FAST Observations of an Extremely Active Episode of FRB 20201124A. III. Polarimetry

Abstract

As the third paper in the multiple-part series, we report the statistical properties of radio bursts detected from the repeating fast radio burst (FRB) source FRB 20201124A with the Five-hundred-meter Aperture Spherical radio telescope (FAST) during an extremely active episode between the 25th and the 28th of September 2021 (UT). We focus on the polarisation properties of 536 bright bursts with $\mathrm{S/N}>50$. We found that the Faraday rotation measures (RMs) monotonically dropped from $-579 \ {\rm rad \ m^{-2}}$ to $-605 \ {\rm rad \ m^{-2}}$ in the 4-day window. The RM values were compatible with the values ($-300$ to $-900\ {\rm rad \ m^{-2}}$ ) reported 4 month ago \citep{Xuheng2021arXiv}. However, the RM evolution rate in the current observation window was at least an order of magnitude smaller than the one ($\sim 500\ {\rm rad \ m^{-2}\, day^{-1}}$) previously reported during the rapid RM-variation phase, but is still higher than the one ($\le 1\ {\rm rad \ m^{-2} day^{-1}}$ ) during the later RM no-evolution phase. The bursts of FRB~20201124A were highly polarised with the total degree of polarisation (circular plus linear) greater than 90\% for more than 90\% of all bursts. The distribution of linear polarisation position angles (PAs), degree of linear polarisation ($L/I$), and degree of circular polarisation ($V/I$) can be characterised with unimodal distribution functions. During the observation window, the distributions became wider with time, i.e. with larger scatter, but the centroids of the distribution functions remained nearly constant. For individual bursts, significant PA variations (confidence level 5-$\sigma$) were observed in 33\% of all bursts. The polarisation of single pulses seems to follow certain complex trajectories on the Poincar\'e sphere, which may shed light on the radiation mechanism at the source or the plasma properties along the path of FRB propagation.

The Statistical Polarization Properties of Coherent Curvature Radiation by Bunches: Application to Fast Radio Burst Repeaters

“…Besides, most of the bursts with low CP for FRB 20201124A (Jiang et al 2022) indicate that most bunches have large opening angles to contribute to the low CP distribution (see the upper panel of Figure 5). Furthermore, the total degree of polarization is higher than 90% for most of the bursts of FRB 20201124A (Jiang et al 2022). It is inferred that the depolarization caused by the propagation effects basically can be neglected.…”

“…The simulation of flux CP shows that most of the bursts with high CP have relatively low flux. The burst energies span for more than 2 orders of magnitude, and artificial sharp cutoffs in the energy distributions are due to the instrumental limitation (Jiang et al 2022). We consider the flux threshold of FRBs 10 −2 times less than the maximum flux (the blue line).…”

“…Therefore, bursts with highly CP are hardly observed since the flux is too low. Some bursts with significant CP fraction can be as high as 75% (Xu et al 2022), and a small fraction of the bursts have CP fraction higher than 70% (Jiang et al 2022). It is suggested that The CP fraction as a function of the angle between LOS and the trajectory plane.…”

“…Surprisingly, the CP fraction can be up to 75% found in a good fraction of bursts (Xu et al 2022). Very recently, Jiang et al (2022) reported most of the bursts with a low CP degree and a small part of bursts have a CP fraction higher than 70%. The highly CP fraction may be caused by an intrinsic radiation mechanism (Tong & Wang 2022;Wang et al 2022bWang et al , 2022c or propagation effects (Beniamini et al 2022).…”

“…The profiles have been predicted by the rotating vector model (Radhakrishnan & Cooke 1969). A significant circular polarization (CP) was observed in an FRB repeater, FRB 20201124A Kumar et al 2022;Xu et al 2022;Jiang et al 2022). A burst with a significant CP fraction can be as high as 47% (Kumar et al 2022).…”