An enigmatic, powerful burst of radio waves coming from far, far away is helping astronomers map the cosmos—while simultaneously adding to the mystery of these peculiar phenomena.
Known as fast radio bursts, fewer than two dozen of these brilliant flashes have been captured by telescopes on Earth. No two bursts are exactly the same, but for a few fractions of a second, they all blaze with seemingly intense amounts of energy. So far, these brief flashes of radio light remain mostly unexplained, with possible sources ranging from evaporating black holes to alien communication.
While aliens are an unlikely explanation (sorry), astronomers still don’t know what kind of astrophysical motor is churning away in the cosmos and sporadically erupting.
“We don’t understand what the population of objects is and how often any given object bursts, or how far away they really are,” says Jim Cordes of Cornell University.
In August 2015, the Parkes telescope in southern Australia recorded the brightest burst yet seen, dubbed FRB 150807. The burst lasted for a few thousandths of a second—a minuscule blip that astronomers were fortunate enough to observe in real time.
As the team reports on November 17 in Science, that burst was bright enough to point toward its cosmic home, as well as illuminate the tangled magnetic fields it traversed on its way to Earth.
Going the Distance
Until relatively recently, it wasn’t clear whether the ultrafast bursts were glitches made by telescopes on Earth, were products of something inside the Milky Way galaxy, or were coming from somewhere much, much farther afield.
Initial distance estimates suggested the weirdly energetic bursts were coming from billions of light-years away. But for a while, that seemed unlikely to some scientists, who thought they might be less energetic events occurring behind cloudy nebulas in our own galaxy.
What astronomers really needed to do was identify a burst’s home in the sky and see whether it was coming from somewhere in the Milky Way or a distant galaxy. That would help determine just how much energy was contained in each burst.
“Are they truly cosmological, or just nearby extragalactic?,” asks Caltech’s Vikram Ravi, a co-author of the paper in Science.
This new observation comes the closest yet to answering that question.
Because FRB 150807 was so spectacularly bright, it lit up two separate detectors in the Parkes telescope. Astronomers could use that information to identify a small region of the sky where the detectors overlapped. When Ravi and his colleagues looked closely at that area, they discovered a handful of galaxies—six, to be exact—and some bright stars.
The team reasoned that the burst likely originated from one of those galaxies. The nearest, brightest galaxy in that handful, which the team says is the best candidate, is a massive red galaxy called VHS7 that’s between 3.2 and 6.5 billion light-years away.
But Ravi and others aren’t quite ready to conclude that VHS7 is the burst’s true origin. For one thing, this analysis assumes the source is something visible in images from the VISTA survey, which the team used for their cosmic hunt. But there’s no reason fast radio bursts can’t come from small, dim galaxies that might be hard to see.
Instead, the team was able to place a lower limit on the burst’s distance, and that limit suggests it came from at least 1.5 billion light-years away.
In other words, FRB 150807 definitely didn’t explode inside the Milky Way.
"I think this is laid to rest for the class of objects. There may be one or two in the 18 published bursts that could still be in our galaxy, but the others could not," Cordes says.
Another lingering mystery is what could be generating such powerful bursts in the first place. There have been a few scarce clues such as recent, if not ironclad, evidence for the bursts being fueled by flares from rapidly rotating, fantastically magnetic dead stars.
Sorting this out means observing many more bursts and finding out what they’re associated with. The good news is that the brightness of FRB 150807 suggests finding more shouldn’t be too difficult.
Scientists estimate that fast radio bursts are going off thousands of times a day, kind of like cosmic popcorn. It’s just that they’re generally hard to catch in the act of bursting.
Two new telescopes may help solve that problem when they come online, perhaps as early as 2017. One, called CHIME, has a large field of view that should be able to snare many, many bursts in its net. Another at Caltech, the Deep Synoptic Array prototype, is a planned array of dishes that should be powerful enough to pinpoint the locations of the bursts it can see.
“If a substantial fraction of the population are really bright events like this one, then maybe we will be finding more [fast radio bursts] soon with large field-of-view, less sensitive radio telescopes,” says astronomer Emily Petroff of the Netherlands Institute for Radio Astronomy.
While the bursts’ true origins remain a mystery, scientists can use fast radio bursts in the meantime to try to map the turbulent, tangled web of intergalactic space. For the few milliseconds that they blaze, fast radio bursts can help astronomers determine the densities of intergalactic material and discern the strength of the magnetic fields they’ve traveled through.
For example, FRB 150807 carries information with it that suggests the magnetic fields between Earth and the burst’s home are more or less what astronomers expect.
“FRB 150807 tells us that models for the magnetization and turbulence of the intergalactic medium along this sightline are not wildly off,” Ravi says. “Our results are hence not surprising in this sense. However, it is surprising that we can say this at all.”