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Recently, some fast radio bursts (FRBs) have been reported to exhibit complex and diverse variations in Faraday rotation measurements (RM) and polarization, suggesting that dynamically evolving magnetization environments may surround them. In this paper, we investigate the Faraday conversion (FC) effect in a binary system involving an FRB source and analyze the polarization evolution of FRBs. For a strongly magnetized high-mass companion binary, when an FRB with ∼100% linear polarization passes through the radial magnetic field of the companion star, the circular polarization (CP) component will be induced and oscillate symmetrically around the point with the degree of CP equal to zero, the rate and amplitude of the oscillation decrease as the frequency increases. The very strong plasma column density in the HMCBs can cause CP to oscillate with frequency at a very drastic rate, which may lead to depolarization. Near the superior conjunction of the binary orbit, the DM varies significantly due to the dense plasma near the companion, and the significant FC also occurs in this region. As the pulsar moves away from the superior conjunction, the CP gradually tends toward zero and then returns to its value before incidence. We also investigate the effect of the rotation of the companion star. We find that a sufficiently significant RM reversal can be produced at large magnetic inclinations and the RM variation is very diverse. Finally, we apply this model to explain some polarization observations of PSR B1744-24A and FRB 20201124A. [ABSTRACT FROM AUTHOR]

The extremely high brightness temperatures of pulsars and fast radio bursts (FRBs) require their radiation mechanisms to be coherent. Coherent curvature radiation from bunches has been long discussed as the mechanism for radio pulsars and recently for FRBs. Assuming that bunches are already generated in pulsar magnetospheres, we calculate the spectrum of coherent curvature radiation under a three-dimensional magnetic field geometry. Different from previous works assuming parallel trajectories and a monoenergetic energy distribution of electrons, we consider a bunch characterized by its length, curvature radius of the trajectory family, bunch opening angle, and electron energy distribution. We find that the curvature radiation spectra of the bunches are characterized by a multisegment broken power law, with the break frequencies depending on bunch properties and trajectory configuration. We also emphasize that in a pulsar magnetosphere, only the fluctuation of net charges with respect to the background (Goldreich–Julian) outflow can make a contribution to coherent radiation. We apply this model to constrain the observed spectra of pulsars and FRBs. For a typical pulsar (, P = 0.1 s), a small fluctuation of the net charge δnGJ ∼ 0.1nGJ can provide the observable flux. For FRBs, the fluctuating net charge may be larger due to its abrupt nature. For δnGJ ∼ nGJ, a neutron star with a strong magnetic field and fast rotation is required to power an FRB in the spindown-powered model. The requirement is less stringent in the cosmic comb model thanks to the larger cross section and compressed charge density of the bunch made by the external astrophysical stream that combs the magnetosphere. [ABSTRACT FROM AUTHOR]

Fast radio bursts (FRBs) are a new kind of extragalactic radio transients. Some of them show repeating behaviors. Recent observations indicate that a few repeating FRBs (e.g., FRB 121102) present time–frequency downward drifting patterns and nearly 100% linear polarization. Following the model of Dai et al. who proposed that repeating FRBs may originate from a slowly rotating, old-aged pulsar colliding with an asteroid belt around a stellar-mass object, we focus on the prediction of time–frequency drifting and polarization. In this scenario, the frequency drifting is mainly caused by the geometric structure of a pulsar magnetosphere, and the drifting rate–frequency index is found to be 25/17. On the other hand, by considering the typical differential mass distribution of incident asteroids, we find that an asteroid with mass m ≳ 1017 g colliding with the pulsar would contribute abundant gravitational energy, which powers an FRB. A broad frequency band of the FRBs would be expected, due to the mass difference of the incident asteroids. In addition, we simulate the linear polarization distribution for the repeating FRBs, and constrain the linear polarization with ≳30% for the FRBs with flux of an order of magnitude lower than the maximum flux. [ABSTRACT FROM AUTHOR]

Pulsars are magnetized rotating compact objects. They spin down due to magnetic dipole radiation and wind emission. If a photon has nonzero mass, the spin-down rate will be lower than in the zero-mass case. We show that an upper limit of the photon mass, i.e., , may be placed if a pulsar with period P is observed to spin down. Recently, a white dwarf (WD)–M dwarf binary, AR Scorpii, was discovered to emit pulsed broadband emission. The spin-down luminosity of the WD can comfortably power non-thermal radiation from the system. Applying our results to the WD pulsar with P = 117 s, we obtain a stringent upper limit of the photon mass between , assuming a vacuum dipole spindown, and , assuming spindown due to a fully developed pulsar wind. [ABSTRACT FROM AUTHOR]