arXiv subjects

Rei Nishiura

Publications and source records attributed to Rei Nishiura.

7 recordsLinked to original sources

arXiv2026

Fast Radio Bursts from Induced Upscattering. I. Magnetar Polar Fireballs

The extreme brightness temperatures of fast radio bursts (FRBs) demand a coherent process that converts magnetar burst energy into millisecond radio emission. We propose that FRBs can be produced within magnetar magnetospheres by induced (stimulated) upscattering in a relativistic fireball outflow. In this paper, we focus on induced Compton upscattering in an electron--positron fireball launched from the polar region of a magnetar. Radiation from the X-ray burst accelerates the polar fireball outflow to Lorentz factors of $Γ\sim10$--$10^3$. Low-frequency seed waves, such as fast magnetosonic or Alfvén waves, entering the outflow from the side or against its motion undergo induced scattering and are Lorentz-boosted by the relativistic bulk motion from $ν_0\sim10\mathrm{kHz}$ to $ν_{\rm obs}\simΓ^2ν_0\sim\mathrm{GHz}$. Combining the polar fireball dynamics with the linear growth and nonlinear saturation of the induced scattering instability, we derive the FRB luminosity $L_{\rm FRB}^{\rm iso}$ and fractional spectral bandwidth $Δν/ν_{\rm c}$. By considering the possible range of pair densities and temperatures, we find that this polar channel can account for the luminosity of Galactic FRB~20200428 and reach luminosities comparable to those of the brightest extragalactic FRBs. If the pair-loaded region is narrower than the relativistic beaming angle $1/Γ$, the model also produces a narrow spectrum and predicts $L_{\rm FRB}^{\rm iso}\propto(Δν/ν_{\rm c})^2$, a trend consistent with observations of repeating FRBs. The induced upscattering model thus connects magnetar X-ray bursts to FRBs through a coherent emission process grounded in plasma physics and makes observationally testable predictions.

astro-ph.HE↗

arXiv2026

Induced Scattering of Fast Radio Bursts in Magnetar Magnetospheres

We investigate induced Compton/Brillouin scattering of electromagnetic waves in magnetized electron and positron pair plasma by verifying kinetic theory with Particle-in-Cell simulations. Applying this to fast radio bursts (FRBs) in magnetar magnetospheres, we find that the scattering--although suppressed by the magnetic field--inevitably enters the linear growth stage before the incident wave amplitude becomes comparable to the background magnetic field during its outward propagation through the magnetosphere. The subsequent evolution bifurcates: full scattering occurs when the density exceeds a critical value, whereas below it the scattering saturates and the FRB can escape without substantial induced-scattering attenuation. This nonlinear saturation eases the tension with observations of compact emission regions and may explain the observed diversity, including the presence or absence of FRBs associated with X-ray bursts.

astro-ph.HE↗

arXiv2026

Unified kinetic theory of induced scattering: Compton, Brillouin, and Raman processes in magnetized electron and positron pair plasma

We extend a unified theoretical framework for induced (stimulated) scattering-parametric instabilities of electromagnetic waves, including induced Compton, stimulated Brillouin, and stimulated Raman scattering (SRS) in strongly magnetized electron-positron pair plasma. By solving the dispersion relations derived from kinetic theory, taking into account the ponderomotive force due to the beat of incident and scattered waves, we obtain analytical expressions for the linear growth rates of the ordinary, neutral, and charged modes of density fluctuations. Our results clarify which type of scattering dominates under different thermal coupling, resonance, and density conditions. In strong magnetic fields, scattering of perpendicularly polarized waves is generally suppressed, but by different powers of the cyclotron frequency. Moreover, SRS, which is forbidden in unmagnetized electron and positron pair plasma, becomes possible in the charged mode. This framework enables a comprehensive evaluation of induced scattering in extreme astrophysical and laboratory plasma, such as fast radio burst (FRB) emission and propagation in magnetar magnetospheres.

astro-ph.HE↗

arXiv2026

Bell Instability and Cosmic-Ray Acceleration in Active Galactic Nuclei Ultrafast Outflow Shocks

We investigate magnetic-field amplification driven by the nonresonant hybrid (NRH or Bell) instability and its impact on cosmic-ray (CR) acceleration at reverse shocks of ultrafast outflows (UFOs) from active galactic nuclei (AGN). Previous kinetic studies by particle-in-cell simulations have demonstrated that when maximum CR energy is near the injection scale, NRH instability efficiently amplifies magnetic field up to the saturation level. However, the efficiency of NRH instability goes down as maximum energy increase since CR current is carried by escaping CRs near the maximum energy. We employ a one-dimensional MHD--CR framework solving telegraph-type diffusion--convection equations to trace the coupled evolution of CRs, magnetic fields, and shock dynamics under realistic parameters. We find a distinct transition with magnetic field strength: for weak background fields ($B_{0}\!\lesssim\!10^{-4}\,\mathrm{G}$), NRH instability efficiently amplifies upstream turbulence, driving a self-regulated state where $E_{\max}$ becomes independent of initial strength of magnetic turbulence. In contrast, for stronger background fields ($B_{0}\!\gtrsim\!10^{-3}\,\mathrm{G}$), the escaping CR current is too weak to drive NRH instability, and magnetic turbulence further decays through parametric instabilities, potentially reducing the acceleration efficiency. We give the physical interpretation for the transition and discuss conditions for PeV--EeV acceleration at UFO reverse shocks.

astro-ph.HE↗

arXiv2026

One-dimensional PIC Simulation of Induced Compton Scattering in Magnetized Electron-Positron Pair Plasma

We investigate induced Compton scattering of a circularly polarized Alfvén wave propagating in a magnetized electron-positron pair plasma using one-dimensional Particle-in-Cell (PIC) simulations. In this system, two distinct modes of density fluctuations, referred to as the charged mode and the neutral mode, are theoretically expected to arise through parametric instabilities. Our simulations confirm these predictions: in the charged mode, the electron and positron densities fluctuate oppositely (Langmuir-like), while in the neutral mode, the charge is Debye-screened and both species fluctuate in phase (acoustic-like). The linear growth rates obtained from the simulations are in good agreement with analytical estimates for both modes. We also find that, in some cases, the linear growth saturates before full scattering occurs, allowing the incident wave to propagate without significant attenuation. Our results allow us to determine whether induced Compton scattering grows linearly in magnetized pair plasmas, offering a foundation for studies of fast radio bursts and laser-plasma experiments.

astro-ph.HE↗

arXiv2025

Induced Compton scattering in magnetized electron and positron pair plasma

A formulation for the parametric instability of electromagnetic (EM) waves in magnetized pair plasma is developed. The linear growth rate of induced Compton scattering is derived analytically for frequencies below the cyclotron frequency for the first time. We identify three modes of density fluctuation: ordinary, charged, and neutral modes. In the charged mode, the ponderomotive force separates charges (electrons and positrons) longitudinally, in contrast to the nonmagnetized case. We also recognize two effects that significantly reduce the scattering rate for waves polarized perpendicular to the magnetic field: (1) the gyroradius effect due to the magnetic suppression of particle orbits, and (2) Debye screening for wavelengths larger than the Debye length. Applying this to fast radio bursts (FRBs), we find that these effects facilitate the escape of X-mode waves from the magnetosphere and outflow of a magnetar and neutron star, enabling 100\% polarization as observed. Our formulation provides a foundation for consistently addressing the nonlinear interaction of EM waves with magnetized plasma in astrophysics and laser physics.

astro-ph.HE↗

arXiv2024

Collective Thomson scattering in magnetized electron and positron pair plasma and the application to induced Compton scattering

We consider collective Thomson scattering of an incident X-mode wave (with the electric vector perpendicular to the background magnetic field) in magnetized electron and positron pair plasma. The collective effects do not exactly cancel out in contrast to the non-magnetized case. Still, the cross-section is comparable to the non-collective one, with the same suppression by the square of the cyclotron frequency in a strong magnetic field. The comparable cross-section holds even though the net current is nearly zero from the drift motion of electrons and positrons. The plasma response does not also affect the cross-section so much. The spectrum of the scattered wave in finite temperature plasma peaks at cyclotron overtones. Based on these results, we also estimate induced Compton scattering in strongly magnetized pair plasma. Implications for pulsars and fast radio bursts are discussed.

astro-ph.HE↗