Until 10 years ago, radio astronomers thought they had assembled an essentially complete picture of the sky. In this view, with telescopes attuned to radio waves rather than visible light, the solar system’s brightest radio sources—the sun and Jupiter—would pale against the Milky Way’s splendor. Aglow with radio emissions from sizzling supernovae debris, gas-shrouded stellar nurseries and the metronomic flashes of pulsars, our galaxy would dominate the vista overhead. Beyond that the entire sky would be speckled with steady, starlike points of luminosity from radio-belching supermassive black holes at the centers of distant galaxies.
It turns out, however, those astronomers had missed something big. The heavens also sparkle with something entirely unexpected: fast radio bursts, or FRBs—flashes of radio waves as “bright” as a half-billion suns, which flare from seemingly random locations and fade in just milliseconds. Because most radio telescopes can only survey small patches of sky for short periods, the phenomenon had gone unnoticed for decades.
Even now, with the study of FRBs becoming the most vibrant subfield of radio astronomy, the phenomenon remains unexplained, and observers have reported detecting less than two dozen in all. But extrapolating those meager results to the entire celestial sphere suggests the radio sky should twinkle with perhaps hundreds of FRBs per day. Astronomers regularly quip that there are more theories for the phenomenon’s physical sources than actual observed FRBs. These range from exploding suns to colliding neutron stars to evaporating black holes—or perhaps even chatty aliens.
TUNING IN TO INTERGALACTIC SPACE
What all the observed FRBs share, however, is a curious smearing-out of their frequencies that matches expectations for how clouds of electrons in deep space would alter the propagation of radio waves journeying through the cosmos. The bigger the smear, the more plasma—ionized gas—an FRB’s waves must have passed through. To date, all detected FRBs display smears greater than what could be easily produced by all the interstellar plasma in our galaxy. This hints that their light has traveled to us across billions of light-years, and that they are among the most luminous phenomena in the universe. If so, astronomers could use FRBs to study new frontiers of high-energy astrophysics and to map the uncharted realms of rarefied, magnetized plasma that suffuse intergalactic space. But denser, closer regions of plasma—like those in the outer layers of flaring stars or in shells of debris surrounding supernovae—could conceivably smear an FRB’s light, too. This leads some researchers to suspect they are actually produced by much more prosaic astrophysical processes right here in our Milky Way or other nearby galaxies, and thus much less promising as new tools for probing the universe’s large-scale structure.
Much is at stake in this great cosmic mystery. Solving it requires pinpointing an FRB’s source to see whether it comes from a distant galaxy, a nearby star or somewhere in between. A key clue emerged in 2016 when astronomers using the Arecibo radio telescope in Puerto Rico reported a repeating FRB, dubbed FRB 121102, sporadically pinging away in the constellation Auriga. That gave researchers a target to scrutinize, and indicated that at least some FRBs are caused by processes that do not result in the cataclysmic destruction of the source.
But whether the repeating FRB’s source was nearby or very far away remained elusive; preliminary investigations could only constrain the source to a relatively large patch of sky as seen from Earth that was one tenth the diameter of the full moon, and packed with stars and background galaxies. But now, at a meeting of the American Astronomical Society in Grapevine, Texas, an international team of astronomers announced they have pinpointed the source, marshaling a global network of radio telescopes to trace an FRB to an enigmatic dwarf galaxy billions of light-years distant. Their findings were published January 4 in Nature and in two additional papers in The Astrophysical Journal Letters. Collectively, these studies all but confirm a decade of speculation that most—probably all—FRBs are extremely bright and energetic events seen across vast cosmic distances.
A BREAKTHROUGH DISCOVERY
Led by Cornell University senior research associate, Shami Chatterjee, the team used the Arecibo telescope and the Very Large Array in New Mexico to monitor multiple bursts from FRB 121102. Next they paired with another team helmed by Benito Marcote of the Joint Institute for VLBI in the Netherlands, using the European Very Long Baseline Interferometry Network of radio telescopes to pinpoint the source to a region of the sky just a hundred-millionth the diameter of the full moon. In the process they also detected a weak, persistent radio emission within 100 light-years of the powerful FRB events. Finally, the two groups worked with a third team, led by Shriharsh Tendulkar of McGill University in Montreal, to zoom in on the source’s location in the sky using the optical Gemini North Telescope in Hawaii, spying a tiny smudge of light that looked to be a dwarf galaxy more than three billion light-years away.
“This is the breakthrough everyone wanted,” says James Cordes, a radio astronomer at Cornell and co-author of two of the studies. “It has broken a logjam of speculation,” he says, adding that FRB 121102 “serves as a prototype for all FRBs until we are compelled to think otherwise.”
According to Chatterjee, Cordes and their co-authors, if this particular extragalactic FRB is indeed representative of all others, it rules out all theories positing FRBs as the product of processes in or around our Milky Way. It would also eliminate all models relying on the one-off cataclysmic explosions or mergers of various types of stars, and would suggest all FRBs will repeat if monitored long enough. The result dictates an agenda for future FRB studies as ambitious as it is necessary, for the discovery of FRB 121102’s cosmic home raises as many questions as it answers. To learn the true nature of FRBs, this seemingly singular repeater would demand even greater scrutiny, and all previously observed FRBs would need to be reexamined for signs of repeats. Most importantly, new networks of high-resolution, wide-field radio telescopes would need to be built to perform all-sky FRB surveys to rapidly detect and localize the elusive eruptions as they occur across the universe. If more are found to repeat and are localized to distant dwarf galaxies, a paradigm-shifting new era of cosmological discovery may be at hand; if not, the curious behavior of FRB 121102 may become just another cosmic cold case in the annals of astronomy’s history.
ONE IS THE LONELIEST NUMBER
For now, Derek Fox, an astronomer at The Pennsylvania State University unaffiliated with the team’s studies, is skeptical that FRB 121102’s repetition can be reconciled with that of another FRB that his team reported late last year. Called FRB 131104, this event appeared to emit a minutes-long blast of gamma rays a billion times more energetic than its milliseconds-long radio pulse. “Among our group, we tend to feel that the much more demanding energy requirements for the gamma-ray emission from FRB 131104 suggest that it cannot be a repeating source,” Fox says, because its progenitor would be destroyed by the process. “So at least two populations [of FRBs] are required, with luminous gamma-ray emission coming only from some fraction of the nonrepeating FRBs.”
Chatterjee acknowledges there could be multiple types of FRBs—some repeating, some not. “So far, we’re reasoning from a sample of one,” he says. “But certainly the more economical explanation is that we’ve just been unlucky with other FRBs so far, and all of them are capable of repeating. If not, we have two (or more) fun mysteries on our hands instead of one!”
There are only a handful of plausible explanations left for FRB 121102’s mysterious repetitions, but none are without problems. Cordes says some of this FRB’s staccato repetitions might be illusory, created by swirling clouds of plasma magnifying or masking a possibly continuous (yet still very mysterious) radio emission from some unknown source.
Presuming instead that the repetitions are intrinsic to this FRB’s source, outbursts from young, highly magnetized neutron stars called “magnetars” could explain such behavior—as could jets expelled from a supermassive black hole gorging on gas and dust at a galaxy’s core. Some hybrid theories even propose that FRBs could be produced by interactions between these two exotic astrophysical objects, and the immediate environment around either a feeding black hole or a newborn neutron star could also produce the persistent background radio emission that appears to underlie the episodic flare-ups from FRB 121102.
DWARF GALAXY, GIANT PROBLEM
The trouble is, neutron stars and supermassive black holes are each typically found in large galaxies—not in small ones like this newly discovered dwarf, which is estimated to be 10 times smaller and a thousand times lighter than our own Milky Way. Furthermore, a supermassive black hole would be expected to lurk near a galaxy’s center, but the outbursts of FRB 121102 appear to originate from the dwarf galaxy’s periphery. Conversely, dwarf galaxies tend to possess greater amounts of pristine primordial gas than larger galaxies do, which promotes the formation of very massive stars. When these stars die, they can create long-duration gamma-ray bursts and superluminous supernovae—cataclysmic explosions that can be seen clear across the universe. FRB 121102’s location in a dwarf galaxy could suggest its repeating outbursts are somehow related to these other energetic and far more destructive events. Or, instead, it could imply astronomers are still missing something fundamental in their analyses.
These accumulating inconsistencies make some experts question whether FRB 121102’s “dwarf galaxy” is really a galaxy at all, or perhaps something stranger and less familiar to science. Steady, compact radio sources like the one detected by Chatterjee and colleagues “are unheard of in such small galaxies,” says Vikram Ravi, a California Institute of Technology astronomer who was not involved with the team’s work. “The takeaway is this,” Ravi says. “A persistent counterpart to [FRB 121102] has been identified, and it is unlike any source we’ve previously known about.”
In a commentary accompanying the Nature paper, Heino Falcke, an astronomer at Radboud University in the Netherlands, speculated that instead of a dwarf galaxy, the object might be “the nucleus of a disrupted galaxy,” “an isolated black hole” or “an exploding star ‘disguised’ to look like a black hole.” Amid that uncertainty, Falcke wrote that for now, “Chatterjee and colleagues, and the rest of the astrophysics community, are left scratching their heads.”