arXiv subjects

Shota Kisaka

Publications and source records attributed to Shota Kisaka.

At least 19 recordsLinked to original sources

arXiv2026

Polarization of Synchrotron Emission from Rotation Powered Pulsars

With the launch of the Imaging X-ray Polarimeter Explorer in 2021, recent observations have provided X-ray polarization data for several rotation-powered pulsars. To fully exploit these data, it is important to better understand the relationship between polarization properties and the magnetospheric structure. In this paper, we investigate polarization properties in the outer gap model, incorporating both outward emission and inward emission toward the rotation axis, which has not been considered previously. We find that the polarization position angle varies at the intensity peaks, typically accompanied by a high polarization degree, as is in the case for outward emission. Moreover, the variation in the polarization position angle tends to exceed 50 degrees when the intensity peaks of outward and inward emissions overlap, compared to cases with only outward emission. We compare the polarization model including inward emission with IXPE observations. For example, for PSR B1509-58 where inward emission is proposed to be dominant in the outer gap model, we find that observed PA swing prefers a line of sight from the southern hemisphere rather than the northern hemisphere.

astro-ph.HE↗

arXiv2026

VLBI Astrometry of Magnetars

The origin of the strongest magnetic fields in the Universe, i.e., the origin of magnetars, is a longstanding question. An enhanced dynamo effect in an irregular supernova explosion is a possible origin, which implies a stronger kick velocity and a higher proper motion of the magnetar compared to those of ordinary pulsars as well as an irregular morphology of the host supernova remnant (SNR). However, this hypothesis is not well studied yet, because there is a lack of precise measurement of the proper motion of magnetar and of identification of the host SNR. VLBI astrometry of magnetars is a unique tool to examine the hypothesis. In this chapter, we introduce the MONSTER (Monitoring Observations of the Neutron Stars That Evolve Rapidly) Project. SKA-VLBI's unprecedented sensitivity and the highest angular resolution will allow us to dramatically expand the survey volume in which we can measure the proper motion of magnetars within a radio outburst period of a few months.

astro-ph.HE↗

arXiv2026

Thermal Spin Polarization Driven by Nuclear Spin-Orbit Coupling in Neutron Star Pasta

We discuss anomalous spin polarization on the surface of nuclear pasta in a neutron star, driven by a nuclear spin-orbit interaction. We present an effective two-band model of surface-localized neutrons near the nuclear pasta. The central point is the emergence of a Rashba-type spin-orbit hybridization generated by the neutron--nucleus spin-orbit force in the presence of the strong density gradient normal to the pasta surface. Starting from a single-particle Hamiltonian with a central potential and a standard nuclear spin-orbit interaction, we show that the surface spin polarization occurs due to the thermal inhomogeneity even in the absence of a magnetic field. Our study links neutron-star physics and solid-state spintronics and would contribute to understanding the interplay between spin dynamics and strong magnetic fields.

nucl-th↗

arXiv2026

Constraining the Pulsar Beaming Fraction with TeV-Selected Galactic Pulsar Wind Nebulae and unidentified TeV Sources

The pulsar beaming fraction is a fundamental quantity for connecting the observed pulsar population to the intrinsic Galactic population and for constraining pulsar emission geometry. In this study, we estimate the beaming fraction in each observational band (radio, $γ$-ray, and X-ray) and for each TeV survey (H.E.S.S., HAWC, and LHAASO) using TeV-selected pulsar wind nebulae (PWNe) and unidentified (Unid) TeV sources, assuming that the TeV emission from PWNe is approximately isotropic and that Unid sources are PWNe powered by pulsars whose beams do not intersect our line of sight. Within each survey, the inferred beaming fractions $\sim 0.1-0.3$ are comparable across bands. In contrast, the values differ by more than a factor of two between H.E.S.S. and HAWC/LHAASO. This discrepancy likely reflects survey-dependent selection effects, including differences in angular resolution and energy range, and is also consistent with the possibility that HAWC/LHAASO selected samples preferentially include older pulsars associated with more extended PWNe than those in the H.E.S.S. sample. We further show that the inferred beaming fractions can be reproduced within a unified framework using a time-dependent opening angle, and that this framework remains compatible with the statistical properties of the observed pulsar population.

astro-ph.HE↗

arXiv2026

Gravitational waves from gaps of neutron stars

The pulsar magnetosphere is a potential source of continuous gravitational waves due to the rapid charge-discharge process in short timescale, varying the electric-field energy density. We estimate the strain of the continuous gravitational waves, considering relativistic effects and different gap regions. We find that the strain from the polar cap is too small, in contrast to previous results. On the other hand, the strain from the outer gap is as large as $\sim2\times10^{-24}$, enough for future gravitational-wave detection such as the Einstein Telescope. Our result presents a new approach for the future detection of gravitational waves to probe the physics in the magnetosphere.

gr-qc↗

arXiv2026

One-week optical observations of pulsed emission from the Crab pulsar with IMONY on the 3.8 m Seimei telescope

We report our optical observations of the Crab pulsar using the Imager of MPPC-based Optical photoN counter from Yamagata (IMONY), a high-time-resolution photon-counting imager with 100 ns timing resolution, mounted on the 3.8 m Seimei telescope in Japan (f/D~6). The detector format was upgraded from a $4\times4$ to an $8\times8$ GAPD array with larger pixels ($100$ to $200~{μm}$), resulting in a 14".5 field of view on the Seimei telescope. We conducted nightly optical observations for one week, including two nights of simultaneous optical and radio observations with the 64 m Usuda radio telescope. Thanks to the large diameter of the Seimei telescope and the high time resolution of IMONY, we successfully detected optical Single Pulses (SPs) emitted in each rotation. Moreover, we found an optical peak timing drift of $30\pm7.9~\mathrm{μs}$ over three days, with a significance of $3.9σ$. The corresponding emission region size is 9.1 km, which is equivalent to 0.006 times the light cylinder radius of the Crab pulsar. We ruled out the possibility of a pulsar glitch and suggested that the optical pulsed emission region of the Crab pulsar may fluctuate due to the spatial drift and variations in the magnetospheric caustics.

astro-ph.IM↗

arXiv2026

Relic of quadrupole deformation produced in a hot neutron star era

A newly born neutron star is expected to exhibit significant deviations from spherical symmetry, which decay with time. Determining how much deformation remains at present is crucial for gravitational-wave astronomy. This study is the first investigation into the evolution of quadrupole deformation during the solid crust formation phase to obtain a plausible value at present. The equilibrium structure before solidification is modeled using a fluid description, and the deformation is introduced through an assumed driving force. As the star cools, this force weakens, leading to a gradual decay of the deformation. Eventually, the deformation vanishes in the fluid region but partially remains in the crust, sustained by elastic forces, after solidification. By comparing the equilibrium models before and after solidification, we estimate the residual ellipticity and demonstrate that the spatial profile of the elastic shear is imprinted in the crust. The relic ellipticity is only a few percent of the original value, with its absolute magnitude depending on the deformation mechanism during the hot era, which cannot be specified owing to the lack of elaborate models. This work provides a first step toward linking early neutron star deformation with future gravitational-wave observations.

gr-qc↗

arXiv2025

Gyromagnetic Angular Momentum Interconversion in Neutron Stars

We propose a novel mechanism for angular momentum (AM) exchange between the crust and core of a neutron star (NS) via the gyromagnetic effect. Using extended hydrodynamics, we model the star by incorporating macroscopic AM and microscopic AM originating from neutron orbital and spin AM. We reveal that macroscopic dynamics in the crust can inform microscopic AM in the core leading to neutron spin polarization, and offer alternative scenario of (anti-)glitches. This work highlights the overlooked multi-scale AM interconversions in NS physics, paving the way for gyromagnetic astrophysics.

astro-ph.HE↗

arXiv2024

The centrifugal acceleration and the Y-point of the Pulsar Magnetosphere

We investigate the centrifugal acceleration in an axisymmetric pulsar magnetosphere under the ideal-MHD approximation. We solved the field-aligned equations of motion for flows inside the current sheet with finite thickness. We find that flows coming into the vicinity of a Y-point become super fast. The centrifugal acceleration takes place efficiently, and most of the Poynting energy is converted into kinetic energy. However, the super fast flow does not provide enough centrifugal drift current to open the magnetic field. Opening of the magnetic field is possible by the plasmas that are accelerated in the azimuthal direction with a large Lorentz factor in the closed field region. We find that this acceleration takes place if the field strength increases toward the Y-point from inside. The accelerated plasma is transferred from the closed field region to the open field region by magnetic reconnection with plasmoid emission. We also estimate the Lorentz factor to be reached in the centrifugal wind.

astro-ph.HE↗

arXiv2024

On the X-ray efficiency of the white dwarf pulsar candidate ZTF J190132.9+145808.7

Strongly magnetized, rapidly rotating massive white dwarfs (WDs) emerge as potential outcomes of double degenerate mergers. These WDs can act as sources of non-thermal emission and cosmic rays, gethering attention as WD pulsars. In this context, we studied the X-ray emissions from ZTF J190132.9+145808.7 (hereafter ZTF J1901+14), a notable massive isolated WD in the Galaxy, using the Chandra X-ray observatory. Our results showed 3.5sigma level evidence of X-ray signals, although it is marginal. Under the assumption of a photon index of 2, we derived its intrinsic flux to be 2.3 (0.9--4.7) $\times 10^{-15}$~erg~cm$^{-2}$s$^{-1}$ and luminosity 4.6 (2.0--9.5) $\times 10^{26}$~erg~s$^{-1}$ for a 0.5--7 keV band in the 90% confidence range, given its distance of 41 pc. We derived an X-ray efficiency (eta) concerning the spin-down luminosity to be 0.012 (0.0022--0.074), a value comparable to that of ordnary neutron star pulsars. The inferred X-ray luminosity may be compatible with curvature radiation from sub-TeV electrons accelerated within open magnetic fields in the magnetosphere of ZTF J1901+14. Conducting more extensive X-ray observations is crucial to confirm whether ZTF J1901+14-like isolated WDs are also significant sources of X-rays and sub-TeV electron cosmic rays, similar to other WD pulsars in accreting systems.

astro-ph.HE↗

arXiv2024

SKA Sensitivity to Sub-GeV Dark Matter Decay: Synchrotron Radio Emissions in White Dwarf Magnetospheres

We investigate the potential of the Square Kilometre Array (SKA) in detecting synchrotron radiation emitted from the decay of sub-GeV dark matter (dark matter with masses below the GeV scale) in the presence of strong magnetic fields. As a concrete setup, we consider scenarios where the magnetosphere of a magnetic white dwarf overlaps with dense dark matter environments, such as those surrounding a primordial black hole. Our study reveals that the encounters of compact objects such as white dwarfs and black holes offer a promising avenue for upcoming radio telescopes to probe the properties of light dark matter, which has been less explored compared with more conventional heavier (masses above the GeV scale) dark matter.

hep-ph↗

arXiv2024

1D GRPIC Simulations of Stellar-Mass Black Hole Magnetospheres: Semi-Analytic Model of Gamma-Rays from Gaps

In the absence of a sufficient amount of plasma injection into the black hole (BH) magnetosphere, the force-free state of the magnetosphere cannot be maintained, leading to the emergence of strong, time-dependent, longitudinal electric field (spark gap). Recent studies of supermassive BH magnetospheres by using analytical methods and particle-in-cell (PIC) simulations propose the possibility of the efficient particle acceleration and consequent gamma-ray emissions in the spark gap. In this work, we perform one-dimensional general relativistic PIC simulations to examine the gamma-ray emission from stellar-mass BH magnetospheres. We find that intermittent spark gaps emerge and particles are efficiently accelerated, in a similar manner to the supermassive BH case. We build a semi-analytic model of the plasma dynamics and radiative processes which reproduces the maximum electron energies and peak gamma-ray luminosities in the simulation results. Based on this model, we show that gamma-ray signals from stellar-mass BHs wandering through the interstellar medium could be detected by gamma-ray telescopes such as the Fermi Large Area Telescope, or the Cherenkov Telescope Array.

astro-ph.HE↗

arXiv2023

Evolutionary deformation toward the elastic limit by a magnetic field confined in the neutron--star crust

Occasional energetic outbursts and anomalous X-ray luminosities are expected to be powered by the strong magnetic field in a neutron star. For a very strong magnetic field, elastic deformation becomes excessively large such that it leads to crustal failure. We studied the evolutionary process driven by the Hall drift for a magnetic field confined inside the crust. Assuming that the elastic force acts against the Lorentz force, we examined the duration of the elastic regime and maximum elastic energy stored before the critical state. The breakup time was longer than that required for extending the field to the exterior, because the tangential components of the Lorentz force vanished in the fragile surface region. The conversion of large magnetic energy, confined to the interior, into Joule heat is considered to explain the power for central compact objects. This process can function without reaching its elastic limit, unless the magnetic energy exceeds $2\times 10^{47}$ erg, which requires an average field strength of $2\times10^{15}$ G. Thus, the strong magnetic field hidden in the crust is unlikely to cause outbursts. Furthermore, the magnetic field configuration can discriminate between central compact objects and magnetars.

astro-ph.HE↗

arXiv2022

Strong toroidal magnetic fields sustained by the elastic crust in a neutron star

We investigate new solutions for magnetized neutron stars with a barotropic core in magnetohydrodynamic (MHD) equilibrium and a magneto-elastic crust, which was neglected by previous studies concerning stars in MHD equilibrium. The Lorentz force of the barotropic star is purely irrotational and the structures of magnetic fields are constrained. By contrast, a solenoidal component of the Lorentz force exists in the elastic crust and the structures of the magnetic fields are less restricted. We find that the minor solenoidal component in the elastic crust is important for sustaining the strong magnetic field in the core. Unlike previous studies, the toroidal magnetic field exists in the entire region of the core, and we obtain equilibrium states with large toroidal magnetic fields, where the toroidal magnetic energy is larger than the poloidal magnetic energy. The elastic force of the crust sustains an order of $10^{15}~\mathrm{G}$ toroidal magnetic field in the core, and the maximum strength of the toroidal magnetic field is approximately proportional to the crust thickness.

astro-ph.HE↗

arXiv2022

Detection of a bright burst from the repeating FRB 20201124A at 2 GHz

We present a detection of a bright burst from FRB 20201124A, which is one of the most active repeating FRBs, based on S-band observations with the 64-m radio telescope at the Usuda Deep Space Center/JAXA. This is the first FRB observed by using a Japanese facility. Our detection at 2 GHz in February 2022 is the highest frequency for this FRB and the fluence of $>$ 189 Jy ms is one of the brightest bursts from this FRB source. We place an upper limit on the spectral index $α$ = -2.14 from the detection of the S band and non-detection of the X band at the same time. We compare an event rate of the detected burst with ones of the previous research, and suggest that the power-law of the luminosity function might be broken at lower fluence, and the fluences of bright FRBs distribute up to over 2 GHz with the power-law against frequency. In addition, we show the energy density of the burst detected in this work was comparable to the bright population of one-off FRBs. We propose that repeating FRBs can be as bright as one-off FRBs, and only their brightest bursts could be detected so some of repeating FRBs intrinsically might have been classified as one-off FRBs.

astro-ph.HE↗

arXiv2022

Magnetically confined mountains on accreting neutron stars with multipole magnetic fields

Magnetically confined mountains on accreting neutron stars are candidates for producing continuous gravitational waves. We formulate a magnetically confined mountain on a neutron star with strong multipole magnetic fields and obtain some sequences of numerical solutions. We find that the mass ellipticity of the mountain increases by one order of magnitude if the neutron star has strong multipole magnetic fields. As matter accretes on to the magnetic pole, the size of the mountain increases and the magnetic fields are buried. If the neutron star has a dipole magnetic field, the dipole magnetic field is buried and transformed into multipole components. By contrast, if the neutron star has both dipole and strong multipole magnetic fields, the multipole magnetic fields are buried and transformed into a negative dipole component. We also calculate magnetically confined mountains with toroidal magnetic fields and find that the ellipticity becomes slightly smaller when the mountain has toroidal magnetic fields. If the multipole magnetic fields are buried, they sustain the intense toroidal magnetic field near the stellar surface, and the ratio of the toroidal magnetic field to the poloidal magnetic field is close to 100. The hidden strong toroidal magnetic fields are sustained by the buried multipole magnetic fields.

astro-ph.HE↗

arXiv2022

Magnetic field sustained by the elastic force in neutron star crusts

We investigate the magneto--elastic equilibrium of a neutron star crust and magnetic energy stored by the elastic force. The solenoidal motion driven by the Lorentz force can be controlled by the magnetic elastic force, so that conditions for the magnetic field strength and geometry are less restrictive. For equilibrium models, the minor solenoidal part of the magnetic force is balanced by a weak elastic force because the irrotational part is balanced by the dominant gravity and pressure forces. Therefore, a strong magnetic field may be confined in the interior, regardless of poloidal or toroidal components. We numerically calculated axially symmetric models with the maximum shear--strain, and found that a magnetic energy $> 10^{46}$ erg can be stored in the crust, even for a normal surface dipole-field-strength ($<10^{13}$ G). The magnetic energy much exceeds the elastic energy ($ 10^{44} -10^{45}$ erg). The shear--stress spatial distribution revealed that the elastic structure is likely to break down near the surface. In particular, the critical position is highly localized at a depth less than 100 m from the surface.

astro-ph.HE↗

arXiv2021

The response of black hole spark gaps to external changes: A production mechanism of rapid TeV flares?

We study the response of a starved Kerr black hole magnetosphere to abrupt changes in the intensity of disk emission and in the global magnetospheric current, by means of 1D general relativistic particle-in-cell simulations. Such changes likely arise from the intermittency of the accretion process. We find that in cases where the pair production opacity contributed by the soft disk photons is modest, as in, e.g., M87, such changes can give rise to delayed, strong TeV flares, dominated by curvature emission of particles accelerated in the gap. The flare rise time, and the delay between the external variation and the onset of the flare emitted from the outer gap boundary, are of the order of the light crossing time of the gap. The rapid, large amplitude TeV flares observed in M87 and, perhaps, other AGNs may be produced by such a mechanism.

astro-ph.HE↗