Strange radio burst raises new questions

Astronomers have found just the second example of a highly active, repetitive Fast Radio Burst (FRB) with a compact source of fainter but persistent radio emission between bursts. The discovery raises new questions about the nature of these mysterious objects, as well as their usefulness as tools for studying the nature of intergalactic space. The scientists used the Karl G. Jansky Very Large Array (VLA) and other National Science Foundation telescopes to study the object, which was first discovered in 2019.

The object, called FRB 190520, was found by the 500-meter-long Aperture Spherical Radio Telescope (FAST) in China. An eruption of the object occurred on May 20, 2019, and was found in data from that telescope in November of that year. Follow-up observations with FAST showed that, unlike many other FRBs, it emits frequent, repetitive bursts of radio waves.

Observations with the VLA in 2020 pinpointed the object’s location, enabling visible-light observations with the Subaru telescope in Hawaii to show it is at the edge of a dwarf galaxy nearly 3 billion light-years from Earth. . The VLA observations also showed that the object continuously emits weaker radio waves between bursts.

“These features make it very similar to the very first FRB whose position was determined — including by the VLA — in 2016,” said Caltech’s Casey Law. This development was a major breakthrough and provided the first information about the environment and distance of an FRB. However, the combination of repeating bursts and sustained inter-burst radio emissions from a compact area set the 2016 object, FRB 121102, apart from all other known FRBs thus far.

“Now we have two like this, and that raises some important questions,” Law said. Law is part of an international team of astronomers reporting their findings in the journal Nature.

The differences between FRB 190520 and FRB 121102 and all others reinforce a previously suggested possibility that there could be two different types of FRBs.

“Are those who repeat different from those who don’t? What about the ongoing radio emission – is that common?” said Kshitij Aggarwal, a graduate student at West Virginia University (WVU).

The astronomers suggest that there are either two different mechanisms that produce FRBs, or that the objects that produce them may behave differently at different stages of their evolution. Leading candidates for the sources of FRBs are the super-dense neutron stars that remain after a massive star explodes as a supernova, or neutron stars with ultra-strong magnetic fields, called magnetars.

A feature of FRB 190520 casts doubt on the usefulness of FRBs as tools for studying the material between them and the Earth. Astronomers often analyze the effects of intervening material on the radio waves emitted by distant objects to learn more about that tenuous material itself. Such an effect occurs when radio waves pass through space containing free electrons. In that case, high-frequency waves travel faster than low-frequency waves.

This effect, called dispersion, can be measured to determine the density of electrons in the space between the object and the Earth, or, if the electron density is known or assumed, give a rough estimate of the distance to the object. The effect is often used to make distance estimates to pulsars.

That didn’t work for FRB 190520. An independent measurement of distance based on the Doppler shift of light from the galaxy, caused by the expansion of the universe, placed the galaxy nearly 3 billion light-years from Earth. However, the burst’s signal shows an amount of dispersion that would normally indicate a distance of about 8 to 9.5 billion light-years.

“This means 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, we can’t count on FRBs to be used as cosmic metrics,” he added.

The astronomers speculated that FRB 190520 could be a “newborn,” still surrounded by dense material expelled from the supernova explosion that left the neutron star. As that material eventually disappears, so would the propagation of the burst signals. In the “newborn” scenario, they said, the repetitive bursts could also be a feature of younger FRBs and decrease with age.

“The FRB field is moving very fast right now and new discoveries are coming out every month. However, there are still big questions and this object gives us challenging clues about those questions,” said Sarah Burke-Spolaor of WVU.

The National Radio Astronomy Observatory is a National Science Foundation facility operated under a collaborative agreement by Associated Universities, Inc.

Reference:

  1. Niu, CH., Aggarwal, K., Li, D. et al. A repetitive fast radio burst associated with a sustained radio source. Nature, DOI 2022: 10.1038/s41586-022-04755-5
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