Polarization Angle Orthogonal Jumps in Fast Radio Bursts

TL;DR

Rapid 90° polarization-angle jumps in FRBs can arise from coherent or incoherent superposition of two orthogonal X- and O-mode waves. The paper shows that coherent superposition conserves the total polarization degree and yields jumps when the amplitude ratio satisfies |E_X|/|E_O| ≈ 1 with a phase offset Δφ ≈ π/2, while incoherent superposition allows jumps with less stringent polarization constraints. Observations of FRB 20201124A favor the incoherent, geometric scenario in which the magnetar’s rotation brings distinct emission regions into the line of sight, and they systematically assess magnetospheric mechanisms. Among intrinsic processes, inverse Compton scattering can plausibly produce two comparable modes and generate millisecond-scale PA jumps; A-O-mode conversion can also generate jumps under specific density-drop conditions, whereas curvature radiation and monster shocks struggle to produce orthogonal jumps. Plasma lensing outside the magnetosphere is deemed unlikely, reinforcing a close-in, magnetospheric origin for the observed PA jumps with implications for magnetar-based FRB models.

Abstract

Recently, polarization angle (PA) orthogonal jumps over millisecond timescales were discovered from three bursts of a repeating fast radio burst source FRB 20201124A by the FAST telescope. In general, PA jumps can arise from the coherent or incoherent superposition of two electromagnetic waves, with total polarization fraction remains constant in the former and not in the latter. The observations seem to be more consistent with incoherent superposition. The amplitudes of the two orthogonal modes are required to be comparable when jumps occur. We provide general constraints on FRB emission and propagation mechanisms based on the data. Physically, it is difficult to produce PA jumps through switching the dominance of the two orthogonal modes within millisecond timescales, and a geometric effect due to the source rotation is more plausible. This requires that the emission region be within the magnetosphere of a spinning central engine, likely a magnetar. The two orthogonal modes in different directions can arise when the source rotation brings two independent emission regions with different dominant modes successively into the line-of-sight, either due to intrinsic radiation mechanisms or the O-mode undergoing a delayed transparency because of the Alfvén-O-mode conversion. Splitting of emission directions for the two modes due to plasma birefringence is not easy to achieve when the plasma is moving relativistically. For intrinsic radiation mechanisms, curvature radiation always predicts $|E_{\rm X}/E_{\rm O}|\gtrsim1$, and is difficult to produce jumps; whereas inverse Compton scattering can achieve the conversion amplitude ratio $|E_{\rm X}/E_{\rm O}|=1$ to allow jumps to occur under special geometric configurations.

Figures (5)

• Figure 1: The polarization properties of the superposed waves after coherent superposition. Upper panel: PA and polarization degree as a function of amplitude ratio $\Lambda$ for $\Delta\phi=0$ (upper left) and $\Delta\phi=\pi/2$ (upper right). Two waves are initially 100% linear polarized. Lower left: PA and polarization degree as a function of relative phase $\Delta\phi$ with a fixed value of $\Lambda=1$. Lower right: The value of PA as a function of $\Lambda$ and $\Delta\phi$.
• Figure 2: The polarization properties of the superposed waves after incoherent superposition. Upper panel: PA and polarization degree as a function of amplitude ratio $\Lambda$ for $\theta_1=0$, $\theta_2=\pi/2$ and $\Delta\phi_1=\Delta\phi_2=0$ (upper left). $\theta_1=0$, $\theta_2=\pi/3$, $\Delta\phi_1=0$ and $\Delta\phi_2=\pi/2$ (upper right). Lower left: PA and polarization degree as a function of relative phase $\Delta\phi$ with a fixed value of $\Lambda=1$, $\theta_1=\theta_2=\pi/4$ and $\Delta\phi_1=\pi/2$ (lower left). The value of PA as a function of $\Lambda$ and $\Delta\phi$ for $\theta_1=\theta_2=\pi/4$ and $\Delta\phi_1=\pi/2$ (lower right).
• Figure 3: General physical and geometrical processes to generate polarization angle jumps of FRBs discussed in Section \ref{['sec:general constraint']}. For each process, close-in (inside the magnetosphere) and far-away (outside the magnetosphere) models are investigated. The favored processes are marked as green, possible process is marked as yellow and the disfavored processes are marked as red.
• Figure 4: A cartoon figure for three scenarios to produce PA jumps of FRBs including (1) intrinsic emission (upper panel), (2) A-O-mode conversion (lower left) and (3) Plasma birefringence (lower right) discussed in geometric effects. In both (2) and (3), the red line and blue line denote X-mode and O-mode of FRBs, respectively. The curved back dashed arrow represents the trajectory of the LOS, with its direction indicating how the LOS evolves over time. The solid black curves denote background magnetic field ($\pmb{B}_{\rm bg}$) in both (1) and (2). In (1), as the source rotates azimuthally, the emission transitions from an O-mode-dominated region to an X-mode-dominated region across the fixed LOS. In (2), two dashed black lines denote the emission radius ($r_{\rm em}$) and critical conversion radius ($r_c$), respectively. In (3), the cyan clumps represent plasma located ahead of the FRBs.
• Figure 5: Left panel: The amplitude ratio of X-mode to O-mode as a function of viewing angle $\theta_v$ for ICS. Right panel: Polarization angle as a function of time via incoherent superposition. The Lorentz factor $\gamma=100$, magnetar spin period $P=1 \ \rm s$ and $\phi_v=60^\circ$ are adopted for the right panel.