Arecibo Observatory Detects Mysterious, Energetic Radio Burst

by Nadia Drake

NATIONAL GEOGRAPHIC 


A brief, blazing burst of radio waves detected by the Arecibo Observatorycould herald a turning of the tide for a peculiar class of cosmic signals. Until recently, the signals had only ever been detected by a telescope in Australia, a pattern that fueled doubts about their origin.

Fewer than a dozen of these bursts, lasting for only a few thousandths of a second, have ever been reported. Called “fast radio bursts,” the signals are cosmic enigmas that appear to come from the very, very distant universe. But since the first burst discovery in 2007, scientists have not only wondered what kind of cosmic object could produce such a tremendously bright, short-lived radio pulse – but have disagreed about whether the bursts are even celestial.

“There are more theories than there are bursts,” says West Virginia University astronomer Duncan Lorimer, an author on the paper describing the burst, posted to the arXiv on April 10.

On November 2, 2012, a blast of radio waves collided with the Arecibo Observatory in Puerto Rico, where the world’s largest single-dish radio telescope lives. Rain or shine, day or night, the 305-meter dish collects radio waves from the cosmos, which are then processed into data for scientists to study.

The data gathered at 6:35 am UT revealed a massive, 3-millisecond spike. Unlike the radio blasts emitted by some pulsars, the burst did not recur. It briefly blazed and then disappeared. Called FRB 121102, the burst was very similar to six earlier events that constitute the entire reported population of ultrafast radio bursts – a population that until November 2012 had only been seen by one telescope, in Australia.

But transience is only part of what makes these signals so weird. Their chief peculiarity lies in just how dang far away they seem to be.

Normally, radio waves travel at the speed of light. This means that all the different wavelengths and frequencies of radio waves emitted by the same object – say, a pulsar – should arrive on Earth in one big batch.

But if something is sufficiently far away, that changes. Longer, lower frequency waves traveling through the cosmos have a trickier time getting to Earth. Clouds of ionized interstellar particles – electrons, primarily – form roadblocks that slow and redirect these longer waves, causing them to follow a more sinuous path. As a result, the longer waves arrive just a bit later than their shorter kin – sometimes, the difference is only a fraction of a second.

That delay in arrival times is called “dispersion,” and it lets astronomers estimate how far away the waves are coming from. The longer the delay, the more intergalactic junk that got in the way. And since scientists think they know how much junk there is, they can use the dispersion measurement to approximate a distance, or at least identify whether an object lives inside or outside the Milky Way.

If astronomers are interpreting the bursts’ dispersion measures correctly, then the bursts came from billions and billions of light-years away – in other words, they’re nowhere near our cosmic neighborhood. And nobody knows what they are.

“The sources of the bursts are undoubtedly exotic by normal standards,” Cornell University astronomer Jim Cordes wrote in Science.

The ultrafast pulses take their name from Lorimer, who spotted and described the first burst in 2007. That mysterious signal, estimated to have traveled roughly 3 billion light-years before colliding with Earth, stunned astronomers. Many of them questioned whether it was an artifact produced by the telescope that detected it, the Parkes Observatory’s 64-meter telescope in Australia.

Site of the first burst found, in 2007. Lorimer et al., NRAO/AUI/NSF)

In the years after the discovery, skepticism grew. A new class of terrestrial radio bursts detected by the Parkes telescope in 2010 cast more doubt on the original Lorimer burst. Those Earth-based signals, called perytons, opened the door to the possibility that even if real, the original burst was actually coming from much closer to home.

Another Parkes-detected burst, reported in 2012, didn’t do much to alleviate doubts.

But that summer, a third Lorimer burst was described at the International Astronomical Union’s general assembly in Beijing, China; as it turned out, this burst would be one member of a quartet that astronomers would announce the next year in Science. By the end of July, 2013, the total reported stood at six.

“The discovery of fast radio bursts at the Parkes Observatory, if confirmed at other observatories, would be a monumental discovery, comparable to that of cosmological gamma-ray bursts and even pulsars,” Shrinivas Kulkarni, an astronomer at Caltech, told Scientific American at the time.

Strength in numbers was helping the bursts achieve legitimacy, but there was no escaping that they’d all been detected by the same telescope. And until another observatory saw something similar, skeptics could easily question whether the signals were a product of the telescope and its location, rather than the cosmos.

“In fairness, it’s not a bad question to ask at all,” Lorimer says. “Whenever you make a new discovery, it’s very important to have it confirmed by different groups, using different equipment.”

Now, the Arecibo detection of FRB 121102 strongly suggests the signals are not a Parkes artifact, and furthermore, that they’re not terrestrial in origin.

“I’m certainly very excited to see such a convincing result from another team using a different observatory,” says astronomer Michael Keith of the University of Manchester, who was not involved in the current study.

So the questions astronomers are asking are: How far have the bursts traveled? And what, exactly, are they?

“My hunch has always been that they’re extragalactic,” Lorimer says. “But that’s really nothing more than a hypothesis at this point.”

Overall, the dispersion measures do seem to suggest an extragalactic origin. There are many more electrons between Earth and the bursts than can be explained by the Milky Way’s interstellar electrons; but it’s still possible that intervening nebulas could be clouding the measurement, Kulkarni says. He suggests the signals could be coming from spinning neutron stars known as radio rotating transients, or RRATs, that live in our galaxy and also emit a single pulse.

Because the signals are so brief and bright, they must be coming from a rather dense source, says astronomer Scott Ransom of the National Radio Astronomy Observatory. “That means a compact object – i.e., a neutron star or a black hole – is likely somehow to blame,” he says.

Just what that compact object is has yet to be explained. One theory suggests that giant flares erupting from highly magnetic neutron stars, known as magnetars, cause the bursts. Others suggest the bursts result from colliding neutron stars or black holes, evaporating primordial black holes, large magnetic stars, or are the death spasms produced when massive, slowly spinning neutron stars collapse into black holes. That last object, proposed in 2013, is known as a blitzar.

More observations should help teams pinpoint the bursts’ origin. Already, more detections from Parkes are coming down the pipeline, and Ransom says he’s looking through the Green Bank Telescope’s data for similar signals. But what astronomers are really hoping for is a way to find the bursts in real-time – then, they might be able to identify an optical source, like a host galaxy. In addition to supporting an extragalactic origin, that would also allow scientists to use the bursts to probe the characteristics of the intervening intergalactic medium and its ions.

“We really need to get their precise positions,” Ransom says. “That will let us see where they originate – hopefully in or near other galaxies where we can get their distances.”