Circularly polarized radio emission from the repeating fast radio burst source FRB 20201124A

Abstract

The mechanism that produces fast radio burst (FRB) emission is poorly understood. Targeted monitoring of repeating FRB sources provides the opportunity to fully characterise the emission properties in a manner impossible with one-off bursts. Here we report observations of the source of FRB 20201124A, with the Australian Square Kilometre Array Pathfinder (ASKAP) and the Ultra-wideband Low (UWL) receiver at the Parkes 64-m radio telescope (Murriyang). The source entered a period of emitting bright bursts during early April 2021. We have detected 16 bursts from this source. One of the bursts detected with ASKAP is the brightest burst ever observed from a repeating FRB source with an inferred fluence of 640 ± 70 Jy ms. Of the five bursts detected with the UWL, none display any emission in the range 1.1-4 GHz. All UWL bursts are highly polarized, and we obtain an average Faraday rotation measure of −613 ± 2 rad m −2 for this source. In one of the UWL bursts, we see evidence of significant circularly-polarized emission with a fractional extent of 47%. Such a high degree of circular polarisation has never been seen before in bursts from repeating FRB sources. We also see evidence for variation in the polarization position angle in this UWL burst. Models for repeat burst emission will need to account for increasing diversity in the burst polarization properties.

Circular polarization of fast radio bursts in the curvature radiation scenario

“…The recently reported circular polarisation in the repeating FRB 20201124A can reach a fraction up to 75% (Hilmarsson et al 2021;Kumar et al 2021;Xu et al 2021). According to the calculations in the previous section, a general picture for the circular polarisation in FRBs is shown in Figure 6.…”

“…Previously, circular polarisation is reported in FRB 181112 (Cho et al 2020, circular polarisation fraction up to 34%), and in FRB 190611 (Day et al 2020, circular polarisation fraction of one subpulse up to 57%). Recently, significant circular polarisation is reported in one repeating FRB 20201124A (Hilmarsson et al 2021;Kumar et al 2021;Xu et al 2021). The circular polarisation fraction can be as high ⋆ E-mail: tonghao@gzhu.edu.cn as 75% (Xu et al 2021), which is very high even compared with that of pulsars and magnetars (Dai et al 2021).…”

“…Therefore, by applying the curvature radiation in neutron star magnetosphere to FRBs, we may tell under what condition the high circular polarisation of FRBs can be produced. Circular polarisation in combination with polarisation position angle swing may give us new clues about the physics of FRBs (Kumar et al 2021;Xu et al 2021).…”

Temporal Scattering, Depolarization, and Persistent Radio Emission from Magnetized Inhomogeneous Environments Near Repeating Fast Radio Burst Sources

“…Another active repeater FRB 20201124A (Lanman et al 2021;Kumar et al 2021;Nimmo et al 2021;Hilmarsson et al 2021) is found in a Milky-Way-sized, metal-rich, barred-spiral host galaxy at z = 0.098 (Xu et al 2021;Fong et al 2021;Ravi et al 2021;Piro et al 2021). This repeater indicates a significant, irregular, variation of the Faraday rotation over 36 days.…”

“…This repeater indicates a significant, irregular, variation of the Faraday rotation over 36 days. Some bursts appear to have circular polarization up to 75% (Kumar et al 2021;Xu et al 2021). In particular, the frequency spectra of both circular polarization and linear polarization of some bursts with moderate circular polarization show clear oscillating structures (Xu et al 2021), which might originate from the polarized absorption or Faraday conversion mechanism (Li et al 2022).…”