Kavli Affiliate: Renxin Xu
| First 5 Authors: Wei-Yang Wang, Jin-Chen Jiang, Jiguang Lu, Heng Xu, Jiangwei Xu
| Summary:
Fast radio bursts (FRBs) are millisecond-duration signals, to be highly
dispersed at distant galaxies, the physical origin of which is still
challenging. Coherent curvature emission by bunches powered, e.g., by
starquakes, has already been proposed for repeating FRBs, with a friendly
nature of understanding the narrowband radiation exhibiting time-frequency
drifting. Recently, a highly active FRB source, FRB 20201124A, was reported to
enter a newly active episode and emit at least some highly circle-polarized
bursts. We revisit the polarized FRB emission here, investigating especially
the production mechanisms of a high circular polarization by deriving intrinsic
and propagation effects. The intrinsic mechanisms by invoking charged bunches
are approached in two scenarios of coherence: curvature radiation (CR) and
inverse Compton scattering (ICS), and consequently, a high circular
polarization could naturally be explained by the coherent summation of outcome
waves, generated or scattered by bunches, with different phases and electric
vectors. Cyclotron resonance can result in an absorption of R-mode photons at
lower altitude region of magnetosphere, and an FRB should then be emitted from
a higher region if the waves are of strong linear polarization. Circularly
polarized components could be produced from Faraday conversion exhibiting a
$lambda^3$-oscillation, but the average circular polarization fraction depends
only on the income wave, indicating a possibility of highly circle-polarized
income wave. The analysis could be welcome if extremely high (e.g., almost
100%) circular polarization from repeating FRBs would be detected in the
future. The production of a bulk of energetic bunches in pulsar-like
magnetosphere is discussed finally, which is relevant to the nature of FRB
central engine.
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