In Qu & Zhang (2024), we investigate that FRBs are generated via coherent inverse Compton scattering (ICS) off low-frequency X-mode electromagnetic waves (fast magnetosonic waves) by bunches at a distance of a few hundred times the magnetar radius.The following findings are revealed: (1) Crustal oscillations during a flaring event would excite kHz Alfv\'en waves. Fast magnetosonic waves with essentially the same frequency can be generated directly or be converted from Alfv\'en waves at a large radius, with an amplitude large enough to power FRBs via the ICS process. (2) The cross section increases rapidly with radius and significant ICS can occur at r$r\gtrsim100R_\star$ with emission power much greater than the curvature radiation power but still in the linear scattering regime. (3) The low-frequency fast magnetosonic waves naturally redistribute a fluctuating relativistic plasma in the charge-depleted region to form bunches with the right size to power FRBs. (4) The required bunch net charge density can be sub-Goldreich–Julian, which allows a strong parallel electric field to accelerate the charges, maintain the bunches, and continuously power FRB emission. (5) This model can account for a wide range of observed properties of repeating FRB bursts, including high degrees of linear and circular polarization and narrow spectra as observed in many bursts from repeating FRB sources.
The figure to the right shows fast magnetosonic waves launched from the magnetar crust quakes.
In Qu & Zhang (2023), we systematically investigate a variety of polarization mechanisms of FRBs within the magnetar theoretical framework considering two emission sites inside and outside the magnetosphere. For each site, we discuss both intrinsic radiation mechanisms and propagation effects. Inside the magnetosphere, we investigate the polarization properties of both coherent curvature radiation and inverse Compton scattering by charged bunches and conclude that both mechanisms produce 100 per cent linear polarization at an on-axis geometry but can produce circular polarization if the viewing angle is off axis. The lack of circular polarization for the majority of bursts requires that the bunches have a large transverse dimension size. Resonant cyclotron absorption within magnetosphere may produce high circular polarization if electrons and positrons have an asymmetric Lorentz factor distribution. Outside the magnetosphere, the synchrotron maser emission mechanism in general produces highly linearly polarized emission. Circular polarization would appear at off-beam angles but the flux is greatly degraded and such bursts are not detectable at cosmological distances. Synchrotron absorption in a nebula with ordered magnetic field may reduce the circular polarization degree. Cyclotron absorption in a strongly magnetized medium may generate significant circular polarization. Faraday conversion in a medium with field reversal can convert one polarization mode to another. The two absorption processes require stringent physical conditions. Significant Faraday conversion may be realized in a magnetized dense environment involving binary systems or supernova remnants.
In Qu, Kumar & Zhang (2022), we study the large-amplitude-wave effect of FRBs in the presence of a strong magnetic field. We find that FRB radiation travelling through the open field line region of a magnetar’s magnetosphere does not suffer much loss due to two previously ignored factors: (1) The plasma is likely to be flowing outwards at a high Lorentz factor in the outer magnetosphere. (2) The angle between the wave vector and the magnetic field line in the outer magnetosphere is likely of the order of 0.1 radian or smaller due in part to the intense FRB pulse that tilts open magnetic field lines so that they get aligned with the pulse propagation direction. Both these effects reduce the interaction between the FRB pulse and the plasma substantially.
We present the numerical results of the scattering optical depth as a function of gamma_p and theta_B. The three black curves are the effective optical depth (tau=1) curves for three multiplicity values xi=1, 10^2 and 10^4, respectively. There is a clear edge on the right side of the diagram with a slopt of theta_B~gamma_p^{-1}. This arises from the 𝐵𝑝 fact that the scattering cross section drops rapidly for omega_B'/omega'> a theta_B'.
In Kumar, Qu & Zhang (2024), we show that when the angular size of the emission region is larger than the Doppler beaming angle, the observed spectral width Delta{nu}/{nu_0}>0.58 due to the high latitude effects for a source outside the magnetosphere, even when the spectrum in the source's comoving frame is monochromatic. The angular size of the source for magnetospheric models of FRBs can be smaller than the Doppler beaming angle, in which case this geometric effect does not influence the observed bandwidth. We discuss various propagation effects to determine if any could transform a broad-spectrum radio pulse into a narrow-spectrum signal at the observer's location. We find that plasma lensing and scintillation can result in a narrow bandwidth in the observed spectrum. However, the likelihood of these phenomena being responsible for the narrow observed spectra with Delta{nu}/{nu_0}<0.58 in the fairly large observed sample of FRBs is exceedingly small.
In Qu & Zhang (2022), we study possible neutrino emission from FRB- emitting magnetars by developing a general theoretical framework. We consider three different sites for proton acceleration and neutrino emission i.e. within the magnetosphere, in the current sheet region beyond the light cylinder, and in relativistic shocks far away from the magnetosphere. The neutrino flux from FRB 200428 and its associated X-ray burst is calculated. The flux of the most optimistic case invoking magnetospheric proton acceleration is still∼4 orders of magnitude below the IceCube sensitivity. We also estimate the diffuse neutrino background from all FRB-emitting magnetars in the universe. The total neutrino flux of magnetars during their FRB-emission phases is a negligible fraction of observed diffuse emission even under the most optimistic magnetospheric scenario for neutrino emission.
