Polarization properties of FRB 20201124A from detections with the Effelsberg 100-m radio telescope

Abstract

The repeating FRB source, FRB 20201124A, was found to be highly active in March and April 2021. We observed the source with the Effelsberg 100-m radio telescope at 1.36 GHz on 9 April 2021 and detected 20 bursts. A downward drift in frequency over time is clearly seen from the majority of bursts in our sample. A structure-maximizing dispersion measure (DM) search on the multi-component bursts in our sample yields a DM of 411.6 ± 0.6 pc cm−3. We find that the rotation measure (RM) of the bursts varies around their weighted mean value of -601 rad m−2 with a standard deviation of 11.1 rad m−2. This RM magnitude is 10 times larger than the expected Galactic contribution along this line of sight (LoS). We estimate a LoS magnetic field strength of 4–6 μG, assuming that the entire host galaxy DM contributes to the RM. Further polarization measurements will help determine FRB 20201124A’s RM stability. The bursts are highly linearly polarized, with some showing signs of circular polarization, the first for a repeating FRB. Their polarization position angles (PAs) are flat across the burst envelopes and vary between bursts. We argue that the varying polarization fractions and PAs of FRB 20201124A are similar to known magnetospheric emission from pulsars, while the observed circular polarization, combined with the RM variability, is hard to explain with Faraday conversion. The high linear polarization fractions, flat PAs, and downward drift from FRB 20201124A bursts are similar to previous repeating sources, while the observed circular polarization is a newly seen behaviour among repeaters.

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.…”

“…In order to understand the circular polarisations in FRBs (Cho et al 2020;Day et al 2020;Hilmarsson et al 2021;Kumar et al 2021;Xu et al 2021) in the curvature radiation scenario, we conclude that:…”

“…( 7)). If the neutron star inside FRB 121102 has a small rotational period, and if the neutron star inside FRB 20201124A has a large rotational period, it may explain why these two FRBs have different circular polarisation characteristics (low for FRB 121102, Michilli et al 2018; high for FRB 20201124A, Hilmarsson et al 2021, Kumar et al 2021, Xu et al 2021. This may also explain why magnetars have higher circular polarisation than normal pulsars (Dai et al 2021, Figure 1 there).…”

“…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).…”

“…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). The circular polarisation may be of intrinsic origin (Hilmarsson et al 2021), especially for the bursts with high circular polarisation fraction (Xu et al 2021).…”