arXiv subjects

Kunihito Ioka

Publications and source records attributed to Kunihito Ioka.

At least 19 recordsLinked to original sources

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

Induced Scattering of Strong Waves in Pair Plasmas

We study induced (stimulated) scattering of linearly polarized, strong electromagnetic waves in pair plasmas, which is crucial for understanding the propagation of fast radio bursts (FRBs). Magnetars are the most promising progenitors of FRBs, and FRBs propagate through the magnetar wind and successfully escape before being significantly scattered. We revisit the steady-state solution of linearly polarized electromagnetic waves in pair plasmas with arbitrary amplitude, and demonstrate that the nonlinearity is characterized by the nonlinearity parameter $a_0ω_{pe}/ω_0$ rather than the dimensionless amplitude $a_0$, where $ω_{pe}$ is the electron plasma frequency and $ω_0$ is the wave frequency. We follow the time evolution of the steady-state solution for the linear regime $a_0ω_{pe}/ω_0 \ll 1$ by performing one-dimensional particle-in-cell simulations, and show that the conventional linear analysis of induced scattering assuming $a_0 \ll 1$ is applicable even for $a_0 > 1$ when the Lorentz boost due to the plasma motion in the incident wave is considered. The saturation level is controlled by $a_0ω_0/ω_{pe}$, which corresponds to the ratio of the wave energy to the plasma energy, and the incident wave is hardly scattered for $a_0ω_0/ω_{pe} \gg 1$. We discuss the application of our results to FRBs.

astro-ph.HE↗

arXiv2026

Revisiting the Growth Rate of the Relativistic Tearing Instability: The Role of the Non-ideal MHD Structure

Magnetic reconnection in magnetically dominated pair plasmas is a key process in high-energy astrophysical systems. We revisit the relativistic tearing instability in a Harris current sheet and derive an improved analytical expression for its linear growth rate and the most unstable wavenumber. The key modification is the treatment of the vector potential perturbation in the non-ideal magnetohydrodynamic (MHD) region. Instead of the conventional constant-A approximation, we use an extrapolated-A approximation, in which the ideal-MHD solution is linearly extrapolated into the non-ideal region. Comparison with two-dimensional particle-in-cell simulations shows that the revised theory improves the prediction of the most unstable wavenumber. The improvement is most pronounced at low particle drift velocities, where the particle gyroradius is smaller than the current-sheet thickness and the fastest-growing mode shifts to longer wavelength. The resulting analytical expressions provide an updated benchmark for magnetically dominated reconnection and its applications to high-energy astrophysical plasmas, including gamma-ray bursts and fast radio bursts.

physics.plasm-ph↗

arXiv2026

Mass and Spin Growth of Very Massive Stars in Star Clusters Potentially Associated with Little Red Dots

Using gravitational $N$-body simulations, we investigate the evolution of mass and spin for very massive stars (VMSs) in dense star clusters, which could subsequently evolve into Little Red Dots (LRDs). Our results show that VMS masses can reach $10^3$--$10^4\,M_\odot$, depending on the initial conditions of the host clusters. Notably, the VMS mass increases by up to a factor of three when accounting for the bloated state at the Hayashi track induced by stellar collisions, provided that this state is maintained at accretion rates exceeding $3 \times 10^{-2}\,M_\odot\,{\rm yr}^{-1}$. In all cases, the spin of the VMS, when normalized to the dimensionless black hole (BH) spin parameter, exceeds $10$, although the mass and spin of VMSs after the post-main sequence phase could be decreased by the stellar evolution process. We nonetheless demonstrate that VMSs formed in dense star clusters can be highly spinning. Such a rapidly spinning VMS is expected to collapse into an intermediate-mass BH surrounded by a massive accretion disk. This BH-disk system could trigger powerful explosions and emit burst gravitational waves, similar to those observed in GW190521 and GW231123, for which the remnant BH masses are estimated to be $\gtrsim 100\,M_\odot$.

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

Cross-correlating galaxies and cosmic dispersion measures: Constraints on the gas-to-halo mass relation from 2MASS galaxies and 133 localized fast radio bursts

We conduct a cross-correlation analysis between large-scale structures traced by the Two Micron All Sky Survey (2MASS) galaxy catalog and the cosmic dispersion measures of 133 localized fast radio bursts (FRBs). The cross-correlation signal is measured as a function of the comoving separation $R$ between 2MASS galaxies and background FRB sightlines, making full use of the available redshift information for both datasets. Our measurements are consistent with a null detection over the range $0.01 < R\, [h^{-1}\mathrm{Mpc}] < 1$. Using a halo-based model in which free-electron density profiles are drawn from the hydrodynamical simulation IllustrisTNG-300 (TNG300), we show that the null signal at $R \sim 0.01\, h^{-1}\mathrm{Mpc}$ is inconsistent with the TNG300 prediction. This discrepancy indicates that the hot-gas mass fraction in halos with masses of $10^{12-13}\, M_\odot$ hosting 2MASS galaxies must be lower than that predicted by TNG300. A simple phenomenological modification of the TNG300 model suggests that the hot-gas mass fraction in halos of $10^{12-13}\, M_\odot$ should be below $\sim 10\%$ of the global baryon fraction in the nearby universe, implying the need for stronger feedback in this mass range. Our constraints are consistent with those inferred from X-ray emission and Sunyaev-Zel'dovich measurements in galaxies, while providing a direct estimate of the hot-gas mass fraction that does not rely on electron-temperature measurements. These results demonstrate that galaxy-FRB cross correlations offer a powerful probe of feedback processes in galaxy formation.

astro-ph.CO↗

arXiv2026

Measurement of angular cross-correlation between the cosmological dispersion measure and the thermal Sunyaev--Zeldovich effect

The dispersion measures (${\rm DMs}$) from fast radio bursts (FRBs) and the thermal Sunyaev--Zeldovich (tSZ) effect probe the free-electron density and thermal pressure, respectively, in the intergalactic medium (IGM) and the intervening galaxies and clusters. Their combination enables disentangling the gas density and temperature. In this work, we present the first detection of an angular cross-correlation between the ${\rm DMs}$ and the Compton $y$ parameter of the tSZ effect. The theoretical expectation is calculated using the halo model $\texttt{HMx}$, calibrated with hydrodynamic simulations. The observational cross-correlation is measured over angular separations of $1^\prime$--$1000^\prime$ using the ${\rm DMs}$ from $133$ localized FRBs and the $y$-maps from the Planck satellite and the Atacama Cosmology Telescope (ACT). We detect a positive correlation with amplitudes of $\mathcal{A}=2.01 \pm 0.50$ ($4.0 σ$) for Planck and $\mathcal{A}=1.23 \pm 0.82$ ($1.5 σ$) for ACT, where $\mathcal{A}=1$ corresponds to the theoretical prediction of the Planck 2018 $Λ$CDM cosmology. Assuming an isothermal gas, the measured amplitude implies an average electron temperature of $\approx 2 \times 10^7 \, {\rm K}$. The correlation is highly sensitive to the matter clustering parameter $σ_8$ and to baryon feedback, and its dependence on other cosmological and astrophysical parameters -- such as the ionized fraction and the Hubble constant -- differs from that of the ${\rm DM}$ alone. This suggests that future joint analyses of the ${\rm DMs}$ and the tSZ effect could help break degeneracies among these parameters.

astro-ph.CO↗

arXiv2026

Little Red Dots as Hidden Neutrino Sources

Little Red Dots (LRDs) are enigmatic, compact, red galaxies at high redshift, $z\sim 4$-$7$, discovered by the James Webb Space Telescope. Broad emission lines in the absence of X-ray and radio counterparts suggest that they host accreting supermassive black holes embedded in dense gaseous envelopes. This black-hole-envelope configuration facilitates efficient photohadronic interactions and neutrino production. Remarkably, their observed source number density and luminosity are compatible with the energetics of the diffuse neutrino background. We consider that relativistic jets and outflows are launched from the black hole and propagate through low-density polar funnels within envelopes, where particle acceleration and neutrino emission occur. This leads to LRDs being effectively hidden sources. Our analytic and numerical calculations show that, in an optimistic scenario, LRDs can contribute $\sim 30\%$ of the observed diffuse background at TeV$-$sub-PeV energies, predominantly through photomeson production. At high neutrino energies, $\gtrsim 10^{5.5}~{\rm GeV}$, inverse-Compton cooling of muons modifies the resulting flavor ratio, providing a distinctive diagnostic for IceCube-Gen2 and other upcoming neutrino telescopes.

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

Black Hole Envelopes in Little Red Dots

Recent observations by the James Webb Space Telescope have uncovered a population of compact, red object ($z\sim 4\text{--}7$) known as little red dots (LRDs). The presence of broad Balmer emission lines indicates active galactic nuclei powered by supermassive black holes (BHs), while LRDs exhibit unusually weak X-ray and radio emission and low variability, suggesting super-Eddington accretion that obscures the central engine. We suggest that such an extreme accretion disc inevitably drives strong outflows, which would disrupt the LRDs themselves unless confined within the nuclear region -- posing a general feedback problem for overmassive BHs. To resolve this, we propose that the BH is embedded in a massive, optically thick envelope that gravitationally confines the outflow, making any outflow a no-go. This envelope, powered by accretion on to the BH, radiates at nearly the Eddington limit, and is sustained by an infall of the interstellar medium at a rate on the order of $\sim 1 M_{\odot}~{\rm yr}^{-1}$. A photosphere emerges either within the envelope or in the infalling medium, with a characteristic temperature of $5000$ - $7000 {\rm K}$, near the Hayashi limit. The resulting blackbody emission naturally explains the red optical continuum of the distinct V-shaped spectrum observed in most LRDs. Furthermore, the dynamical time-scale at the photosphere, $\sim 0.01~{\rm pc}$, is consistent with the observed year-scale variabilities. The nuclear structure and spectral features of LRDs are shaped by this envelope, which not only regulates feedback but also acts as a gas reservoir that sustains rapid BH growth in the early universe.

astro-ph.HE↗

arXiv2025

Confined Circumstellar Material as a Dust Formation Site in Type II Supernovae

We propose a model for dust formation in Type II supernovae (SNe) interacting with confined circumstellar material (CSM), motivated by recent time-domain surveys that have revealed a substantial fraction of SN progenitors to be surrounded by CSM ejected shortly before core-collapse. We simulate the pre-SN mass eruption and the resulting confined CSM using the open-source code CHIPS, and follow the subsequent evolution of the SN ejecta and its interaction with the CSM. We show that a cold dense shell (CDS) is formed at the radiative shock under a wide range of conditions and later undergoes rapid adiabatic cooling during free expansion, leading to efficient dust condensation. The resulting dust mass ranges from $\sim10^{-3}\,M_\odot$ to $0.1\,M_\odot$, depending on the mass and spatial extent of the CSM. We further calculate the infrared (IR) emission from the newly formed dust and find broad consistency with observations of SN~1998S. Notably, the IR light curve exhibits a rapid rise within $\lesssim10\,{\rm d}$, closely resembling that of kilonovae (KNe). This suggests that dust emission powered by confined CSM interaction may be also discovered in KN searches. Moreover, the high-density environment of the CDS may allow dust grains to grow to larger sizes, enhancing their survivability against destruction by reverse shocks propagating from the interstellar medium at later times.

astro-ph.SR↗

arXiv2025

Gamma-Ray Burst Jets in Circumstellar Material: Dynamics, Breakout, and Diversity of Transients

Recent observations indicate that stripped-envelope core-collapse supernovae are often surrounded by dense circumstellar material (CSM). Motivated by this, we develop an analytic model to systematically study the dynamics of long gamma-ray burst (LGRB) jet propagation in various CSM environments. We derive a general expression for the jet head velocity ($β_{\rm h}$) and breakout time ($t_{\rm b}$) valid across Newtonian, relativistic, and intermediate regimes, accounting for a previously unrecognized dependence on $1 - β_{\rm h}$. Our results highlight a fundamental distinction between jet propagation in massive stars, where $β_{\rm h}\ll 1$, and in extended CSM, where $1-β_{\rm h}\ll 1$. We establish an analytic success/failure criterion for jets and express it in terms of jet and CSM parameters, revealing a strong dependence on CSM radius. To quantify the relativistic nature of the jet-cocoon system, we introduce the energy-weighted proper velocity $\overline{Γβ}$. We identify three possible jet outcomes-(a) successful jets ($\overline{Γβ} \sim 10-100$), (b) barely failed jets ($\overline{Γβ} \sim 1$), and (c) completely failed jets ($\overline{Γβ} \sim 0.1$)-and constrain their respective jet/CSM parameter spaces. We show that in (b) and (c), large CSM radii can result in luminous fast blue optical transients via cocoon cooling emission. This theoretical framework provides a basis for future observational and theoretical studies to understand the link between LGRBs, intermediate GRBs, low-luminosity LGRBs, and their environments.

astro-ph.HE↗

arXiv2025

EP240414a: A Gamma-Ray Burst Jet Weakened by an Extended Circumstellar Material

The recent Einstein Probe (EP) event EP240414a exhibits several unusual observational features. Its prompt and afterglow emissions place it between long gamma-ray bursts (LGRBs) and low-luminosity GRBs (LLGRBs). The event is followed by a fast optical transient (AT 2024gsa), initially exhibiting a thermal-like spectrum but later evolving into an unusually red peak at $\sim 3-5$ days, which is difficult to explain with thermal emission. Using our generalized analytic framework for jet propagation in a circumstellar material (CSM; Hamidani et al. 2025), we explore a scenario in which a conventional LGRB jet is launched in a progenitor surrounded by a dense CSM. For a CSM of $\sim 0.03 M_\odot$ extending to $\sim 3\times 10^{13}$ cm, we find that the jet is significantly weakened before breaking out, becoming "barely failed", an intermediate state between successful (LGRB) and completely failed (LLGRB) jets. This scenario naturally explains EP240414a's multi-wavelength observations, with the early thermal component produced by cocoon cooling emission, and the red peak explained by non-thermal afterglow emission from the mildly relativistic barely failed jet (and its inner-cocoon). Our work demonstrates the important role of extended CSM in shaping GRB jets and illustrates how early multi-wavelength follow-up observations can reveal the physically diverse nature of jet-driven transients.

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↗

arXiv2025

Self-consistent scenario for jet and stellar explosion in collapsar: General relativistic magnetohydrodynamics simulation with dynamo

A resistive magnetohydrodynamics simulation with a dynamo term is performed for modeling the collapsar in full general relativity. As an initial condition, a spinning black hole and infalling stellar matter are modeled based on a stellar evolution result, superimposing a weak toroidal magnetic field. After the growth of a massive torus around the black hole, the magnetic field is amplified in it, developing poloidal fields via dynamo. In an early stage of the torus growth, magnetic fluxes that fall to the vicinity of the central black hole are swallowed by the black hole and global poloidal magnetic fields that can be the source of the Blandford-Znajek mechanism are not developed. However, in a later stage in which the ram pressure of the infalling matter becomes weak, the magnetic field amplified by the black hole spin via the winding becomes large enough to expel the infalling matter by the magnetic pressure, and subsequently, a global poloidal magnetic field that penetrates the black hole is established, launching a jet along the spin axis by the Blandford-Znajek mechanism with the luminosity suitable for explaining typical long gamma-ray bursts. Together with the jet launch, the effectively viscous effect in the inner region of the torus and the magnetocentrifugal effect drive the stellar explosion with the explosion energy comparable to typical or powerful supernovae. We also find large amounts of synthesized $^{56}$Ni and Zn associated with the stellar explosion. In the presence of jet launching, $r$-process elements are weakly synthesized. The numerical results of the explosion energy, ejecta mass, and $^{56}$Ni mass are in a good agreement with those for observed broad-lined type Ic supernovae. Our result illustrates a self-consistent scenario for the gamma-ray-burst-associated broad-lined type Ic supernovae.

astro-ph.HE↗

arXiv2025

The origin of very-high-energy gamma-rays from GRB 221009A: implications for reverse shock proton synchrotron emission

Recently, GRB 221009A, known as the brightest of all time (BOAT), has been observed across an astounding range of $\sim 18$ orders of magnitude in energy, spanning from radio to VHE bands. Notably, the Large High Altitude Air Shower Observatory (LHAASO) recorded over $60000$ photons with energies exceeding $0.2\rm~TeV$, including the first-ever detection of photons above $10\rm~TeV$. However, explaining the observed energy flux evolution in the VHE band alongside late-time multi-wavelength data poses a significant challenge. Our approach involves a two-component structured jet model, consisting of a narrow core dominated by magnetic energy and a wide jet component dominated by matter. We show that the combination of the forward shock electron synchrotron self-Compton emission from both jets and reverse shock proton synchrotron emission from the wide jet could account for both the energy flux and spectral evolution in the VHE band, and the early TeV lightcurve may be influenced by prompt photons which could explain the initial steep rising phase. We noticed the arrival time of the highest energy photons detected by LHAASO-KM2A coincident with the peak of the reverse shock proton synchrotron emission, especially a minor flare occurring about $\sim500-800$ seconds after the trigger, coinciding with the observed spectral hardening and arrival time of the $\sim 13\rm~TeV$ photons detected by LHAASO. These findings imply that the GRB reverse shock may serve as a potential accelerator of ultra-high-energy cosmic rays, a hypothesis that could be tested through future multimessenger observations.

astro-ph.HE↗

arXiv2024

Probing the Origin of the Star Formation Excess Discovered by JWST through Gamma-Ray Bursts

The recent observations by the James Webb Space Telescope (JWST) have revealed a larger number of bright galaxies at $z\gtrsim10$ than was expected. The origin of this excess is still under debate, although several possibilities have been presented. We propose that gamma-ray bursts (GRBs) are a powerful probe to explore the origin of the excess and, hence, the star and galaxy formation histories in the early universe. Focusing on the recently launched mission, Einstein Probe (EP), we find that EP can detect several GRBs annually at $z\gtrsim10$, assuming the GRB formation rate calibrated by events at $z\lesssim6$ can be extrapolated. Interestingly, depending on the excess scenarios, the GRB event rate may also show an excess at $z\simeq10$, and its detection will help to discriminate between the scenarios that are otherwise difficult to distinguish. Additionally, we discuss that the puzzling, red-color, compact galaxies discovered by JWST, the so-called ``little red dots'', could host dark GRBs if they are dust-obscured star forming galaxies. We are eager for unbiased follow-up of GRBs and encourage future missions such as HiZ-GUNDAM to explore the early universe.

astro-ph.HE↗

arXiv2024

Parametric decay instability of circularly polarized Alfvén wave in magnetically dominated plasma

We investigate parametric decay instability (PDI) of circularly polarized Alfvén wave into daughter acoustic wave and backward Alfvén wave in magnetically-dominated plasma, in which the magnetization parameter $σ$ (energy density ratio of background magnetic field to matter) exceeds unity. We analyze relativistic magnetohydrodynamics (MHD), focusing on wave frequencies sufficiently lower than the plasma and cyclotron frequencies. We derive analytical formulae for the dispersion relation and growth rate of the instability as a function of the magnetization $σ$, wave amplitude $η$, and plasma temperature $θ$. We find that PDI persists even in high magnetization $σ$, albeit with a decreased growth rate up to $σ\to\infty$. Our formulae are useful for estimating the decay of Alfvén wave into acoustic wave and heat in high magnetization $σ$ plasma, which is a ubiquitous phenomenon such as in pulsars, magnetars, and fast radio bursts.

astro-ph.HE↗