Scientists detect a rare circular polarization in active repeating fast radio bursts

Till now, only one repeating fast radio burst - FRB20201124A, has been reported with circular polarization.
Jijo Malayil
A magnetar type of neutron star
A magnetar type of neutron star

Naeblys/iStock 

The latest research by scientists at the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) has identified circular polarization in active repeating fast radio bursts (FRBs). The results were based on precise observations of the Five-hundred-meter Aperture Spherical Telescope (FAST).

The study findings, led by Prof. Li Di, were published in Science Bulletin

FRBs hold the distinction of emitting the most luminous radio flashes in the universe, with the energy released in one such incident rivaling the Sun's output "over a whole day or even a month to a year," according to phy.org

The first FRB was recorded in 2007, and around 600 such events have been identified since then. The majority of such events are only detected once, with less than five percent of all FRBs recording repeated activity. The active number of FRBs reported is under 10. 

A rare state of polarization 

Polarization is a fundamental property of FRBs, mainly consisting of electromagnetic waves, and its study helped researchers trace FRBs' radiation mechanisms and propagation processes. Linear polarization has been detected in almost all repeating FRBs. Circular polarization, however, remains relatively rare, according to phy.org

Till now, only one repeating FRB - FRB20201124A, has been reported with circular polarization. "Circular polarization has been detected in about half of non-repeating FRBs for which the polarization was detectable. Linear polarization has been detected in almost all repeating FRBs. In contrast, circular polarization is only seen in one repeating source FRB 20201124A," said the researchers in the study. 

Researchers could use data captured by FAST to record active episodes for these two FRBs, allowing the team to identify the precise characterization of their polarization.

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Only two out of the 600 known FRBs - FRB 20121102A and FRB 20190520B, are found to coincide with a compact persistent radio source. Researchers attribute their young age to their extreme activity. 

The frequency of polarizations

A thorough analysis of the data captured by the team concluded that only five percent of the bursts from the two FRBs contained circular polarization, with the largest degree of circular polarization being around 64 percent. 

Researchers attribute two possible mechanisms behind circular polarization, and these are various processes during propagation and a radiation mechanism intrinsic to the FRB source. "During propagation, multipath propagation and Faraday conversion could, in some circumstances, generate circular polarization." said the team. 

Multipath propagation is a result of electromagnetic radiation traveling in an inhomogeneous magneto-ionic environment. "Faraday conversion is a relatively weak effect and generates observable circular polarization only when propagating through an extremely magnetized region with magnetic field reversals or propagating through strongly magnetized plasma consisting of relativistic electrons," said the researchers.