arXiv subjects

J. Takata

Publications and source records attributed to J. Takata.

87 records · Page 5Linked to original sources

arXiv2010

Gamma-ray spectral properties of mature pulsars: a two-layer model

We use a simple two-layer outer gap model, whose accelerator consists of a primary region and a screening region, to discuss $γ$-ray spectrum of mature pulsars detected by $Fermi$. By solving the Poisson equation with an assumed simple step-function distributions of the charge density in these two regions, the distribution of the electric field and the curvature radiation process of the accelerated particles can be calculated. In the our model, the properties of the phase-averaged spectrum can be completely specified by three gap parameters, i.e. the fractional gap size in the outer magnetosphere, the gap current in the primary region and the gap size ratio between the primary region and the total gap size. We discuss how these parameters affect the spectral properties. We argue that although the radiation mechanism in the outer gap is curvature radiation process, the observed gamma-ray spectrum can substantially deviate from the simple curvature spectrum because the overall spectrum consists of two components, i.e. the primary region and screening region. In some pulsars the radiation from the screening region is so strong that the photon index from 100MeV to several GeV can be as flat as $\sim 2$. We show the fitting fractional gap thickness of the canonical pulsars increases with the spin down age. We find that the total gap current is about 50~\% of the Goldreich-Julian value and the thickness of the screening region is a few percent of the total gap thickness. We also find that the predicted γ$-ray luminosity is less dependent on the spin down power ($L_{sd}$) for the pulsars with $L_{sd}\ga 10^{36}$~erg/s, while the $γ$-ray luminosity decreases with the spin down power for the pulsars with $L_{sd}\la 10^{36}$~erg/s.

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arXiv2010

Outer gap accelerator closed by magnetic pair-creation process

We discuss outer gap closure mechanism in the trans-field direction with the magnetic pair-creation process near the stellar surface. The gap closure by the magnetic pair-creation is possible if some fraction of the pairs are produced with an outgoing momentum. By assuming that multiple magnetic field will affect the local field near the stellar surface, we show a specific magnetic field geometry near the stellar surface resulting in the outflow of the pairs. Together with the fact that the electric field is weak below null charge surface, the characteristic curvature photon energy emitted by incoming particles, which were accelerated in the outer gap, decreases drastically to $\sim 100$MeV near the stellar surface. We estimate the height measured from the last-open field line, above which 100MeV photons is converted into pairs by the magnetic pair-creation. We also show the resultant multiplicity due to the magnetic pair-creation process could acquire $M_{e^{\pm}}\sim 10^4-10^5$. In this model the fractional outer gap size is proportional to $P^{-1/2}$. The predicted gamma-ray luminosity ($L_γ$) and the characteristic curvature photon energy ($E_c$) emitted from the outer gap are proportional to $B^2P^{-5/2}$ and $B^{3/4}P^{-1}$ respectively. This model also predicts that $L_γ$ and $E_c$ are related to the spin down power ($L_{sd}$) or the spin down age of pulsars ($τ$) as $L_γ \propto L_{sd}^{5/8}$ or $L_γ \propto τ^{-5/4}$, and $E_c \propto L_{sd}^{1/4}$ or $E_c \propto τ^{-1/2}$ respectively.

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arXiv2010

Asteroseismic model of paramagnetic neutron star

We investigate an asteroseismic model of non-rotating paramagnetic neutron star with core-crust stratification of interior pervaded by homogeneous internal and dipolar external magnetic field, presuming that neutron degenerate Fermi-matter of the star core is in the state of Pauli's paramagnetic permanent magnetization caused by polarization of spin magnetic moments of neutrons along the axis of magnetic field of collapsed massive progenitor. The magnetic cohesion between metal-like crust and permanent-magnet-like core is considered as playing a main part in the dynamics of starquake. Focus is laid on the post-quake relaxation of the star by node-free torsional vibrations of highly conducting crustal solid-state plasma, composed of nuclei embedded in the degenerate Fermi-gas of relativistic electrons, about axis of magnetic field frozen in the immobile paramagnetic core. Two scenarios of these axisymmetric seismic vibrations are examined, in first of which these are considered as maintained by combined action of Lorentz magnetic and Hooke's elastic forces and in second one by solely Lorentz force. Based on the energy variational method of magneto-solid-mechanical theory of elastic continuous medium, the spectral formulae for the frequency and lifetime of this toroidal mode are obtained and discussed in the context of theoretical treatment of recently discovered quasi-periodic oscillations of the X-ray outburst flux from SGR 1806-20 and SGR 1900+14 as being produced by above seismic vibrations.

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arXiv2008

Nodeless differentially rotational Alfvén oscillations of crustal solid-state plasma in quaking neutron star

The two-component, core-crust, model of a neutron star with homogenous internal and dipolar external magnetic field is studied responding to quake-induced perturbation by substantially nodeless differentially rotational Alfvén oscillations of the perfectly conducting crustal matter about axis of fossil magnetic field frozen in the immobile core. The energy variational method of the magneto-solid-mechanical theory of a viscoelastic perfectly conducting medium pervaded by magnetic field is utilized to compute the frequency and lifetime of nodeless torsional vibrations of crustal solid-state plasma about the dipole magnetic-moment axis of the star. It is found that obtained two-parametric spectral formula for the frequency of this toroidal Alfven mode provides fairly accurate account of rapid oscillations of the X-ray flux during the flare of SGR 1806-20 and SGR 1900+14, supporting the investigated conjecture that these quasi-periodic oscillations owe its origin to axisymmetric torsional oscillations predominately driven by Lorentz force of magnetic field stresses in the finite-depth crustal region of the above magnetars.

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arXiv2008

Non-thermal emissions from outer magnetospheric accelerators of middle-aged pulsars

We discuss $γ$-ray emissions from the outer gap accelerators of middle-aged pulsars for part of the series of our studies. A two-dimensional electrodynamic model is used to solve the distribution of accelerating electric fields with electron and positron pair creation and radiation processes in the magnetic meridional plane. We compute the curvature radiation and the synchrotron radiation by solving the evolution of the Lorentz factor and the pitch angle. The calculated spectra are compared with observed phase-averaged spectra. We also use a three-dimensional geometrical model to discuss the pulse profiles. We argue that the outer gap of middle-aged pulsars occupies the whole region between the last-open field lines and the critical magnetic field lines, which are perpendicular to the rotational axis at the light cylinder. We assume that there is no outer gap accelerator inside the light cylinder between the rotational axis and the critical magnetic field lines. For the Geminga pulsar, we demonstrate that the outward curvature radiation dominates in the spectrum above 10 MeV, while the inward synchrotron radiation dominates below 10 MeV. We find that the computed spectrum is consistent with the observations in X-ray through $γ$-ray bands. With the pulse morphology of the $γ$-ray emissions, we argue that the inclination angle and the viewing angle for the Geminga pulsar are $α\sim 50^{\circ}$ and $ξ\sim 90^{\circ}$, respectively.

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arXiv2008

Particle Acceleration and Non-Thermal Emission in Pulsar Outer Magnetospheric Gap

A two-dimensional electrodynamic model is used to study particle acceleration and non-thermal emission mechanisms in the pulsar magnetospheres. We solve distribution of the accelerating electric field with the emission process and the pair-creation process in meridional plane, which includes the rotational axis and the magnetic axis. By solving the evolutions of the Lorentz factor, and of the pitch angle, we calculate spectrum in optical through $γ$-ray bands with the curvature radiation, synchrotron radiation, and inverse-Compton process not only for outgoing particles, but also for ingoing particles, which were ignored in previous studies. We apply the theory to the Vela pulsar. We find that the curvature radiation from the outgoing particles is the major emission process above 10 MeV bands. In soft $γ$-ray to hard X-ray bands, the synchrotron radiation from the ingoing primary particles in the gap dominates in the spectrum. Below hard X-ray bands, the synchrotron emissions from both outgoing and ingoing particles contribute to the calculated spectrum. The calculated spectrum is consistent with the observed phase-averaged spectrum of the Vela pulsar. We show that the observed five-peak pulse profile in the X-ray bands of the Vela pulsar is reproduced by the inward and outward emissions, and the observed double-peak pulse profile in $γ$-ray bands is explained by the outward emissions.

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arXiv2007

A re-visit of the phase-resolved X-ray and γ-ray spectra of the Crab pulsar

We use a modified outer gap model to study the multi-frequency phase-resolved spectra of the Crab pulsar. The emissions from both poles contribute to the light curve and the phase-resolved spectra. Using the synchrotron self-Compton mechanism and by considering the incomplete conversion of curvature photons into secondary pairs, the observed phase-averaged spectrum from 100 eV - 10 GeV can be explained very well. The predicted phase-resolved spectra can match the observed data reasonably well, too. We find that the emission from the north pole mainly contributes to Leading Wing 1. The emissions in the remaining phases are mainly dominated by the south pole. The widening of the azimuthal extension of the outer gap explains Trailing Wing 2. The complicated phase-resolved spectra for the phases between the two peaks, namely Trailing Wing 1, Bridge and Leading Wing 2, strongly suggest that there are at least two well-separated emission regions with multiple emission mechanisms, i.e. synchrotron radiation, inverse Compton scattering and curvature radiation. Our best fit results indicate that there may exist some asymmetry between the south and the north poles. Our model predictions can be examined by GLAST.

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arXiv2007

Torsional shear oscillations in the neutron star crust driven by restoring force of elastic stresses

We present several exact solutions of the eigenfrequency problem for torsional shear vibrations in homogeneous and non-homogeneous models of the neutron star crust governed by canonical equation of solid mechanics with a restoring force of Hookean elasticity. Particular attention is given to regime of large lengthscale nodeless axisymmetric differentially rotational oscillations which are treated in spherical polar coordinates reflecting real geometry of the neutron star crust. Highlighted is the distinction between analytic forms and numerical estimates of the frequency, computed as a function of multipole degree of nodeless torsional oscillations and fractional depth of the crust, caused by different boundary conditions imposed on the toroidal field of material displacements. The relevance of considered models to quasiperiodic oscillations, recently detected during the flare of SGR 1806-20 and SGR 1900+14, is discussed.

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arXiv2007

Pulse Profiles, Spectra and Polarization Characteristics of Non-Thermal Emissions from the Crab-Like Pulsars

We discuss non-thermal emission mechanism of the Crab-like pulsars with both a two-dimensional electrodynamical study and a three-dimensional model. We investigate the emission process in the outer gap accelerator. In the two-dimensional electrodynamical study, we solve the Poisson equation of the accelerating electric field in the outer gap and the equation of motion of the primary particles with the synchrotron and the curvature radiation process and the pair-creation process. We show a solved gap structure which produces a consistent gamma-ray spectrum with EGRET observation. Based on the two-dimensional model, we conduct a three-dimensional emission model to calculate the synchrotron and the inverse-Compton processes of the secondary pairs produced outside the outer gap. We calculate the pulse profiles, the phase-resolved spectra and the polarization characteristics in optical to $γ$-ray bands to compare the observation of the Crab pulsar and PSR B0540-69. For the Crab pulsar, we find that the outer gap geometry extending from near the stellar surface to near the light cylinder produces a complex morphology change of the pulse profiles as a function of the photon energy. This predicted morphology change is quite similar with that of the observations. The calculated phase-resolved spectra are consistent with the data through optical to the $γ$-ray bands. We demonstrate that the 10$\sim$20 % of the polarization degree in the optical emissions from the Crab pulsar and the Vela pulsar are explained by the synchrotron emissions with the particle gyration motion.

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arXiv2007

The Phase-resolved High Energy Spectrum of the Crab Pulsar

We present a modified outer gap model to study the phase-resolved spectra of the Crab pulsar. A theoretical double peak profile of the light curve containing the whole phase is shown to be consistent with the observed light curve of the Crab pulsar by shifting the inner boundary of the outer gap inwardly to $\sim 10$ stellar radii above the neutron star surface. In this model, the radial distances of the photons corresponding to different phases can be determined in the numerical calculation. Also the local electrodynamics, such as the accelerating electric field, the curvature radius of the magnetic field line and the soft photon energy, are sensitive to the radial distances to the neutron star. Using a synchrotron self-Compton mechanism, the phase-resolved spectra with the energy range from 100 eV to 3 GeV of the Crab pulsar can also be explained.

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arXiv2007

Study on polarization of high-energy photons from the Crab pulsar

We investigate polarization of high-energy emissions from the Crab pulsar in the frame work of the outer gap accelerator. The recent version of the outer gap, which extends from inside the null charge surface to the light cylinder, is used for examining the light curve, the spectrum and the polarization characteristics, simultaneously. The polarization position angle curve and the polarization degree are calculated to compare with the Crab optical data. We show that the outer gap model explains the general features of the observed light curve, the spectrum and the polarization by taking into account the emissions from inside of the null charge surface and from tertiary pairs, which were produced by the high-energy photons from the secondary pairs. For the Crab pulsar, the polarization position angle curve indicates that the viewing angle of the observer measured from the rotational axis is greater than 90 degrees.

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arXiv2006

Polarization of high-energy emissions from the Crab pulsar

We investigate polarization of high-energy emissions from the Crab pulsar in the frame work of the outer gap accelerator, following previous works of Cheng and coworkers. The recent version of the outer gap, which extends from inside the null charge surface to the light cylinder, is used for examining the synchrotron radiations from the secondary and the tertiary pairs, which are produced outside the gap. We calculate the light curve, the spectrum and the polarization characteristics, simultaneously, by taking into account gyration motion of the particles. The polarization position angle curve and the polarization degree are calculated to compare with the Crab optical data. We demonstrate that the radiations from inside the null charge surface make outer-wing and off-pulse emissions in the light curve, and the tertiary pairs contribute to bridge emissions. The emissions from the secondary pairs explain the main features of the observed light curve and spectrum. On the other hand, both emissions from inside the null charge surface and from the tertiary pairs are required to explain the optical polarization behavior of the Crab pulsar. The energy dependence of the polarization features is expected by the present model. For the Crab pulsar, the polarization position angle curve indicates that the viewing angle of the observer measured from the rotational axis is greater than $90^{\circ}$.

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arXiv2005

A two-dimensional electrodynamical outer gap model for gamma-ray pulsars: Gamma-ray spectrum

A two-dimensional electrodynamical model is used to study particle acceleration in the outer magnetosphere of a pulsar. The charge depletion from the Goldreich-Julian charge density causes a large electric field along the magnetic field lines. The charge particles are accelerated by the electric field and emit $γ$-rays via the curvature process. Some of the emitted $γ$-rays may collide with $X$-ray photons to make new pairs, which are accelerated again on the different field lines in the gap and proceed similar processes. We simulate the pair creation cascade in the meridional plane using the pair creation mean-free path, in which the $X$-ray photon number density is proportional to inverse square of radial distance. With the space charge density determined by the pair creation simulation, we solve the electric structure of the outer gap in the meridional plane and calculate the curvature spectrum. Because the two-dimensional model can link both gap width along the magnetic field line and trans-field thickness with the spectral cut-off energy and flux, we can diagnose both the current through the gap and inclination angle between the rotational and magnetic axes. We apply the theory to the Vela pulsar. By comparing the results with the $EGRET$ data, we rule out any cases that have a large particle injection at the outer boundary. We also suggest the inclination angle of $α_{inc}\geq65^{\circ}$. The present model predicts the outer gap starting from near the conventional null charge surface for the Vela pulsar.

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arXiv2004

A pulsar outer gap model with trans-field structure

We investigate the electrodynamics of an outer gap in the meridional plane of the aligned-rotator. The charge depletion from the Goldreich-Julian charge density causes a large electric field along the magnetic field line. The electrons or the positrons are accelerated by the field-aligned electric field and radiate the $γ$-rays tangentially to the local magnetic field line. Some of such $γ$-rays collide with $X$-rays to materialize as the electron-positron pairs on different field lines from the field line on which they were emitted. As a result, the electric field structure is expected to change across the field lines. Including these trans-field effects, we solve the formation of the electric field self-consistently with the curvature radiation and the pair creation processes. The $γ$-ray emission and the pair creation are treated by use of Monte Carlo technique. We demonstrate that the distribution of the electric field along the field lines is affected by both the gap geometry and the external currents coming into the gap through the boundaries. In the electrodynamical model, it has been known that the solution disappears if the current density carried by the electron-positron pairs produced in the gap exceeds a critical value. We show that the critical current density is significantly increased when the trans-field structure is taken into account. We also find that the location of the inner boundary of the gap shifts toward the stellar surface from the conventional null surface as the current density increases. The reason for the shift is derived from the stability condition of the inner boundary. We also argue that the ideal-MHD condition holds outside of the gap only when the low energy particles coexist with the high energy particles migrating from the gap.

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arXiv2003

Outer-magnetospheric model for Vela-like pulsars:formation of sub-GeV spectrum

We investigate the $γ$-ray emission from a outer-gap accelerator, which is located in the outer-magnetosphere of a pulsar. The charge depletion from the Goldreich-Julian density causes a large electric field along the magnetic field lines. The electric field accelerates electrons and positrons to high energies. We solve the formation of the electric field self-consistently with the radiation and the pair-creation processes in one-dimensional geometry along the magnetic field lines, and calculate curvature spectrum to compare with the observations. We find that because the particles escape from the accelerating region with large Lorentz factors as $10^{7.5}$, curvature radiation from the outside of the gap, in which there is no electric field, still has a important contribution to the spectrum. Including the outside emission, the model spectra are improved between 100 MeV and 1 GeV for the Vela-type pulsars. Since the field line curvature close to the light cylinder affects the spectrum in several hundred MeV, there is a possibility to diagnose the field structure by using the spectrum in these bands. The difference found in the $γ$-ray spectra of the twin pulsars, PSR B0833-45 (Vela) and B1706-44, is deduced from the dimensionless electric current running through the gap. The extension to the three-dimensional modeling with self-consistent electrodynamics is necessary to understand the difference in the observed flux between the twin pulsars.

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