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Something in Space Is Blasting Earth with Mysterious 'Fast Radio Bursts'

Confirming the unknown origin of the quick electromagnetic bursts thought to be bombing Earth 10,000 times a day.
Image: Semeis 147, near the directional source of the 2012 signal/Nobuhiko Miki

That the sky is full of strange radio bursts isn't big news. Pulsars, the rotating "radio lighthouses," of the universe, cause a steady flicker of signals in the eyes of Earth's radio observatories, and astrophysicists are quite good at sorting them all out, pinning individual bursts to specific pulsars and using this data to further describe our galaxy in general. While these pulsars are typically thought of as rapidly spinning tops, completing rotations in timespans ranging from milliseconds to seconds, some of them spin so slowly that we've yet to see them completing a full rotation at all. These are called rotating radio transients (RRATs), which are signals from pulsars that spin very irregularly or have a very irregular magnitude of electromagnetic emission, sort of like a garden hose that keeps getting stepped on.

RRATs were first described only in 2006, and just a year later the sky got even weirder with the discovery of fast radio bursts (FRBs). This category of signal is distinctly non-repeating, or at least in any observation so far. While this wouldn't eliminate a pulsar necessarily (as they might still be shown to repeat in future observations), they also appear to originate outside of our galaxy, which we can infer based on their relatively slow speed and wider dispersion (think electrons per unit of volume) when they reach us here on Earth, compared to typical pulsar bursts that don't have to make the journey across the intergalactic medium and are much sharper (less dispersed) on arrival.

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A number of additional FRB observations have been made over the past several years, but, confoundingly, they've all come from a single telescope, the Parke Observatory in Australia, with no confirmation from any of the many other radio observatories here on Earth. The implication of this single-source restriction is a high likelihood that the signals are in fact just coming from Earth or near-Earth sources, and not actually very deep space. A new study published in the Astrophysical Journal, however, describes a 2012 observation of FRB activity, a burst known unceremoniously as FRB 121102, made at the Arecibo radio telescope in Puerto Rico—an event that in effect validates the phenomenon as a real cosmic mystery.

Pulsars, it should be noted, are a rather local phenomenon. Only 20 of them have so far been discovered beyond the Milky Way, and those are all within the Magellanic Clouds. And the Clouds aren’t particularly extragalactic either, relatively—they’re a  pair of galaxies close enough to our own to be distorted by its gravitational forces and to be visible from Earth’s southern hemisphere, appearing (to the eye) almost as a part of the Milky Way.

This isn’t to say that there aren’t deeper extragalactic pulsars, just that we haven’t been able to observe them yet. The problem with calling FRBs a pulsar-related phenomenon is that, well, it would just be very strange for the only non-Magellanic extragalactic pulsars yet observed to be the very bizarre sort that rotate slowly enough such that we’ve never seen a single one of them complete a rotation. It’s possible that deep space harbors a uniquely powerful pulsar variety able to get to reach Earth with the necessary property of also being very, very slow. “Another possibility is that they are bursts much brighter than the giant pulses seen from some pulsars,” noted James Cordes, one of the new study's co-authors, in an emailed statement.

The super-powerful, super-slow pulsar hypothesis might sound like a stretch, but there really aren’t many less contrived, or at least bold, extragalactic explanations. The paper, which is also posted in unpaywalled form at the arXiv preprint server, summarizes them as such: “Proposed extragalactic sources of non-repeating, fast radio transients include evaporating primordial black holes, merging neutron stars, collapsing supramassive neutron stars, and superconducting cosmic strings.” In any of these cases, we’re assured of at least learning something new about the universe.

The two galactic, local explanations are a bit less exciting. One is again the possibility of an Earth-based signal, but, as the paper explains, this is unlikely given that the 2012 burst was seen in only a single telescope beam, while bursts caused by radio-frequency interference are, "generally seen in several or all beams simultaneously due to their local origin,” the paper explains. It is also possible that the signals are coming from inside the galaxy after all and might just be slowed down for some other reason, like pockets of higher densities acting as interstellar speed bumps. Indeed, the team behind the current study attempted to find said pockets, but, “we find no evidence for previously unmodeled dense gas along the line-of-sight that would explain the excess [dispersion,” they write.

So, we’re mostly left with extragalactic explanations, which is great. Not only do we have these high-energy pulses traveling through the intergalactic medium toward Earth, a readymade tool for studying those peculiar not-quite-voids, but we have the greater mystery of where they’re coming from in the very first place. And with an estimated 10,000 of these bursts occurring per day over the whole Earth sky, there should (eventually) be no shortage of FRB data to study. In the meantime, have fun thinking about the aliens.