satellites pointing to the sky at the Very Large Array

The Karl G. Jansky Very Large Array

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Mysterious radio bursts originate outside the Milky Way

Scientists have identified the source of mysterious flashes of cosmic radio waves known as fast radio bursts (FRBs): a surprisingly small galaxy more than 3 billion light-years away. The discovery may help researchers understand one of the biggest mysteries in astronomy. 

”It’s an observational breakthrough,” says Neil Gehrels, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who was not involved in the new discovery. ”We now have definitive proof” of their extragalactic nature, adds team member Jason Hessels of ASTRON, the Netherlands Institute for Radio Astronomy in Dwingeloo.

FRBs have been a hot subject since 2007. That year, a team led by radio astronomer Duncan Lorimer of West Virginia University in Morgantown accidentally found the first FRB by analyzing old observations of the 64-meter radio telescope in Parkes, Australia. Since then, astronomers have stumbled upon 17 more FRBs, which usually last for at most a few milliseconds.

Fortunately, one discovered in 2012 with the 305-meter radio telescope in Arecibo, Puerto Rico, turned out to repeat at irregular intervals. Known as FRB 121102, its location on the sky has now been monitored for many tens of hours by the National Radio Astronomy Observatory’s Karl G. Jansky Very Large Array (VLA) in Socorro, New Mexico (an array of 27 radio dishes), and the European VLBI Network (EVN)—a continent-wide collaboration of radio telescopes.

Between 23 August and 18 September 2016, the VLA detected nine bursts. Those observations, published today in Nature, reveal that the location of the bursts coincides with a faint, remote galaxy that also hosts a faint, persistent source of radio waves. Four additional bursts from the same source were found on 20 September 2016 by the EVN, which, along with data from the Arecibo dish, helped provide an even more precise localization within the galaxy, according to a paper published today in Astrophysical Journal Letters.

Using the optical 8.1-meter Gemini North telescope on Mauna Kea in Hawaii, astronomers then managed to determine the galaxy’s distance: more than 3 billion light-years, as reported in a second paper in the same issue of Astrophysical Journal Letters. “Surprisingly, the host galaxy [of FRB 121102] is a puny, star-forming dwarf system,” says ASTRON’s Cees Bassa, who led the optical observations together with Shriharsh Tendulkar of McGill University in Montreal, Canada. Because dwarf galaxies contain so few stars, this suggests that whatever is responsible for FRB 121102 has a better chance of forming in tiny galaxies than large, spiral ones.

Astronomers had thought that FRBs come from one-off, cataclysmic events—for instance the formation of a black hole by the merger of two neutron stars, the compact remains of supernova explosions. But the repeating nature of FRB 121102 reveals that whatever is producing the bursts cannot be destroyed in the process.

Hessels thinks the culprit may be occasional explosions from extremely rapidly spinning, highly magnetized neutron stars. But given the energies involved, he admits, those would need to have spin and magnetic properties beyond anything seen in the Milky Way so far.

FRBs are probably not directly related to long gamma ray bursts (another type of explosive event that preferentially occurs in dwarf galaxies), because there are just too few gamma ray bursts and too many FRBs. Alternative explanations, like matter falling into black holes, cannot yet be ruled out. Rapid follow-up studies of FRBs with NASA’s Swift and Fermi spacecraft could help solve the mystery by finding x-ray or gamma ray light accompanying the radio bursts, Gehrels says.

Another riddle is why FRB 121102 appears to be the only repeater. Hessels suspects that all FRBs observed to date are from the same type of source. But Lorimer is not so sure. “My guess at the moment is that [FRB 121102] is not representative of all FRBs, and that there are multiple classes.”