New Twists Add More Mystery to the Fast Radio Burst Enigma

If there’s something that the Universe is really good at, it’s its ability to pull the rug from under our feet when we think we have grasped the basics of a cosmic process. One such prominent example that lately caught everyone’s attention is the otherwise inconspicuous star KIC 8462852 with its weird and utterly anomalous brightness variations, which had been originally suggested to most likely be caused by the presence of a large swath of orbiting cometary fragments, an explanation that has subsequently been met with doubt. Not to be outdone, an equally mystifying and different phenomenon, called “Fast Radio Bursts,” or FRBs, has been nagging astronomers for almost a decade, with its elusive and transient nature. The hallmark of fast radio bursts discovered to date was that they were all one-off events that weren’t repeatable … until now.

Fast radio bursts were first discovered quite by chance in 2007 by a research team, led by Dr. Duncan Lorimer, who was shifting through archival data previously collected with the 64-m Parkes radio telescope in Australia. Originally suspected of being the result of noise artifacts in the data or instrument glitches at the radio telescope itself, FRBs were later confirmed to have a cosmic origin when subsequent surveys from independent teams of astronomers started digging up more such sporadic events with the help of other radio telescopes, in various different parts of the sky. An important milestone in the study of these elusive phenomena was met in May 2014, when researchers managed to observe a fast radio burst in almost real-time, just seconds after it had went off. This detection, which was accompanied by immediate follow-up observations with a host of ground- and space-based observatories, helped astronomers to narrow down the list of possible causes behind these ultra-brief and sporadic cosmic radio flashes that last for no more than a few milliseconds, yet without offering any definitive explanations as to their exact nature.

Another important breakthrough on the road toward solving this nagging astrophysical mystery occurred a year later, in April 2015, when an international team of astronomers led by Dr Evan Keane, lead scientist for the proposed Square Kilometre Array project, first detected FRB 150418 as part of the Survey for Pulsars and Extragalactic Radio Bursts at the Parkes radio telescope. Just hours after their initial detection, Keane’s team homed in on the direction in the sky of the newly found fast radio burst with the Australia Telescope Compact Array, which allowed the researchers to identify two variable compact radio sources, one of which was found to fade gradually over the course of the next six days.

At the same time, a set of additional observations were also carried out with the 8.2-m optical Subaru telescope in Hawaii, in order to see how the exact region around FRB 150418 appeared in visible wavelengths. What the astronomers found was an elliptical galaxy sitting right in the middle of the fast radio burst’s point of origin, which led them to identify it as the source of FRB 150418. Spectroscopic measurements of the light coming from the galaxy revealed that the latter was a staggering 6 billion light-years away—almost half the way to the edge of the observable Universe.

Up till now, the only way for astronomers to have a sense of FRBs’ approximate distances was by measuring their dispersion through space. More specifically, when radio waves are emitted by a cosmic radio source along our line of sight, they are dispersed by the ionised gas of the interstellar and intergalactic medium from which they pass through, causing their lower frequencies to arrive on Earth later than their higher ones. This time delay in the arrival of the radio wavelengths is dependent on the density of the intervening ionised gas. The longer the delay, the greater the density of the intervening medium and the longer the distance that the radio waves have to travel to reach Earth.

The identification of a fast radio burst’s exact distance is an important accomplishment, not least for the fact that most of the 16 fast radio bursts that have been discovered to date were only seen as fleeting bright radio flashes at random points in the sky, while astronomers were digging through archival observations long after the flashes themselves had occurred, without offering any more clues to their exact whereabouts. “In the past FRBs have been found by sifting through data months or even years later,” Dr. Keane said in a statement, following the publication of his team’s study at the journal Nature late last month. “By that time it is too late to do follow up observations.”

“For the first time, we have identified the host galaxy and measured the distance to a fast radio burst,” adds Dr. Tomonori Totani, a professor of astronomy at the University of Tokyo who led the team behind the optical observations of FRB 150418 at the Subaru telescope.

Elliptical galaxies like the one where FRB 150418 was found have very low star formation rates, being populated mostly by aging, red giant stars instead. In addition, the quick fading of FRB 150418’s associated afterglow, which was observable for no more than six days, indicates that this particular fast radio burst wasn’t caused by violent cataclysmic events like Type II supernova explosions, which mostly appear in spiral galaxies and whose afterglows can typically last for weeks or months. The best possible explanation for FRB 150418’s origins, according to Keane’s team, is the merging of compact objects, like two neutron stars or black holes instead.

Does this mean the mystery of fast radio bursts is finally settled? Not so fast, says an independent study by Peter Williams and Edo Berger, both astronomers at Harvard University, who also studied the host galaxy of FRB 150418 with the Very Large Array in New Mexico. What Williams’ and Berger’s observations showed was that the inferred host galaxy for FRB 150418 radio emission was more typical of an active galactic nucleus that harbors a supermassive black hole feeding off neighboring interstellar material. Furthermore, the original radio afterglow that had been detected by Keane’s team seemed to brighten up, something that is expected from an active galactic nucleus that has a long-term variability in brightness. “If the galaxy’s bright radio emission is not due to star formation, the alternate source is AGN activity,” write the researchers in their study. “This is immediately worrisome because AGN are both intrinsically and extrinsically variable and could thus falsely appear as a transient radio source. We argue that the radio light curve reported by Keane et al, with only five measurements and three within eight days of each other, is insufficient to reject long-term variability of the host [galaxy], and is not atypical of AGN variability given the sampling.”

The latest (and more interesting) twist in the fast radio burst saga came from an international team of astronomers led by Dr. Laura Spitler, a post-doctoral researcher at the Max Planck Institute for Radio Astronomy in Germany. Using the 305-m Arecibo radio telescope in Puerto Rico, Dr. Spitler and colleagues revisited FRB 121102 throughout May and June of 2015, a fast radio burst that had been first detected back in 2012 very close to the galactic plane in the direction of the northern constellation Auriga. Wanting to determine if fast radio bursts were one-off events as had been the census among the scientific community to date, Spitler’s team was on the lookout for signs of repeatability from FRB 121102. And sure enough they found just that, in the form of 10 additional radio bursts that had exactly the same dispersion measure with the original one that had been detected in 2012. More interestingly, these repeating signals from FRB 121102 exhibited a set of widely spectra, while occurring at random intervals with no fixed periodicity. Some bursts were brighter toward higher radio frequencies, while others were brighter toward lower frequencies, and some occurred within minutes and others within hours of each other. “The repeat signals were surprising – and very exciting,” said in a press release Paul Scholz, a graduate student at the McGill University in Montreal, Canada and member of Spitler’s team. “I knew immediately that the discovery would be extremely important in the study of FRBs.”

What these findings also indicate is that the original FRB 121102 radio burst and its successors cannot be traced back to single cataclysmic events like the death of very massive stars and the merging of neutron stars, contrary to the results of a different study of another fast radio burst called FRB 110523, which had suggested supernova explosions as one of the possible candidates for these elusive cosmic phenomena. “Not only does this source repeat, but the brightness and spectra of the bursts also differs from those of other FRBs,” says Spitler. As to the nature of FRB 110523, the scenario favored by Spitler’s team is that it linked to a peculiar brand of highly spinning, highly magnetised stars called magnetars, yet none of the magnetars that have been detected to date have produced the immense energy output that is so characteristic of fast radio bursts.

So what’s the deal with fast radio bursts? One possibility, according to Spitler’s team, is that they come in different flavors, like their cosmic cousins, gamma-ray bursts, which are also very powerful and energetic events that have also been observed in distant galaxies and could both ultimately turn out to be different expressions of the same cosmic phenomena. “While the FRB 121102 bursts share many similarities to [other] FRBs detected using the Parkes and Green Bank telescopes, it is unclear whether FRB 121102 is representative of all FRBs,” write the researchers in their study which appeared on the 2 March edition of the journal Nature. “The 10 bursts from FRB 121102 in 2015 were detected near the best-known position in 3 hrs of observations. In contrast, follow-up observations of the Parkes FRBs, again using the Parkes telescope, range in total time per direction from a few hours to almost 100 hours and have found no additional bursts … FRB 121102 may be fundamentally different from the FRBs detected at Parkes and Green Bank. As was the case for supernovae and gamma-ray bursts, multiple astrophysical processes may be required to explain a diversity of observational properties of FRBs.”

Whatever the case may be, one thing that’s sure is that astronomers have their work cut out for them.

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