Correction

CORE CONCEPTSCorrection for “Core Concept: Unraveling the enigma of fast radiobursts,” by Adam Mann, which appeared in issue 13, March28, 2017, of Proc Natl Acad Sci USA (114:3269–3271; 10.1073/pnas.1703512114).Victoria Kaspi was incorrectly identified as the principal

investigator of the Canadian Hydrogen Intensity MappingExperiment. Kaspi should have been identified as the principalinvestigator of the project’s fast radio bursts search. The onlineversion has been corrected.

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CORE CONCEPTS

Unraveling the enigma of fast radio burstsAdam Mann, Science Writer

The mystery began in 2007, when astrophysicistDuncan Lorimer and his undergraduate physics stu-dent were combing through archival data from theParkes Observatory in Australia. After a month ofanalysis, the two noticed something unusual: anextreme burst from 2001 that briefly became one ofthe brightest radio objects in the night sky (1). “Weestimated that it put out as much energy in 5 millisec-onds as the sun does in a month,” says Lorimer, a pro-fessor atWest Virginia University (WVU) inMorgantown.

Exactly what could be producing such prodigiousamounts of power remains unknown. But since 2001,scientists have found around 18 similar astronomicalevents. They call them “fast radio bursts” (FRBs). The-ories abound as to their origin: everything from highlymagnetized neutron stars to energetic young nebulato evaporating black holes. “The joke is that the num-ber of theories outnumber the number of knownbursts,” says astrophysicist Emily Petroff of The Neth-erlands Institute for Radio Astronomy.

Now, researchers believe they might be turningthe corner on understanding these strange flashes. Atthe American Astronomical Society conference inJanuary, a team announced that they had traced arepeating FRB back to a distant dwarf galaxy, the firsttime such an object has been triangulated to a specificspot in the sky (2). Beginning later this year, severaltelescope projects will be coming online that promiseto uncover dozens of FRBs per day. Besides resolvinga long-standing head-scratcher, scientists hope thesediscoveries will help them survey and study the ion-ized gas between galaxies, giving them insight intodark matter, dark energy, and the large-scale structureof the universe.

Peculiar PulseThe story of FRBs begins with pulsars, the highlymagnetized compact husks of former massive starsthat emit regular electromagnetic beams. For de-cades, astronomers optimized their pulsar searches byseeking only periodic signals from space. But in 2006,astrophysicist Maura McLaughlin, also of West VirginiaUniversity, found that some pulsars in our galaxy canoccasionally send out an exceptionally bright singleradio emission, an observation she and her colleaguescalled “rotating radio transients” (3). Lorimer and hisstudent were looking for such one-off pulses whenthey ran into their FRB.

Lorimer found that the burst they detected was toostrong to be a rotating radio transient. And it pos-sessed another strange characteristic: it was highlydispersed, meaning that the high-frequency portion ofits electromagnetic signal reached the telescope a fewmilliseconds before the lower-frequency portion. Thatcould only happen if the radio waves were encoun-tering a huge amount of free electrons out in deepspace. “That was a pretty strong indication there was alot of stuff between us and the object,” says astro-physicist Casey Law of the University of California,Berkeley. “It had to come from far outside our galaxy,perhaps billions of light years away.”

The finding got muddied in 2011 when physicistSarah Burke Spolaor, then at the National Radio

Sightings of an FRB in 2015 at the Green Bank Radio Telescope in West Virginiahelped mitigate doubts about their existence. Image courtesy of EnglishWikipedia/Geremia.

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Astronomy Observatory in Socorro, New Mexico, andher team reported on events seen at Parkes, calledperytons, that looked a lot like Lorimer’s signal butwere clearly coming from somewhere on Earth (4).“That really freaked people out and turned peopleoff,” recalls Petroff. “They were saying maybe it wasn’treal after all.” It wasn’t until 2 years later that anotherteam found four new FRBs and confirmed that theywere celestial in origin (5). Even then, some skepticismremained, because only the Parkes telescope seemedto be recording these odd bursts.

The doubts were finally assuaged when the AreciboObservatory in Puerto Rico and the Green Bank RadioTelescope in West Virginia spotted their own FRBsin 2014 and 2015, respectively (6, 7). Also in 2015,Petroff explained the peryton mystery by noticing thatthey tended to appear around noon at Parkes, whenstaff on lunch break would be opening and closingtheir kitchen microwave, hence, allowing electro-magnetic energy to leak out (8). “Even though theLorimer burst was 10 years ago, it’s only in the last2 years that the radio astronomy community has beenconvinced that these things are real and actually as-trophysical,” says astronomer Scott Ransom of the

National Radio AstronomyObservatory in Charlottesville,Virginia.

Astronomical OriginsEfforts to truly understand FRBs received a majorbreakthrough last year, when the Arecibo discoverywas found to repeat, sending out multiple irregularpulses rather than a single burst (9). By honing in onthe flashes, researchers pinpointed them to a metal-poor dwarf galaxy ∼3 billion light-years away. “We’renow able to place constraints on models for thesources,” says McLaughlin. “Where they’re comingfrom; what kind of [physical processes] might bedriving them; but there’s still tons we don’t know.”

Foremost among the unknowns is what causesFRBs. Many models involve catastrophic processes,such as the collision of two neutron stars, or a neutronstar and a black hole. One exotic suggestion hasbeen that the bursts represent the death rattles ofsmall black holes that formed shortly after the BigBang. These would have been emitting a type of ra-diation known as Hawking radiation, after physicistStephen Hawking, who realized that quantum effectsat the edge of a black hole can cause the spontane-ous appearance of subatomic particle pairs. Oneparticle would fall into the black hole while the otherwould whiz away, sapping some energy from theblack hole. Over the course of cosmic time, the blackhole would gradually shrink until it could no longersustain itself gravitationally, eventually releasing itsenormous mass energy back into the universe as atitanic electromagnetic surge.

But the repeating FRB—and the possibility that allof the bursts similarly repeat if stared at for longenough—seems to argue against one-off processes.Many researchers now prefer models involving pulsarsbecause they are already known to give off repetitivebeams. The most-favored culprit is a magnetar, a rareclass of pulsar with an unusually strong magnetic fieldand the ability to produce solar-flare–like bursts ofradiation. Another possibility is pulsar wind nebulas,which form just after a giant star explodes as a su-pernova, leaving a rapidly spinning pulsar inside acloud of gas and dust. The Crab Nebula in our owngalaxy is known to occasionally produce sporadicelectromagnetic flashes, but neither it nor any knownmagnetar have ever been seen emitting signals strongenough to be an FRB. “None of the models put for-ward seem perfectly adequate,” says astrophysicistVictoria Kaspi of McGill University in Montreal, Quebec.“Not one really explains all the observations.”

Further advances are likely to come only when re-searchers have captured many more bursts than theroughly 18 cataloged to date. That modest total partlystems from radio observatories’ limited scope; theParkes telescope, for example, can only stare at anarea about half the size of the full moon at any giventime. Based on the frequency of known events, as-tronomers have inferred that FRBs are happening “at arate of 5 to 10 thousand bursts every day,” says as-tronomer Shami Chatterjee of Cornell University in

The Karl Jansky Very Large Array in New Mexico was able to pinpoint a fast radioburst to a dwarf galaxy within the small white square. The Arecibo radio telescopehad only been able to localize the fast radio burst to the area inside the twocircles. Reprinted with permission from ref. 2.

“None of the models put forward seem perfectlyadequate. Not one really explains all the observations.”

—Victoria Kaspi

3270 | www.pnas.org/cgi/doi/10.1073/pnas.1703512114 Mann

Ithaca, New York. “That gives you some sense of howlittle of the sky we’re seeing.”

New instruments should help. The upcomingCanadian Hydrogen Intensity Mapping Experiment(CHIME), slated to start gathering data by year’s end,is a ground-based telescope consisting of four20-meter by 100-meter cylindrical reflectors, “or, inCanadian units, the area of six hockey rinks,” says Kaspi,the principal investigator of the project’s fast radioburst search. This affords enormous collecting power.In addition to its original mission of mapping neutralhydrogen to better understand dark energy, CHIME isexpected to spot dozens of FRBs per day. Upgrades tothe Westerbork Synthesis Radio Telescope in TheNetherlands and the Molonglo Observatory SynthesisTelescope in Australia should also greatly increasethe FRB discovery rate. Both are expected to becompleted by early next year.

Astronomers want to know not only what createsFRBs, but also whether the repeating FRB is typicalor not. It’s possible that astronomers are simplyclassifying similar-looking but unrelated processesunder the same name, as was the case with gamma-ray bursts, a 1960s discovery that turned out to havemultiple causes.

Once researchers have observed thousands of FRBsignals, they hope to use the dispersion patterns as anintergalactic probe. The diffuse ionized gas betweengalaxies is extremely difficult to see with currenttechniques, but light from FRBs will illuminate thismedium. In bulk, such intergalactic gas plays an im-portant role in shaping the universe’s large-scalestructure, and astronomers think mapping it will givethem a better understanding of dark matter and darkenergy. “They really have the potential to come intotheir own as cosmological tools,” says Lorimer.

1 Lorimer DR, Bailes M, McLaughlin MA, Narkevic DJ, Crawford F (2007) A bright millisecond radio burst of extragalactic origin. Science318(5851):777–780.

2 Chatterjee S, et al. (2017) A direct localization of a fast radio burst and its host. Nature 541(7635):58–61.3 McLaughlin MA, et al. (2006) Transient radio bursts from rotating neutron stars. Nature 439(7078):817–820.4 Burke-Spolaor S, Bailes M, Ekers R, Macquart JP, Crawford F (2011) Radio bursts with extragalactic spectral characteristics showterrestrial origins. Astrophys J 727(1):18–22.

5 Thornton D, et al. (2013) A population of fast radio bursts at cosmological distances. Science 341(6141):53–56.6 Spitler L, et al. (2014) Fast radio burst discovered in the Arecibo pulsar ALFA survey. Astrophys J 790(2):101–109.7 Masui K, et al. (2015) Dense magnetized plasma associated with a fast radio burst. Nature 528(7583):523–525.8 Petroff E, et al. (2015) Identifying the source of perytons at the Parkes radio telescope. Mon Not R Astron Soc 451(4):3933–3940.9 Spitler LG, et al. (2016) A repeating fast radio burst. Nature 531(7593):202–205.

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