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Cosmic firecrackers: The mystery of fast radio bursts

These bright but fleeting explosions unleash more energy than the Sun produces in a day — and yet we don't even know what causes them.
RELATED TOPICS: ASTROPHYSICS
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A powerful outburst builds along the surface of a magnetar. The atmosphere and crust of these objects are coupled magnetically. Rapid release of energy can fracture the crust, leading to a starquake and potentially producing a burst of radio energy: a fast radio burst.
Don Dixon for Astronomy
Duncan Lorimer says he will never forget the day in 2007 when he stumbled upon the first bolt from the blue. The West Virginia University astronomer had tasked an undergraduate student, David Narkovic, with combing through pulsar survey data from Parkes Observatory in Australia, and one day Narkovic walked into Lorimer’s office with an unusual signal unlike anything anyone had seen or predicted before. It was one of the brightest astronomical sources in the sky for a scant few milliseconds, and it bore signatures of an origin beyond the galaxy. “I was stunned,” recalls Lorimer. “Frankly, I didn’t know what to make of it.”

Lorimer had discovered the first fast radio burst, or FRB. He published his find later that year. At first, no one else in the astronomical community knew what to make of it, either. Surely such a signal was some sort of man-made interference, many argued, or some phenomenon like lightning. Researchers even produced evidence of man-made signals that looked similar to Lorimer’s FRB but were in fact created by a microwave oven at Parkes. “Even my own wife [radio astronomer Maura McLaughlin] was on a paper arguing the first burst wasn’t real,” recalls Lorimer.

 But as the years passed, other astronomers discovered FRBs, first at Parkes and then using radio telescopes around the world. Evidence that FRBs were, in fact, from deep space began to mount. And scientific skepticism grew into excitement upon realizing that FRBs were very real, and perhaps one of the greatest new discoveries in astronomy in decades.

It’s been 10 years since the first FRB discovery. It’s generated so much buzz that in 2017, a few dozen astronomers held the first conference on FRBs, and millions of dollars in funding have been devoted to finding more. But as more bursts come in, the mystery has only deepened. To travel the distance between galaxies, FRBs must have an insane amount of energy — in the few brief milliseconds it shines, an FRB can generate more energy than the Sun does in a day. And yet despite the tremendous energy, no one has a clue about where they come from.

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The Parkes Observatory in New South Wales, Australia, is home to the 210-foot (64 meters) Parkes radio telescope. This telescope not only discovered the first recorded fast radio burst, FRB 010724, but also most of the currently known pool of 30-odd FRBs as well.
CSIRO/David McClenaghan

What we know

So far, we’ve seen 33 bursts, and they last a few milliseconds at most. We know they are, for that brief period, one of the brightest radio sources in our sky, and they have been identified in all areas of the sky, rather than originating in a single direction. But astronomers are cautious about drawing many conclusions from such a small sample as we have now.

We know FRBs come from beyond the galaxy. This distance information is based on a radio astronomy trick relying on the fact that space is not a perfect vacuum — while it is better than any vacuum on Earth, even the void between galaxies has a few hydrogen atoms per cubic meter. Radio waves traveling through space from a single source will interact with those few atoms’ electrons they pass, causing a slight delay in the waves depending on their frequency. Measuring this exact delay, known as the dispersion measure (DM), can tell you how much material the signal has passed through. A higher DM means a signal has traveled a greater distance, and the FRB DMs are decidedly extragalactic.

But things get weirder from there. So far, only one FRB repeats, despite many hours of follow-up observations. This exception is FRB 121102, the “repeater,” as astronomers colloquially call it. Arecibo Observatory in Puerto Rico first detected it November 2, 2012 (hence its name). Since then, astronomers have observed periods of calm where nothing is seen for months at a time from this source, and periods of outburst where it gives off over two dozen bursts in two hours, with no distinguishable pattern. No one knows whether the repeater is a type of FRB different from the others, or if all FRBs repeat and the Arecibo Observatory’s 1,000-foot (305 meters) radio dish is the only telescope sensitive enough to easily find repeating bursts.

The repeater has been crucial in providing the first clues on where FRBs come from. Radio waves differ greatly from visible light. The wavelength of light varies from 400 nanometers (violet) to 700 nm (red); a nanometer is one-billionth of a meter, or 40-billionths of an inch. But radio waves can vary from a millimeter to hundreds of meters in length.

This has important applications for the telescopes astronomers use because the angular resolution of a telescope on the sky — that is, the level of detail the telescope can see — depends on the wavelength of light observed and the diameter of the telescope. Point a 1-meter optical telescope on the sky, and its diameter is 2 million times bigger than the light waves it observes, and its resolution is 0.3" or 1/12,000°. On the other hand, a radio telescope with a 210-foot (64 meters) dish — the diameter of Parkes Observatory, which has discovered the majority of FRBs — will yield a resolution of 900" (¼°), about half the diameter of the Full Moon in the sky. That area may sound small, but it’s enough to hold hundreds or even thousands of distant sources. Because of this, a definitive identification of an FRB’s origin with a single-dish telescope like Parkes or Arecibo is impossible.

Luckily, astronomers have a few tricks to get around the resolution problem. The first is to link multiple radio telescopes together in a technique known as interferometry. By combining simultaneous observations from multiple telescopes, astronomers effectively create a radio telescope with a diameter equal to the distance between dishes.

Interferometry can be carried out using telescopes thousands of miles apart, giving resolutions down to 10-millionths of a degree. You wouldn’t want to do a survey for new objects with such a small field of view on the sky — an FRB could be going off less than a Moon-width away, and you’d never know — but it is a wonderful technique for pinpointing a single signal like the FRB repeater.

Using such an interferometer — the Karl G. Jansky Very Large Array (VLA), consisting of twenty-seven 28-foot (25 m) dishes in New Mexico — astronomers in fall 2016 detected several outbursts from the repeating FRB 121102. This allowed them to narrow down its area of origin and make detailed radio images of the region with other telescopes — as well as image it in optical light using the Gemini North Telescope in Hawaii — to identify the source of the repeater.

What the team found was a surprise. The repeater appears to originate from a dwarf galaxy 3 billion light-years away, which would appear completely uninteresting if something weren’t repeatedly throwing out insane amounts of radio energy. The galaxy is about the size of the Small Magellanic Cloud, a satellite galaxy of the Milky Way with about 1 percent the mass of our own. Astronomers used the Hubble Space Telescope and the Spitzer Space Telescope for further follow-up, and it appears the bursts originate from a star-forming region on the outskirts of its host galaxy. No one knows the source more specifically than that, but the bursts keep coming from that location — over 150 of them at last count.
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FRB 121102 originates from a tiny dwarf galaxy 3 billion light-years distant in the constellation Auriga. This image, taken in optical light with the Gemini North Telescope, doesn’t reveal much about the host, but radio observations have led some to believe the FRB may be associated with a supermassive black hole or a young neutron star.
Gemini Observatory/AURA/NRC/NSF/NRAO

The mystery remains

There’s a lot we don’t know about FRBs. Most glaringly, we don’t know what causes them. “There are more theories than bursts,” observes Lorimer, who organized the first FRB conference where several dozen theories were put forth.

One popular theory for the repeater, at least, suggests the bursts originate from a magnetar, a neutron star dominated by an extremely strong magnetic field. A magnetar’s field can be so powerful that even at more than 600 miles (1,000 kilometers) away, it alone would kill you by compressing the electron clouds in your atoms. They are also known to give off enormous bursts of high-energy radiation. In 2004, a magnetar called SGR 1806–20 experienced a starquake, or tiny shift in its crust, that shook its magnetic field so violently that the event would have registered as a 23 on the Richter scale. (By comparison, the 2004 Boxing Day tsunami in Indonesia was triggered by an earthquake registering 9.1 on this scale.) Even though SGR 1806–20 is 50,000 light-years away, the tremor caused a 0.2-second flare brighter than the Full Moon. It knocked research and communication satellites briefly offline and temporarily altered the shape of Earth’s upper atmosphere. The burst had enough energy in it to power the Sun for 150,000 years and could have caused billions of dollars in damage had the magnetar been closer.
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Magnetars are young neutron stars whose intense magnetic fields are a quadrillion times stronger than Earth’s and a thousand times stronger than the average neutron star’s. They are believed to periodically give off optical light and gamma rays as sources called soft gamma repeaters
ESO/L.Calçada
Some astronomers argue a similar magnetar flare in a far-off galaxy could also create FRBs, but the magnetar theory has its own problems, one of which originates from the SGR 1806–20 event. Parkes Observatory happened to be observing at the time of that flare — the opposite part of the sky, granted, but an FRB created within our own galaxy would be bright enough to swamp the telescope regardless of where it was pointing. Nothing unusual was recorded.

Another popular theory argues that FRBs could come from young neutron stars, just a few years after their birth. A neutron star is created at the core of a supernova as the star dies in a fiery explosion, with a mass as large as two Suns squeezed into a ball of neutrons just 10 miles (16 km) across. Pulsars, a subset of neutron stars, give off a beam of radio radiation, seen to regularly flicker from Earth as the neutron star rapidly rotates (as fast as a thousand times a second).

Supernovae are rare events. A Milky Way-sized galaxy averages one such explosion per century, and none has been recorded in our galaxy since the invention of the telescope. But one of the brightest radio sources in the sky is the Crab Pulsar, known to give off random giant pulses lasting a fraction of a nanosecond and suddenly exceeding the brightness of normal pulses by a factor of several thousand. And we happen to know its age: The Crab Pulsar was created in a supernova explosion recorded as a “guest star” in A.D. 1054 by Chinese astronomers, so bright it was visible in daytime. Could FRBs be caused by similar giant pulses from even younger pulsars, just a few decades old? No one knows.
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The starquake that shook the crust of SGR 1806–20, a magnetar in the Milky Way, launched a flare that was recorded in December 2004. The gamma rays from this quake were incredibly powerful, briefly lighting up Earth’s upper atmosphere. Although SGR 1806–20’s outburst did not cause an associated FRB, similar bursts from extragalactic magnetars remain one theorized source of these signals.
University of Hawaii
 Some astronomers have even speculated that FRBs could originate from other intelligent life in the universe. “Should you consider it as a first explanation? No,” emphasizes Joe Lazio of the California Institute of Technology. “But I think we can say, based on our own capabilities in our own civilization, that we can’t rule it out.” Lazio argues that FRBs could still prove to be transmissions from alien radar systems, developed by alien species in distant galaxies that we incidentally pick up.

It may sound far-fetched, but Lazio cites how Arecibo Observatory often doubles as the world’s most powerful radar transmitter, bouncing radio beams off asteroids and other solar system objects in order to map them. Arecibo’s radar signals could conceivably be detected, for a brief moment, by any stars behind the body being mapped. Even our own radio dishes could pick up an Arecibo-like signal from within a few light-years of Earth. So, based on our current capabilities, who’s to rule out that we aren’t incidentally picking up signals from a sophisticated intergalactic radar system?

Finally, FRBs could come from more than one source. The field of gamma-ray bursts (GRBs) is an example of how this scenario could play out. U.S. military satellites designed to detect gamma radiation from nuclear weapons tests first observed GRBs in the 1960s; the existence of GRBs from deep space was declassified in 1973. By 1994, no fewer than 128 models of GRBs were published, many of which were quickly discarded when a newly discovered GRB didn’t fit the bill. In the end, however, this was hasty because it turns out GRBs from deep space fall into two major categories (and a few more rare ones). About 70 percent of all GRBs are so-called long GRBs, which occur when a supermassive star at the end of its life explodes as a supernova and subsequently forms a new black hole. The remaining category is made up of short GRBs, which originate when two neutron stars merge. These were confirmed in October 2017 by the gravitational wave detector LIGO and telescope follow-up.

The moral of GRBs is no single theory can cover all angles of their origin. Many astronomers therefore argue one should be cautious to assume all FRBs are from the same sources.

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The first identified FRB, FRB 010724, lasted less than 5 milliseconds. This observation of FRB 010724, taken with the 13-beam receiver of the Parkes radio telescope, shows flux in beam 07 as a function of time. It appeared in data taken in 2001 but was not discovered and published until 2007.
Duncan Lorimer

Aiming for answers

It is clear to many astronomers that finding more FRBs is the key to decoding their secrets. “Every FRB right now is like an individual snowflake, where we admire the individual characteristics and details we can see,” explains Emily Petroff, an American astronomer at ASTRON, the Netherlands Institute for Radio Astronomy. Petroff has discovered several FRBs and created the first-ever catalog for the signals. “In the future, we want an FRB snowbank, where there are so many FRBs you no longer care about an individual one,” she says.

Many groups are interested in contributing to that FRB snowbank, with new instruments and telescopes under development to search for them. One of the standouts is the Canadian Hydrogen Intensity Mapping Experiment (CHIME), predicted to spot as many as several dozen FRB signals each day. As the name implies, CHIME was not initially conceived as an FRB-detecting machine — its primary science goal is to precisely map hydrogen in distant galaxies to learn about the expansion history and acceleration of the universe. But it does have an ideal field of view for FRB hunting, and when Victoria Kaspi of McGill University heard about the first FRBs, she acquired funding to look for them as well. “I was first thinking about pulsars,” Kaspi confesses, “but it soon became clear that CHIME would be ideal for FRBs.”

Ten years after discovering the first FRB, Lorimer is optimistic about the future. He predicts that by 2020, the first hundred FRBs will be found thanks to CHIME, and by 2025, thousands of FRBs will be known with many radio telescopes around the world searching for them. He even speculates that by 2030, FRBs could be essential cosmological tools, taking advantage of the vast distances they travel to probe distant parts of the universe.

We are at the dawn of the fast radio burst era. For now, we will have to wait to see where this new cosmic mystery takes us.

Yvette Cendes is a radio astronomer at the Dunlap Institute for Astronomy and Astrophysics, University of Toronto. Her website is www.yvettecendes.com
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