Mechanism of Circular Polarization in Giant Pulses and Fast Radio Bursts

• Published in: Astrophysical journal. Letters Vol. 974; no. 1; p. L21
• Main Author: Wu, Hui-Chun
• Format: Journal Article
• Language: English
• Published: Austin The American Astronomical Society 01-10-2024; IOP Publishing
• Subjects: Asymmetry; Circular polarization; Cyclotron resonance; Escape velocity; Faraday effect; Linear polarization; Magnetic fields; Magnetic resonance; Plasma astrophysics; Polar caps; Polar regions; Polarization; Propagation modes; Propagation velocity; Pulsar magnetospheres; Pulsars; Pulse propagation; Radio bursts; Radio pulsars; Radio transient sources
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/ad8154

Abstract Some giant pulses and fast radio bursts (FRBs) exhibit notable circular polarization, which remains unexplained and carries significant implications for their emission mechanisms. In this study, we identify multiple nanoshot pairs uniformly spaced by approximately 21 μ s within a giant pulse emitted by the Crab pulsar. Among these pairs, a subset displays left-hand and right-hand circular polarization in two distinct nanoshots. We propose that the occurrence of such nanoshot pairs with dual circular polarizations arises from the fragmentation of a linearly polarized nanoshot along the magnetic field lines under the extreme Faraday effect, leveraging highly asymmetrical pair plasma and the ultra-intense field of nanoshots. The asymmetry in pair plasmas is likely linked to discharge activities in pulsars. Moreover, the intense field of nanoshots induces cyclotron resonance within the magnetosphere, effectively slowing down the propagation velocity of the circularly polarized mode. Our findings suggest that Crab giant pulses composing nanoshots originate in its polar cap region and escape the magnetosphere along the polar magnetic field. This mechanism can also elucidate the origin of circular polarization in some FRBs and thus lends support to their magnetospheric origin.