A research team from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) discovered circular polarization in active repeating fast radio bursts (FAST). The research was led by Prof. Li Di of NAOC. LI Di made this discovery by studying precise observations of the Five-hundred-meter Aperture Spherical Telescope.  

Their research was published in the journal Science Bulletin.

The most luminous radio flashes in the universe are fast radio bursts (FRBs). The estimated equivalent energy of a single FRB event can rival the sun’s energy output over an entire day or even a month to a year.

Since the first FRB was reported in 2007, over 600 FRB sources have been discovered. The majority of them being detected only once. Fewer than 5% of all FRBs have repeated bursts, of which less than 10 are active.

Polarization is one of the fundamental properties of electromagnetic waves. It conveys critical information about the intrinsic properties of FRBs and their environments. Unpolarized light is emitted by many common light sources. It includes incandescent light bulbs and the majority of stars, including our sun. Almost all repeating FRBs exhibit linear polarization. Circular polarization, on the other hand, is uncommon. With circular polarization, only one repeating FRB which is FRB20201124A, has been reported.

The first known repeater is FRB20121102A. The first persistently active repeater is FRB20190520B. It was discovered by the Commensal Radio Astronomy FAST Survey (CRAFTS). They are the only repeaters discovered to be associated with persistent radio sources (PRSs). These discoveries could be attributed to their youth and hyperactive nature. FAST was able to capture extremely active episodes for these two FRBs. These allowed researchers for precise polarization characterization.

The researchers discovered circular polarization in less than 5% of the bursts from both FRBs using systematic data analysis. The highest degree of circular polarization was 64%. The high degree of circular polarization rules out multi-path propagation as a possible cause. Faraday conversion and a radiation mechanism intrinsic to the source are currently viable hypotheses.

Circular polarization appears to be more common in non-repeaters than in repeaters at the moment. Circular polarization conditions in repeating FRBs should thus be rarer.

The number of repeating FRBs with circular polarization increased from one to three in this study. The discovery of circular polarization in FRB20121102A, 20190520B, and 20201124A suggests that circular polarization is a common trait in repeating FRBs.

FAST’s continued systematic and precise polarization characterization will shed new light on the emission mechanism of FRBs. It eventually revealed the origin of such mysterious events in our dynamic universe.

More information: Yi Feng et al, Circular polarization in two active repeating fast radio bursts, Science Bulletin (2022). DOI: 10.1016/j.scib.2022.11.014