McLaughlin calls the "cataclysmic scenarios" that could also be at work here. The first is the collision between two neutron stars, and the second is a black hole evaporating.
Specialized software helped extract the fast radio burst from more than 40 terabytes of survey data, and gave the sliver more of information astronomers now have. But the mystery of the fast radio burst still isn't solved. In this case, the astronomers aren't even sure where it came from. Green Bank's telescope is one of the largest radio telescopes in the world, but even then, it wasn't precise enough to sort out the point of origin. That's because it came from a region where numerous galaxies reside.
"We really don't know exactly how far away it is or the direction very well," McLaughlin says. "There's so many galaxies in the region that we just can't really associate it well."
But with a little information and a few more tools, radio astronomers can dig deeper into the mystery. The culprit may be a rare cosmic event, or it may be the quirk of a pulsar in a particular kind of nebula. Or neutron stars colliding. Or some other giant, energy producing event that lets out linearly polarized light. Future surveys may even yield a fast radio burst closer to home, and one that hasn't spread out from a journey that takes millions of light years.
"I'm kind of guessing this happens in our galaxy, but it's rare," McLaughlin says.