FRB 190520 is only the second example of a repeating fast radio burst with a compact source of persistent radio emission between bursts.
An artist’s conception of a neutron star with an ultra-strong magnetic field, called a magnetar, emitting radio waves (red). Image credit: Bill Saxton, NRAO / AUI / NSF.
Fast radio bursts (FRBs) are mysterious and rarely detected bursts of radio waves from space.
The first FRB was discovered in 2007, although it was actually observed some six years earlier, in archival data from a pulsar survey of the Magellanic Clouds.
These events have durations of milliseconds and exhibit the characteristic dispersion sweep of radio pulsars.
They emit as much energy in one millisecond as the Sun emits in 10,000 years, but the physical phenomenon that causes them is unknown.
Theories range from highly magnetized neutron stars, blasted by gas streams near to a supermassive black hole, to suggestions that the burst properties are consistent with signatures of technology developed by an advanced civilization.
The discovery of FRB 190520 raises new questions about the nature of these mysterious objects and about their usefulness as tools for studying the nature of intergalactic space.
The FRB 190520 event occurred on May 20, 2019, and was found in data from that the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in November of that year.
Follow-up observations showed that, unlike many other FRBs, it emits frequent, repeating bursts of radio waves.
Observations with NSF’s Karl G. Jansky Very Large Array (VLA) in 2020 pinpointed the object’s location, and that allowed visible-light observations with the Subaru telescope in Hawaii to show that it is in the outskirts of a dwarf galaxy nearly 3 billion light-years from Earth.
The VLA observations also found that the object constantly emits weaker radio waves between bursts.
“These characteristics make this one look a lot like the very first FRB whose position was determined — also by the VLA — back in 2016,” said Dr. Casey Law, an astronomer in the Cahill Center for Astronomy and Astrophysics and the Owens Valley Radio Observatory at Caltech.
“That development was a major breakthrough, providing the first information about the environment and distance of an FRB. However, its combination of repeating bursts and persistent radio emission between bursts, coming from a compact region, set the 2016 object, called FRB 121102, apart from all other known FRBs, until now.”
“Now we have two like this, and that brings up some important questions.”
Optical, infrared, and radio images of the field of FRB 190520. Image credit: Niu et al., arXiv: 2110.07418.
The differences between FRB 190520 and FRB 121102 and all the others strengthen a possibility suggested earlier that there may be two different kinds of FRBs.
“Are those that repeat different from those that don’t? What about the persistent radio emission — is that common?” said Kshitij Aggarwal, a graduate student in the Department of Physics and Astronomy and the Center for Gravitational Waves and Cosmology at West Virginia University.
The astronomers suggest that there may be either two different mechanisms producing FRBs or that the objects producing them may act differently at different stages of their evolution.
One characteristic of FRB 190520 calls into question the usefulness of FRBs as tools for studying the material between them and Earth.
Astronomers often analyze the effects of intervening material on the radio waves emitted by distant objects to learn about that tenuous material itself.
One such effect occurs when radio waves pass through space that contains free electrons. In that case, higher-frequency waves travel more quickly than lower-frequency waves.
This effect, called dispersion, can be measured to determine the density of electrons in the space between the object and Earth, or, if the electron density is known or assumed, provide a rough estimate of the distance to the object. The effect often is used to make distance estimates to pulsars.
That didn’t work for FRB 190520. An independent measurement of the distance based on the Doppler shift of the galaxy’s light caused by the expansion of the Universe placed the galaxy at nearly 3 billion light-years from Earth.
However, the burst’s signal shows an amount of dispersion that ordinarily would indicate a distance of roughly 8 to 9.5 billion light-years.
“This means that there is a lot of material near the FRB that would confuse any attempt to use it to measure the gas between galaxies,” Aggarwal said.
“If that’s the case with others, then we can’t count on using FRBs as cosmic yardsticks.”
The astronomers speculated that FRB 190520 may be a ‘newborn,’ still surrounded by dense material ejected by the supernova explosion that left behind the neutron star.
As that material eventually dissipates, the dispersion of the burst signals also would decline.
Under the ‘newborn’ scenario, the repeating bursts also might be a characteristic of younger FRBs and dwindle with age.
“The FRB field is moving very fast right now and new discoveries are coming out monthly,” said Dr. Sarah Burke-Spolaor, an astronomer in the Department of Physics and Astronomy and the Center for Gravitational Waves and Cosmology at West Virginia University and the Canadian Institute for Advanced Research.
“However, big questions still remain, and this object is giving us challenging clues about those questions.”
The findings appear in the journal Nature.
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C.-H. Niu et al. 2022. A repeating fast radio burst associated with a persistent radio source. arXiv: 2110.07418
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