Astronomers have now captured numerous ultra-short but extremely powerful radio pulses from space. However, the cause of these fast radio bursts (FRB) is unclear. Now a newfound source of repeated radio bursts raises more questions. Because the object in a dwarf galaxy almost three billion light-years away emits a mixture of strong, short pulses and a much weaker sustained radio emission. So far, astronomers have only known such a combination from another source of repeated radio bursts. What kind of cosmic object creates them and why they differ so much from other radio bursts is still unclear.
Astronomers have been puzzling and discussing the origin of ultrashort cosmic radio bursts for years. These Fast Radio Bursts (FRB) 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 researchers initially believed a technical or atmospheric disturbance. In the meantime, however, hundreds more such radio pulses have also been captured by other radio telescopes. Many of them come from other galaxies, but some also from our Milky Way. While most of these radio pulses occur only once, there are also a few “repeat offenders” – sources that sporadically emit entire series of radio bursts. One of them is FRB 121102, which has been known since 2016 and had a unique combination of features. According to measurements, it emanates from a compact object that, in addition to the pulses, also emits weak but persistent radiation. What this is about, however, is unknown.
A second exotic
Now Chenhui Niu of the National Radio Observatories of China and his colleagues have discovered another repetitive fast radio burst that bears striking similarities to FRB 121102 in many respects. The astronomers captured these radio pulses for the first time in May 2019 with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in China (FAST). The telescope detected four pulses in just 24 seconds. Another 85 pulses were later captured in a time window of around 18.5 hours. The distribution of the radio pulses suggested that entire series of radio bursts are released in quick succession. “Similar to other repetitive fast radio bursts, this FRB also shows a complex structure with multi-component profiles, sub-pulse drift and scattering,” the astronomers report.
To locate the source of these pulses, the team targeted the region of origin using the Very Large Array radio telescopes in the United States and also searched for an optical counterpart of the radio source using the Canada-France-Hawaii telescope. The analyzes revealed that the source of these new radio pulses, dubbed FRB 190520, lies at the edge of a dwarf galaxy around three billion light-years away. In addition, similar to 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, as the team explains. Instead, the characteristics of the radio pulses suggest that they emanate from a compact object. “More than a dozen sources of fast radio bursts have already been located prior to FRB 190520, including five sources of repeated FRBs. But only FRB 121102 has been associated with a compact, persistent source so far,” write Niu and his colleagues.
Young neutron star as originator?
With this, astronomers now know two sources of repeated radio bursts that differ from all other known FRBs. “This raises some important questions,” says co-author Casey Law of the California Institute of Technology. “Are the causes of these repeaters fundamentally different from those of the individual radio bursts? And what about the persistent radio emission – is it an exception or common?” So far, astronomers can only speculate about which cosmic objects produce the ultrashort radio pulses. Due to the polarization and scattering of some FRBs, magnetars, neutron stars with strong magnetic fields, have so far been considered possible originators of the FRBs. But a neutron star in the immediate vicinity of a black hole could also produce the necessary energy.
The scattering rates of FRB 190520 also suggest that these newly discovered radio pulses come from an environment with the highest electron density observed for such a radio burst so far, as the astronomers report. This could indicate that their originator is still surrounded by complex plasma clouds, such as those that form immediately after a particularly luminous supernova. The source of FRB 190520 could therefore be a newborn neutron star still surrounded by the remains of its parent star and its explosion, Niu and his colleagues speculate. They hope that discoveries of more fast radio bursts will shed more light on the causes of these still-puzzling phenomena. “There’s a lot going on in the FRB field right now, and new discoveries are being made almost monthly,” 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