Exotic radio pulses from a galaxy far, far away

Astronomers have now captured numerous ultrashort but extremely powerful radio pulses from space. However, the cause of these fast radio pulses (FRBs) is unclear. Now the newly discovered source of repetitive radio pulses raises more questions. Because an object in a dwarf galaxy nearly three billion light-years away, it emits a mixture of strong, short pulses and much weaker constant radio emission. Until now, astronomers have only known such a combination from a different source of repeating radio bursts. What kind of cosmic object creates them, and why they are so different from other radio flares, is still unclear.

Astronomers have been wondering and discussing the origin of ultra-short cosmic radio flares for years. These fast radio pulses (FRBs) are very intense, but only last milliseconds. Initially, these pulses were only detected in data from the Parkes Radio Telescope in Australia, which is why scientists initially believed in technical or atmospheric disturbances. In the meantime, however, hundreds of such radio pulses were picked up by other radio telescopes. Many of them come from other galaxies, but some also come from our Milky Way. While most of these radio pulses only occur once, there are also a few “repeat perpetrators” – sources that occasionally emit entire bursts of radio pulses. One of them is FRB 121102, which has been known since 2016 and has a unique combination of features. According to measurements, it emanates from a compact object which, in addition to pulses, also emits weak but persistent radiation. However, it is not known what is going on.

The second exotic

Now Chenhui Niu of the National Radio Observatories in China and his colleagues have discovered another repetitive high-speed radio outburst that in many ways is strikingly similar to FRB 121102. Astronomers first captured these radio pulses in May 2019 using the 500-meter Aperture radio telescope Spherical (FAST) in China (FAST). The telescope detected four pulses in just 24 seconds. Another 85 pulses were later captured in a time window of approximately 18.5 hours. The distribution of radio pulses suggested that entire bursts of radio pulses are released in quick succession. “Like other recurring high-speed radio bursts, this FRB also shows a complex structure with multi-component profiles, sub-impulse drift and scattering,” reported the astronomers.

To locate the source of these pulses, the team tracked their region of origin with the Very Large Array radio telescopes in the United States, and searched for the optical equivalent of the radio source with the Canada-France-Hawaii telescope. The analysis revealed that the source of these new radio pulses, dubbed FRB 190520, lies at the edge of a dwarf galaxy about three billion light-years away. Moreover, like FRB 121102, it also generates a constant radio emission parallel to the pulses. However, this does not appear to be due to the star-forming activity of the host galaxy, the team explains. Instead, the characteristics of the radio pulses suggest that they emanate from a compact object. “More than a dozen sources of high-speed radio pulses have already been located prior to FRB 190520, including five sources of repeating FRBs. But so far only FRB 121102 has been associated with a compact, durable source, ”write Niu and his colleagues.

Young neutron star as an originator?

As a result, astronomers now know two sources of repeating radio bursts that differ from all other known FRBs. “That raises some important questions,” says co-author Casey Law of the California Institute of Technology. “Are the causes of these repeaters fundamentally different from the causes of single radio pulses? What about persistent radio emission – is it an exception or a common phenomenon? ” Until now, astronomers can only speculate which space objects produce ultrashort radio pulses. Due to the polarization and scattering of some FRBs, magnetars, neutron stars with strong magnetic fields, have so far been considered potential FRB makers. But a neutron star in the immediate vicinity of a black hole can also generate the necessary energy.

As reported by astronomers, the scattering coefficients of FRB 190520 also suggest that these newly discovered radio pulses come from the environment with the highest electron density observed for such a radio burst. This may indicate that their creator is still surrounded by complex plasma clouds, such as those that form immediately after a particularly bright supernova. The source of FRB 190520 could therefore be a newborn neutron star, still surrounded by the remnants of the parent star and its explosion, Niu and his colleagues speculate. They hope that the discoveries of faster radio flares will shed more light on the causes of these still puzzling phenomena. “There is a lot going on in the FRB field right now and new discoveries are made almost every month,” says co-author Sarah Burke-Spolaor of West Virginia University.

Source: Chenhui Niu (Chinese Academy of Sciences, Beijing) et al., Nature, doi: 10.1038 / s41586-022-04755-5

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