Single flashes of cosmic radio waves appear to have a different source than repeated bursts
Australian Academy of Science
Astronomers have long wondered what triggers brief, brilliant blasts of radio waves from other galaxies. Now, new observations suggest these events, known as fast radio bursts, or FRBs for short, may emanate from two completely different phenomena.
For the first time, astronomers have identified the home galaxy of a one-off FRB. Until now, only the repeating FRB 121102 had been pegged to a particular galaxy — a tiny, highly active dwarf galaxy about 2.5 billion light-years away (SN: 2/4/17, p. 10). In contrast, the newly discovered lone FRB hailed from a much more massive and tranquil host, researchers report online June 27 in Science.
“You have to be somewhat wary” about drawing broad conclusions from just the two examples, says Duncan Lorimer, an astrophysicist at West Virginia University in Morgantown who helped discover the first-ever FRB in 2007. Though not involved in the latest work, Lorimer says the new findings offer compelling evidence that a one-and-done FRB is a different animal than an FRB that flickers on and off.
Astronomers spotted the new FRB using the Australian Square Kilometer Array Pathfinder, a cadre of 36 radio dishes spread up to six kilometers apart in the Australian outback. By combining data from all these dishes using a technique called interferometry, astronomers pinpointed the FRB’s location with much higher accuracy than they could have with observations from a single telescope (SN: 4/27/19, p. 7).
The new FRB’s discovery was somewhat serendipitous. Scientists had already spent two weeks in September scouting for FRBs, with no luck. “We were just about to give the telescope back,” but the next person scheduled for telescope time called in sick, says Keith Bannister, a radio astronomer at Australia’s Commonwealth Scientific and Industrial Research Organization in Sydney. So the team got an extra observation day, and caught the 1.3-millisecond burst of radio waves. “In his honor, for being sick that day … we called the FRB ‘Tony,’” Bannister says.
Tony — officially named FRB 180924 for the date of its discovery — hailed from the galaxy DES J214425.25–405400.81, about 4 billion light-years away in the constellation Grus.
The FRB 180924, nicknamed “Tony,” came from a disk-shaped galaxy about 4 billion light-years away (yellow). To astronomers’ surprise, Tony (black circle) originated not from the active galactic center, but the outskirts of the galaxy — peripheral to about 90 percent of the disk’s starlight.
Compared with the repeating FRB’s origin, “you couldn’t think of two galaxies that are more different,” Bannister says. FRB 121102, which flickers on and off at seemingly erratic intervals, is nestled in a dense, highly magnetized region near the center of a faint dwarf galaxy that is furiously forging new stars. Tony’s home, however, sits on the edge of a disk-shaped galaxy about the size of the Milky Way that is undergoing very little star formation.
That position, about 13,000 light-years from the galaxy’s center, surprised Bannister’s team. “Centers of galaxies are usually exciting places” with energetic sources that could power something like an FRB, Bannister says, but Tony “seems to come from the suburbs. Not a lot is happening out there, that we know of.”
FRB 121102’s rapid blinks have led to speculation that it’s generated by a rapidly rotating neutron star called a pulsar (SN: 2/3/18, p. 6). Given its intense environs, that neutron star could either be interacting with a black hole, or be an unusually magnetic breed of neutron star called a magnetar, says Sarah Burke-Spolaor, an astrophysicist at West Virginia University who helped track FRB 121102’s origins but was not involved in the new study. Tony’s quiet home isn’t a galaxy where you’d expect to find such energetic objects, she says.
That leaves astronomers to wonder what could trigger a single-flash FRB like Tony. Lorimer thinks these FRBs might originate from neutron star mergers. Burke-Spolaor says, however, that such cataclysmic mergers don’t occur often enough to explain the dozens of radio bursts astronomers have observed.
Over the next few years, telescopes around the world are expected to provide a more comprehensive census of FRBs throughout the universe (SN: 2/2/19, p. 12). Those observations should “paint a clearer picture about what’s going on with these mysterious events,” Burke-Spolaor says.
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