Astronomers have discovered the second example of a highly active, repeating “fast radio emission” or 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 and also about their usefulness as tools for studying the nature of intergalactic space.

Scientists used the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) and other telescopes to study the object, first discovered in 2019.

The object, called FRB 190520, was spotted by China’s Five Hundred Meter Aperture Spherical Radio Telescope (FAST). An explosion of the object occurred on May 20, 2019, and was discovered in data from this telescope in November of that year. Subsequent observations with FAST showed that, unlike many other FRBs, it emits frequent and repeated bursts of radio waves.

VLA observations in 2020 pinpointed the object’s location, and this allowed visible-light observations with the Subaru Telescope in Hawaii to show that it is on the outskirts of a dwarf galaxy nearly 3 billion light-years from Earth. VLA observations also found that the object constantly emits weaker radio waves between bursts.

New questions about the nature of these mysterious objects

“These features make it very similar to the first FRB whose position was determined — also by the VLA — in 2016,” said Caltech’s Casey Law. That evolution was a major advance, providing the first information about the environment and distance of an FRB. However, its combination of repeated 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 so far.

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

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

“Are repeats different from non-repeats? What’s going on with the persistent radio emission—is it common?” said Kshitij Aggarwal, a student at West Virginia University (WVU).

The astronomers suggest that there could either be two different mechanisms that produce FRBs, or that the objects that produce them could act differently at different stages of their evolution, they write EurekAlert.

Frequent and repeated bursts of radio waves

The main candidates for FRB sources are superdense neutron stars, left over after a massive star explodes as a supernova, or neutron stars that have ultra-strong magnetic fields, called magnetars.

One feature of FRB 190520 calls into question the usefulness of FRBs as tools for studying the material between them and Earth. Astronomers often look at the effects of the intervening material on the radio waves emitted by distant objects to learn more about that thin material itself.

Such an effect occurs when radio waves pass through space containing free electrons. In this case, higher frequency waves travel faster than lower frequency ones.

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

Can we count on using FRBs as a cosmic benchmark?

That didn’t work for FRB 190520. An independent distance measurement based on the displacement of the galaxy’s light caused by the expansion of the Universe placed the galaxy nearly 3 billion light-years from Earth. However, the burst signal shows a scatter 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 confound any attempt to use it to measure the gas between galaxies,” Aggarwal said. “If it’s the same for others, then we can’t count on using FRBs as a cosmic benchmark,” he added.

Astronomers have speculated that FRB 190520 may be a “newborn,” still surrounded by dense material ejected from the supernova explosion that left the neutron star behind. As this material eventually dissipates, the dispersion of the explosion signals would also decrease. Under the “newborn” scenario, they said, repeated bursts could also be a feature of younger FRBs and decrease with age.

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