A radio burst with the energy of a hundred million Suns has finally been placed on the cosmic map, enabling scientists to investigate the origin of these mysterious bursts.
|Update (March 4, 2016): Since this paper's publication, a number of counter-arguments have been proposed that suggest the radio burst is not actually associated with the radio glow spotted in an elliptical galaxy. These arguments include a paper that suggests the afterglow is actually due to a gobbling supermassive black hole known as an active galactic nucleus. In addition, the likelihood of a chance association between the fast radio burst and the galaxy put forward in the initial Nature paper may have been too low. A flurry of proposals have been submitted to perform further analysis.|
Thousands of times a day, a burst of radio waves hundreds of millions of times more energetic than the Sun, washes over Earth. In less than a second the burst disappears and the radio sky returns to an eerie quiet. Astronomers have struggled to explain the origin of these so-called fast radio bursts (FRBs) for nearly a decade — the mysterious events are difficult to pinpoint, and they often elude detection altogether. But, the bursts provide tantalizing hints that they might represent something genuinely new to science.
Now, for the first time, an FRB has been spotted at a shorter radio wavelength, enabling astronomers to pin down its exact location in the universe and speculate on its potentially exotic origins. The results published February 25th in Nature not only shed light on the burst’s mysterious origin, they also open a new window on the cosmos.
On the Hunt
The first of these ultrafast, ultrabright pulses was discovered in 2007 when Duncan Lorimer (West Virginia University) was parsing archival data from the 64-meter Parkes Radio Telescope in Australia. A 5-millisecond burst on August 24, 2001, was so bright that it caught his eye.
There was something unexpected in the dispersion of the burst’s wavelengths. The event’s short-wavelength components arrived at the telescope a fraction of a second before its long-wavelength components, an effect that can only be caused if the light had traveled through some sort of medium such as ionized gas. The longer the delay, the higher the number of electrons between us and the source — and generally, the greater its distance.
The Lorimer burst, as it is now called, must have traveled through a few billion light-years of intergalactic space. It would have been very short, tremendously bright, and potentially cataclysmic (the source, whatever it was, didn’t burst again). But what could have sparked such an extreme event?
Lorimer suspected that it might represent an entirely new, previously undetected astronomical source, one that astronomers could glimpse again. But the radio sky above the Parkes dish remained frustratingly quiet. Astronomers started to find handfuls of FRBs buried deep within archival data, but arguments ensued over whether these bursts were actually extragalactic. Perhaps our models of how ionized gas is distributed within the Milky Way were incorrect. Or maybe the bursts originated in flare-prone stars that were enveloped in so much plasma that it, too, could cause the same delay signature.
To be sure, astronomers needed to pinpoint fast radio bursts on the sky. And in order to do that, they needed to catch one in real time.
Pinpointing a Fast Radio Burst
So, with the help of a little funding, the team now sends all of the data from Parkes to a supercomputer at Swinburne University in Melbourne, Australia. Within 30 seconds of the initial flash, the supercomputer pings a series of other telescopes, alerting them to the burst’s location. This enables observers across the world to slew their telescopes toward the FRB’s location with the hope of catching a burst’s afterglow.
Several follow-up searches came up empty. Then on April 18, 2015, “it all just clicked into place,” says Evan Keane (Square Kilometer Array Organization and Swinburne University of Technology). The Swinburne supercomputer had caught an FRB red-handed. Within hours, observers at the Australia Telescope Compact Array — a collection of 22-meter dishes — searched the sky where Parkes had spotted the burst. It caught a slowly fading radio signal, an afterglow surely associated with the FRB.
But it still wasn’t clear where both signals had originated. Parkes could narrow down the burst to within 15 arcminutes, about half the angular size of the full Moon, but hundreds of galaxies easily fit within such a region. The array further narrowed the afterglow down to about 1 arcsecond, a location 900 times more precise. Still, the team eventually relied on the 8.2-meter Subaru telescope in Hawaii to match the afterglow’s location.
The FRB had burst forth from an elliptical galaxy roughly 6 billion light-years away.
The galaxy is old, well past its prime period for star formation — a useful hint as to the origin of this FRB. Although there are more theories on FRBs than discoveries, the most compelling theories can be lumped into two broad categories: relatively rare explosive collisions between aging stars and periodic outbursts of younger stars.
The fact that the galaxy is older suggests that — at least this time — an explosive collision produced the FRB. Both Keane and Lorimer place their money on the inspiral of two neutron stars. (If you’ve been following your physics news this month you might know that this would also produce a gravitational wave signal, which is true, but at 6 billion light-years it would be too faint to see with LIGO).
A New Window on the Cosmos
Although this burst doesn’t represent a previously unknown astrophysical source, as Lorimer had initially hoped, he is excited about how fast radio bursts might be able to probe the cosmos.
“As these radio signals traverse the cosmos, they're imprinted with the characteristics of the universe,” says Keane. Those smeared-out radio waves tell astronomers how much material the burst has traveled through. Now, with the host galaxy’s corresponding distance, astronomers can compute the density of electrons in the intergalactic medium and better map the distribution of matter throughout the universe.
When Keane and his colleagues did just that for this single discovery, they found that the result matched current cosmological models perfectly. “Our grasp on the reality of the universe just got a bit better,” says Keane. And with more FRBs pinpointed in the years to come, astronomers will be able to nail down more lines of sight, until they create a 3D model of the cosmos.
Lorimer, who has been studying FRBs the longest, thinks the field is really poised to take off now as a standalone research field. (Before it piggybacked on pulsar research.) “The mist is gradually clearing,” he says. “We still have a long way to go — we're not out of the woods yet — but these are very exciting times.”
E. F. Keane et al. “The Host Galaxy of a Fast Radio Burst.” Nature. February 25, 2016.