Tuesday, July 5, 2022

The second example of a highly active, repeating Fast Radio Burst detected

The discovery raises new questions about the nature of these mysterious objects.

Using the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in China, astronomers discovered the second example of a highly active, repeating Fast Radio Burst (FRB). The object first discovered in 2019, has a compact source of weaker but persistent radio emissions between bursts.

A burst from the object called FRB 190520 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, repeating bursts of radio waves. The discovery raises new questions about the nature of these mysterious objects and their usefulness as tools for studying the nature of intergalactic space.

Observations with the VLA pinpointed the object’s location. These observations 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.

Casey Law of Caltech said, “These characteristics make this one look a lot like the very first FRB whose position was also determined by the VLA in 2016. That development was a 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.”

FRB 190520
VLA image of Fast Radio Burst FRB 190520 (red), combined with optical image, when the FRB is bursting. Credit: Niu, et al.; Bill Saxton, NRAO/AUI/NSF; CFHT

Law said, “Now we have two like this, which raises some important questions.”

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.

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

According to scientists, FRBs could be produced by two separate mechanisms or objects that behave differently at different stages of their existence. The superdense neutron stars left over after a giant star explodes as a supernova, or neutron stars with ultra-strong magnetic fields, known as magnetars, are the most likely producers of FRBs.

One characteristic of FRB 190520 calls into question the usefulness of FRBs as tools for studying the material between them and Earth. Astronomers frequently study the effects of intervening material on radio waves radiated by faraway objects to understand more about that flimsy material. When radio waves flow across space containing free electrons, one such effect called dispersion happens, in which higher-frequency waves travel faster than lower-frequency waves in this scenario.

Measuring dispersion can help 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.

For FRB 190520, that didn’t work. The galaxy is roughly 3 billion light-years away from Earth, according to an independent assessment based on the Doppler shift of the galaxy’s light induced by the Universe’s expansion. On the other hand, the burst’s signal has a level of dispersion that would normally indicate a distance of 8 to 9.5 billion light-years.

Aggarwal said, “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. If that’s the case with others, then we can’t count on using FRBs as cosmic yardsticks.”

According to astronomers, FRB 190520 may be a newborn surrounded by dense material ejected by the supernova explosion that left behind the neutron star.

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

Journal Reference:

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

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