Abstract

ABSTRACT The repeating fast radio burst (FRB) source, FRB 20201124A, was found to be highly active in 2021 March and April. We observed the source with the Effelsberg 100-m radio telescope at 1.36 GHz on 2021 April 9 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 multicomponent 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 an 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.

Highlights

  • Fast radio bursts (FRBs) are short-duration radio transients of extragalactic origin

  • From the roughly 600 FRBs detected so far,1 repeating bursts have been detected from 24 FRB sources (Spitler et al 2016; CHIME/FRB Collaboration 2019, 2021b)

  • High levels of activity from a repeating FRB source discovered by CHIME, FRB 20201124A,2 were announced in 2021 March (CHIME/FRB Collaboration 2021a)

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Summary

INTRODUCTION

Fast radio bursts (FRBs) are short-duration radio transients of extragalactic origin. From the roughly 600 FRBs detected so far, repeating bursts have been detected from 24 FRB sources (Spitler et al 2016; CHIME/FRB Collaboration 2019, 2021b). FRB 20180916B’s RM is orders of magnitude lower than that of FRB 20121102A, consistent with a several hundred year old SNR (Marcote et al 2020) Bursts from both FRB 20121102A and FRB 20180916B are highly linearly polarized, with flat polarization angles across the bursts, and exhibit a downward drift in the burst frequency structure (CHIME/FRB Collaboration 2019; Hessels et al 2019; Chawla et al 2020).

O B S E RVAT IONSAND DATA P RO CESSING
BURST PROPERTIES AND ANALYSIS
Periodicity and burst rate
DM and RM
Polarimetry
Spectro-temporal properties
Findings
DISCUSSION
SUMMARY
Full Text
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