Core Concept: Unraveling the Enigma of Fast Radio Bursts

Home , Fast radio burst, Peryton (astronomy), Rotating radio transient, Very Small Array, Victoria Kaspi

Correction

CORE CONCEPTS Correction for “Core Concept: Unraveling the enigma of fast radio bursts,” by Adam Mann, which appeared in issue 13, March 28,2017,ofProc 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 Mapping Experiment. Kaspi should have been identified as the principal investigator of the project’s fast radio bursts search. The online version has been corrected.

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Unraveling the enigma of fast radio bursts CORE CONCEPTS Adam Mann, Science Writer

The mystery began in 2007, when astrophysicist Duncan Lorimer and his undergraduate physics student were combing through archival data from the Parkes Observatory in Australia. After a month of analysis, the two noticed something unusual: an extreme burst from 2001 that briefly became one of the brightest radio objects in the night sky (1). “We estimated that it put out as much energy in 5 milliseconds as the sun does in a month,” says Lorimer, a pro- fessor at West Virginia University (WVU) in Morgantown. Exactly what could be producing such prodigious amounts of power remains unknown. But since 2001, scientists have found around 18 similar astronomical events. They call them “fast radio bursts” (FRBs). The- ories abound as to their origin: everything from highly magnetized neutron stars to energetic young nebula to evaporating black holes. “The joke is that the number of theories outnumber the number of known bursts,” says astrophysicist Emily Petroff of The Neth- erlands Institute for Radio Astronomy. Now, researchers believe they might be turning the corner on understanding these strange flashes. At the American Astronomical Society conference in January, a team announced that they had traced a repeating FRB back to a distant dwarf galaxy, the first time such an object has been triangulated to a specific spot in the sky (2). Beginning later this year, several telescope projects will be coming online that promise to uncover dozens of FRBs per day. Besides resolving Sightings of an FRB in 2015 at the Green Bank Radio Telescope in West Virginia a long-standing head-scratcher, scientists hope these helped mitigate doubts about their existence. Image courtesy of English discoveries will help them survey and study the ion- Wikipedia/Geremia. ized gas between galaxies, giving them insight into dark matter, dark energy, and the large-scale structure of the universe. Lorimer found that the burst they detected was too strong to be a rotating radio transient. And it pos- Peculiar Pulse sessed another strange characteristic: it was highly The story of FRBs begins with pulsars, the highly dispersed, meaning that the high-frequency portion of magnetized compact husks of former massive stars its electromagnetic signal reached the telescope a few that emit regular electromagnetic beams. For de- milliseconds before the lower-frequency portion. That cades, astronomers optimized their pulsar searches by could only happen if the radio waves were encoun- seeking only periodic signals from space. But in 2006, tering a huge amount of free electrons out in deep astrophysicist Maura McLaughlin, also of West Virginia space. “That was a pretty strong indication there was a University, found that some pulsars in our galaxy can lot of stuff between us and the object,” says astro- occasionally send out an exceptionally bright single physicist Casey Law of the University of California, radio emission, an observation she and her colleagues Berkeley. “It had to come from far outside our galaxy, called “rotating radio transients” (3). Lorimer and his perhaps billions of light years away.” student were looking for such one-off pulses when The finding got muddied in 2011 when physicist they ran into their FRB. Sarah Burke Spolaor, then at the National Radio

www.pnas.org/cgi/doi/10.1073/pnas.1703512114 PNAS | March 28, 2017 | vol. 114 | no. 13 | 3269–3271 National Radio Astronomy Observatory in Charlottesville, Virginia.

Astronomical Origins Efforts to truly understand FRBs received a major breakthrough last year, when the Arecibo discovery was found to repeat, sending out multiple irregular pulses rather than a single burst (9). By honing in on the flashes, researchers pinpointed them to a metal- poor dwarf galaxy ∼3 billion light-years away. “We’re now able to place constraints on models for the sources,” says McLaughlin. “Where they’re coming from; what kind of [physical processes] might be driving them; but there’s still tons we don’t know.” Foremost among the unknowns is what causes FRBs. Many models involve catastrophic processes, such as the collision of two neutron stars, or a neutron star and a black hole. One exotic suggestion has been that the bursts represent the death rattles of small black holes that formed shortly after the Big Bang. These would have been emitting a type of radiation known as Hawking radiation, after physicist Stephen Hawking, who realized that quantum effects at the edge of a black hole can cause the spontane- ous appearance of subatomic particle pairs. One particle would fall into the black hole while the other The Karl Jansky Very Large Array in New Mexico was able to pinpoint a fast radio would whiz away, sapping some energy from the burst to a dwarf galaxy within the small white square. The Arecibo radio telescope black hole. Over the course of cosmic time, the black had only been able to localize the fast radio burst to the area inside the two circles. Reprinted with permission from ref. 2. hole would gradually shrink until it could no longer sustain itself gravitationally, eventually releasing its enormous mass energy back into the universe as a Astronomy Observatory in Socorro, New Mexico, and titanic electromagnetic surge. her team reported on events seen at Parkes, called But the repeating FRB—and the possibility that all ’ perytons, that looked a lot like Lorimer s signal but of the bursts similarly repeat if stared at for long were clearly coming from somewhere on Earth (4). enough—seems to argue against one-off processes. “ That really freaked people out and turned people Many researchers now prefer models involving pulsars ” “ ’ off, recalls Petroff. They were saying maybe it wasn t because they are already known to give off repetitive ” ’ real after all. It wasn t until 2 years later that another beams. The most-favored culprit is a magnetar, a rare team found four new FRBs and confirmed that they class of pulsar with an unusually strong magnetic field were celestial in origin (5). Even then, some skepticism and the ability to produce solar-flare–like bursts of remained, because only the Parkes telescope seemed radiation. Another possibility is pulsar wind nebulas, to be recording these odd bursts. which form just after a giant star explodes as a su- pernova, leaving a rapidly spinning pulsar inside a “None of the models put forward seem perfectly cloud of gas and dust. The Crab Nebula in our own adequate. Not one really explains all the observations.” galaxy is known to occasionally produce sporadic —Victoria Kaspi electromagnetic flashes, but neither it nor any known magnetar have ever been seen emitting signals strong The doubts were finally assuaged when the Arecibo enough to be an FRB. “None of the models put for- ” Observatory in Puerto Rico and the Green Bank Radio ward seem perfectly adequate, says astrophysicist Telescope in West Virginia spotted their own FRBs Victoria Kaspi of McGill University in Montreal, Quebec. “Not one really explains all the observations.” in 2014 and 2015, respectively (6, 7). Also in 2015, Further advances are likely to come only when re- Petroff explained the peryton mystery by noticing that searchers have captured many more bursts than the they tended to appear around noon at Parkes, when roughly 18 cataloged to date. That modest total partly staff on lunch break would be opening and closing stems from radio observatories’ limited scope; the their kitchen microwave, hence, allowing electro- Parkes telescope, for example, can only stare at an “ magnetic energy to leak out (8). Even though the area about half the size of the full moon at any given ’ Lorimer burst was 10 years ago, it s only in the last time. Based on the frequency of known events, as- 2 years that the radio astronomy community has been tronomers have inferred that FRBs are happening “at a convinced that these things are real and actually as- rate of 5 to 10 thousand bursts every day,” says as- trophysical,” says astronomer Scott Ransom of the tronomer Shami Chatterjee of Cornell University in

3270 | www.pnas.org/cgi/doi/10.1073/pnas.1703512114 Mann Ithaca, New York. “That gives you some sense of how Astronomers want to know not only what creates little of the sky we’re seeing.” FRBs, but also whether the repeating FRB is typical New instruments should help. The upcoming or not. It’s possible that astronomers are simply Canadian Hydrogen Intensity Mapping Experiment classifying similar-looking but unrelated processes (CHIME), slated to start gathering data by year’s end, under the same name, as was the case with gamma- is a ground-based telescope consisting of four ray bursts, a 1960s discovery that turned out to have 20-meter by 100-meter cylindrical reflectors, “or, in multiple causes. Canadian units, the area of six hockey rinks,” says Kaspi, Once researchers have observed thousands of FRB the principal investigator of the project’s fast radio signals, they hope to use the dispersion patterns as an burst search. This affords enormous collecting power. intergalactic probe. The diffuse ionized gas between In addition to its original mission of mapping neutral galaxies is extremely difficult to see with current hydrogen to better understand dark energy, CHIME is techniques, but light from FRBs will illuminate this expected to spot dozens of FRBs per day. Upgrades to medium. In bulk, such intergalactic gas plays an im- the Westerbork Synthesis Radio Telescope in The portant role in shaping the universe’s large-scale Netherlands and the Molonglo Observatory Synthesis structure, and astronomers think mapping it will give Telescope in Australia should also greatly increase them a better understanding of dark matter and dark the FRB discovery rate. Both are expected to be energy. “They really have the potential to come into completed by early next year. their 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. Science 318(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 show terrestrial 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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