What causes fast radio bursts? No-one knows, but the only source producing repeated FRB signals provided plenty to talk about at the American Astronomical Society meeting in January. Sue Bowler reports. Fast radio bursts are usually one-off, very powerful bursts of radio-frequency radiation, lasting milliseconds. Both the source of the signal and the environment in which it forms remain unknown. But the first such source to produce repeated bursts is helping researchers to explore its source and the extreme environment in which it resides: possibly a neutron star close to a massive black hole, a wind nebula or a supernova remnant near a young neutron star. Whatever it is, this source, FRB 121102, came under scrutiny at the 231st meeting of the American Astronomical Society in Washington DC in January this year. While there is not yet enough data to provide an unambiguous model of what and where the source is, it seems to be in an active phase and may provide more data with which to uncover its secrets. And, while alien intelligence is unlikely to be the source of these short-duration radio signals, they do offer an opportunity to test one of the big data handling systems that are used for radio-frequency searches for extraterrestrial intelligence (SETI). First discovery The first fast radio burst to be detected was discovered in archive data from the Parkes radio telescope in 2007 (Lorimer et al. 2007). Many more have been detected since, through data mining and new observations. They appeared to be one-off events; this and their short duration suggested that they originated in some sort of cataclysmic event akin to a supernova. They were also thought to be extragalactic (Thornton et al. 2013), and hence very bright. Laura Spitler, then a postdoctoral researcher at Cornell University, New York, discovered FRB 121102 (named from the date it was recorded) in 2014, while examining 2012 data from the Pulsar Arecibo L-Band Feed Array survey, using the 305 m William E Gordon telescope at the Arecibo Observatory in Puerto Rico (Spitler et al. 2014). She found a 3 ms signal, similar to other FRBs and equally intriguing. Then, in 2015, data from Arecibo showed 10 additional short-duration radio pulses (Spitler et al. 2016). These signals did not repeat with a regular period, but they all came from the same part of the sky; Spitler et al. concluded from the common features of the signals that they came from the same, single source: FRB 121102. That source is definitely extragalactic. The signals all show a similarly high dispersion, a propagation effect that results in the high-frequency parts of the signal arriving before lower frequencies. The spread of the signal arises from the distance the signal has travelled and the density of the material it travelled through – the integrated column density of free electrons between source and detector. FRB 121102 has a dispersion indicating a path length larger than the radius of our galaxy. Observations in 2017 showed that the signal came from a dwarf galaxy (Chaterjee et al. 2017, Tendulkar et al. 2017), in a region of intense star formation (Bassa et al. 2017). This galaxy is at redshift 0.193 (about 3 million light-years from Earth) and associated with a persistent radio source, also of unknown nature. Repeated bursts from the same place ruled out a catastrophic origin; whatever the source was, it was not destroyed in the process of producing the signal, but continued to generate the bursts. It was also powerful. These short bursts from outside our galaxy release roughly as much energy in a millisecond as the Sun does in a day. Their source remains unknown. The short duration of the bursts indicates that they come from a small source region, probably less than 10 km across, suggesting a compact object such as a neutron star. Now further targeted observations with Arecibo and the Green Bank Telescope, West Virginia, have shown more signals from FRB 121102: 16 more bursts picked up at Arecibo and a sequence of five bursts in 30 minutes picked up six months later at GBT. These detections are at higher frequencies: 4.1–4.9 GHz for the Arecibo data and 4–8 GHz at GBT. These data also included polarization measurements, which indicate that the radio emission had passed through a region with a strong magnetic field. The new data recorded at higher frequencies are also shorter than the first repeated bursts, and show complex structure. One of them was so short – just 30 μs – that it can only have come from a compact region. FRB 121102 also shows much more complicated structures within the bursts. “The profiles of the other known FRBs are simple, with just one or occasionally two peaks in time. We've observed bursts from FRB 121102 with as many as seven peaks, and the bursts peak in radio frequency as well as time,” said Spitler, now at the Max Planck Institute for Radioastronomy. “We are trying to understand whether the bursts' structure is an intrinsic feature of the process that generates the radio emission, or the result of the propagation through the dense plasma local to the source.” Source or surroundings? This is the big question and the evidence for the contribution of each of these is difficult to unravel. The bursts showed varying degrees of linear polarization, and that the direction of polarization was rotated by passing through a region of strongly magnetized plasma: an effect known as Faraday rotation. In the case of FRB 121102, it is extremely high, at 105 rad m−2, among the highest measured. The Milky Way would be expected to produce effects of a few tens of rad m−2, and the intergalactic effects would be less than 100 rad m−2. Daniele Michilli, a postgraduate candidate at the University of Amsterdam/ASTRON (Netherlands Institute for Radio Astronomy) and lead author, noted that the level of rotation measured was extreme, 500 times more than usual. “Faraday rotation takes place when radio waves cross a magnetized plasma region. The polarization direction of the radiation is rotated and the rotation is larger if the magnetic field is stronger,” he said. “The only other plasma we know to be comparable is that near Sagittarius A, around a supermassive black hole. Maybe that's important here, but we can tell that the source is located in an extreme environment with high magnetic fields.” This led Michilli et al. (2018) to suggest that FRB 121102 could have originated in the vicinity of a supermassive black hole, with accretion onto the black hole producing the persistent radio source that they also detected from the host galaxy. But this is not the only possibility for the origin of these bursts: the source may also exist in a powerful nebula like the Crab Nebula – albeit one a million times brighter. Michilli et al. (2018) also suggest that a compact region of dense plasma with a high magnetic field is responsible for the rotation, and that it is this that has changed in the six months between the Arecibo and GBT detections. It is also likely to be close to the source – the chances of two dense magnetic regions at a distance from the source just happening to fall in the path of the emission, six months apart, are small. The suggested region of magnetic plasma is probably less than 0.7 pc across and probably changes in time, to give the variations in rotation noted in the Arecibo and GBT data. Polarization and Faraday rotation have been measured for five other FRBs, with varied results. The extreme rotation measured for these signals has suggested that FRBs with no detectable linear polarization may in fact have very large rotations, undetectable at the limited frequency resolution where they were detected. The frequency and time structure of the bursts is important in understanding the properties of the source and/or its environment. But they can be difficult to examine in the standard frequency–time plots. The structure is much clearer in three dimensions – and translates well to the field needed to print models. Anne Archibald, an astronomer at the University of Amsterdam, has produced models of these FRBs with a 3D printer that make the evolving frequency patterns clear and easy to see. “I have a 3D printer and I like tinkering,” she said. “I had access to this data and thought it was interesting data to visualize.” She has made available the files needed to print out your own FRB on the open source website Thingiverse (http://www.thingiverse.com/thing:2723399). The models trace the intensity of the signal in frequency and time, showing peaks and troughs as it evolves. Burst sequence The pattern of bursts is intriguing. “We saw 18 bursts in the first 30 minutes of our 5 hours observations, then nothing,” said Vishal Gajjar of the University of California at Berkeley, working at the GBT as part of the Breakthrough Listen project. “There's variable activity, but why so many pulses, some strong at low frequencies, some strong at high frequencies? And we saw weird bursts with odd structures.” Part of the reason for the detail in this dataset is that the researchers were using the Breakthrough Listen Digital Backend (MacMahon et al. 2018), which collected data across the 4 GHz bandwidth of the receiver. What Breakthrough Listen's involvement means is extremely powerful kit on the GBT: a 32-node computing cluster collecting data at a rate equivalent to 18 000 DVDs per hour, with analytical power to match. “That gives us the edge we need to explore FRBs and extreme pulsars,” said Gajjar. Breakthrough Listen is a project to develop SETI methods; its involvement does not suggest that alien intelligence is considered a likely origin for FRBs. In the spirit of extraordinary claims requiring extraordinary evidence, the Breakthrough Listen team sees FRBs as a chance to examine an unexplained phenomenon. The first pulsar signals were considered a possible sign of extraterrestrial civilizations, in part because no known astrophysical process could explain them; with more data from more examples, they led into a new area of astrophysical exploration, as FRBs may well do. The compact nature of the source suggests that it may be a neutron star, in an extreme environment such as close to a supermassive black hole like that at the centre of the Milky Way. A nearby black hole is an appealing model, in part because the persistent radio emission could arise from black hole accretion. Pulsars in our galaxy, such as the magnetar PSR J1745-2900 close to the galactic centre, also show high rotation measures (−7 × 104 rad m−2 in this case). But there is a scale problem: FRB 121102 produces bursts that are many orders of magnitude more energetic than this or any other pulsar in our galaxy. Given the distance, FRB 121102 is about a million times more energetic than the Crab Pulsar, for example. There is also the problem of periodicity. Pulsars are noted for their regularity, arising from their rotation. No periodicity has been found in this FRB, and Michilli et al. (2018) also infer that the structure within the bursts is more likely to come from source variability than rotation. Another suggestion is that this FRB originated from the neutron star in a supernova remnant and nebula powered by a central neutron star. The density of filaments measured in the Crab Nebula could be enough to give the Faraday rotation, and such a structure would perhaps account for some of the variability seen in the bursts. The team also suggests more exotic models. At the moment, FRBs are raising a lot of questions. “They look like pulsars, but they're much more powerful,” said Jason Hessels of ASTRON. “And what about the periodicity? Is there an underlying periodicity that we can't see yet? They're probably near a supermassive black hole, to give the persistent radio emission, but do black holes exist in dwarf galaxies? What about a young, powerful nebula to provide the persistent radio source? If this is it, it would have to be about a million times brighter than the Crab Nebula, which is bright.” Even the best guesses do not quite fit. The major unknowns are the nature of the periodicity of FRB 121102 and whether it is worth investing in longer duration observing campaigns. There is also the nature of the persistent radio source – observations using very long baseline interferometry may be able to identify it, if it is a supermassive black hole. A big step forward would be the discovery of more repeating FRBs, allowing the identification of shared features and shedding light on the contributions of source and local environment to the complex signals received. Conclusion At present, both the individual FRBs and the repeated signals remain enigmatic. “FRB 121102 was already unique because it repeats, which hasn't yet been observed in any other FRBs; now the huge Faraday rotation we have detected singles it out yet again,” said Michilli. “We're curious as to whether these two unique aspects are linked.” Whether or not FRB 121102 is representative of other, single, FRBs, it offers an intriguing glimpse into a powerful radio source and its surroundings. But that glimpse does not yet offer a conclusive picture of its origins, how a mechanism might relate to models of pulsars, blazars and energetic nebulae. Until there are more examples, a lot of these questions will remain unanswered. The next generation of detectors – such as ASKAP and CHIME – should provide more discoveries, including more repeating FRBs, and refine or reject some of the ideas for their origin. “These may not be the right models,” noted Hessels. “At the moment we have more theories than FRBs.” REFERENCES Bassa C Get al. 2017 Astrophys. J. 843 L8 CrossRef Search ADS Chaterjee Set al. 2017 Nature 541 58 CrossRef Search ADS PubMed Lorimer D Ret al. 2007 Science 318 777 CrossRef Search ADS PubMed MacMahon D H Eet al. 2018 Proc. Astron. Soc. Pacific in press Michilli Det al. 2018 Nature 553 182 CrossRef Search ADS PubMed Spitler L Get al. 2014 Astrophys. J. 790 101 CrossRef Search ADS Spitler L Get al. 2016 Nature 531 202 CrossRef Search ADS PubMed Tendulkar S Pet al. 2017 Astrophys. J. 834 L7 CrossRef Search ADS Thornton Det al. 2003 Science 341 53 CrossRef Search ADS © 2018 Royal Astronomical Society
Astronomy & Geophysics – Oxford University Press
Published: Apr 1, 2018
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