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Search Results: 1 - 10 of 300 matches for " Kouichi Hirotani "
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High-energy Emission from Pulsar Outer Magnetospheres
Kouichi Hirotani
Physics , 2003, DOI: 10.1086/517510
Abstract: We investigate a stationary pair production cascade in the outer magnetosphere of an isolated, spinning neutron star. The charge depletion due to global flows of charged particles, causes a large electric field along the magnetic field lines. Migratory electrons and/or positrons are accelerated by this field to radiate gamma-rays via curvature and inverse-Compton processes. Some of such gamma-rays collide with the X-rays to materialize as pairs in the gap. The replenished charges partially screen the electric field, which is self-consistently solved together with the energy distribution of particles and gamma-rays at each point along the field lines. By solving the set of Maxwell and Boltzmann equations, we demonstrate that an external injection of charged particles at nearly Goldreich-Julian rate does not quench the gap but shifts its position and that the particle energy distribution cannot be described by a power-law. The injected particles are accelerated in the gap and escape from it with large Lorentz factors. We show that such escaping particles migrating outside of the gap contribute significantly to the gamma-ray luminosity for young pulsars and that the soft gamma-ray spectrum between 100 MeV and 3 GeV observed for the Vela pulsar can be explained by this component. We also discuss that the luminosity of the gamma-rays emitted by the escaping particles is naturally proportional to the square root of the spin-down luminosity.
Three-dimensional non-vacuum pulsar outer-gap model: Localized acceleration electric field in the higher altitudes
Kouichi Hirotani
Physics , 2014, DOI: 10.1088/2041-8205/798/2/L40
Abstract: We investigate the particle accelerator that arises in a rotating neutron-star magnetosphere. Solving the Poisson equation for the electro-static potential, the Boltzmann equations for relativistic electrons and positrons, and the radiative transfer equation simultaneously, we demonstrate that the electric field is substantially screened along the magnetic field lines by the pairs that are created and separated within the accelerator. As a result, the magnetic-field-aligned electric field is localized in the higher altitudes near the light cylinder and efficiently accelerates the positrons created in the lower altitudes outwards but not the electrons inwards. The resulting photon flux becomes predominantly outwards, leading to typical double-peak light curves, which are commonly observed from many high-energy pulsars.
Collimation of Highly Variable Magnetohydrodynamic Disturbances around a Rotating Black Hole
Kouichi Hirotani
Physics , 1997, DOI: 10.1086/305742
Abstract: We have studied non-stationary and non-axisymmetric perturbations of a magnetohydrodynamic accretion onto a rotating (Kerr) black hole. Assuming that the magnetic field dominates the plasma accretion, we find that the accretion suffers a large radial acceleration resulting from the Lorentz force, and becomes highly variable compared with the electromagnetic field there. In fact, we further find an interesting perturbed structure of the plasma velocity with a large peak in some narrow region located slightly inside of the fast-magnetosonic surface. This is due to the concentrated propagation of the fluid disturbances in the form of fast-magnetosonic waves along the separatrix surface. If the fast-magnetosonic speed is smaller in the polar regions than in the equatorial regions, the critical surface has a prolate shape for radial poloidal field lines. In this case, only the waves that propagate towards the equator can escape from the super-fast-magnetosonic region and collimate polewards as they propagate outwards in the sub-fast-magnetosonic regions. We further discuss the capabilities of such collimated waves in accelerating particles due to cyclotron resonance in an electron-positron plasma.
Composition of active galactic nuclei jets: pair-plasma dominance in the 3C 345 and 3C 279 jets
Kouichi Hirotani
Physics , 2000,
Abstract: We investigate whether the parsec-scale jets of quasars 3C 345 and 3C 279 are dominated by a normal (proton-electron) plasma or a pair (electron-positron) plasma. We first present a new method to compute the kinetic luminosity of a conical jet by using the core size observed at a single very long baseline interferometry frequency. The deduced kinetic luminosity gives electron densities of individual radio-emitting components as a function of the composition. We next constrain the electron density independently by using the theory of synchrotron self-absorption. Comparing the two densities, we can discriminate the composition. We then apply this procedure to the five components in the 3C 345 jet and find that they are pair-plasma dominated at 14 epochs out of the total 19 epochs at which the turnover frequencies are reported, provided that the bulk Lorentz factor is less than 15 throughout the jet. We also investigate the composition of the 3C 279 jet and demonstrate that its two components are likely pair-plasma dominated at all the four epochs, provided that their Doppler factors are less than 10, which are consistent with observations. The conclusions do not depend on the lower cutoff energy of radiating particles.
Does a strong particle accelerator arise very close to the light cylinder in a pulsar magnetosphere?
Kouichi Hirotani
Physics , 2014, DOI: 10.1093/mnrasl/slu042
Abstract: We examine if an efficient particle acceleration takes place by a magnetic-field-aligned electric field near the light cylinder in a rotating neutron star magnetosphere. Constructing the electric current density with the actual motion of collision-less plasmas, we express the rotationally induced, Goldreich-Julian charge density as a function of position. It is demonstrated that the 'light cylinder gap', which emits very high energy photons via curvature process by virtue of a strong magnetic-field-aligned electric field very close to the light cylinder, will not arise in an actual pulsar magnetosphere.
Luminosity Evolution of Rotation-powered Gamma-ray Pulsars
Kouichi Hirotani
Physics , 2013,
Abstract: We investigate the electrodynamic structure of a pulsar outer-magnetospheric particle accelerator and the resultant gamma-ray emission. By considering the condition for the accelerator to be self-sustained, we derive how the trans-magnetic-field thickness of the accelerator evolves with the pulsar age. It is found that the thickness is small but increases steadily if the neutron-star envelope is contaminated by sufficient light elements. For such a light element envelope, the gamma-ray luminosity of the accelerator is kept approximately constant as a function of age in the initial ten thousand years, forming the lower bound of the observed distribution of the gamma-ray luminosity of rotation-powered pulsars. If the envelope consists of only heavy elements, on the other hand, the thickness is greater but increases less rapidly than what a light element envelope has. For such a heavy element envelope, the gamma-ray luminosity decreases relatively rapidly, forming the upper bound of the observed distribution. The gamma-ray luminosity of a general pulsar resides between these two extreme cases, reflecting the envelope composition and the magnetic inclination angle with respect to the rotation axis.
Luminosity Evolution of Gamma-ray Pulsars
Kouichi Hirotani
Physics , 2013, DOI: 10.1088/0004-637X/766/2/98
Abstract: We investigate the electrodynamic structure of a pulsar outer-magnetospheric particle accelerator and the resultant gamma-ray emission. By considering the condition for the accelerator to be self-sustained, we derive how the trans-magnetic-field thickness of the accelerator evolves with the pulsar age. It is found that the thickness is small but increases steadily if the neutron-star envelope is contaminated by sufficient light elements. For such a light element envelope, the gamma-ray luminosity of the accelerator is kept approximately constant as a function of age in the initial ten thousand years, forming the lower bound of the observed distribution of the gamma-ray luminosity of rotation-powered pulsars. If the envelope consists of only heavy elements, on the other hand, the thickness is greater but increases less rapidly than what a light element envelope has. For such a heavy element envelope, the gamma-ray luminosity decreases relatively rapidly, forming the upper bound of the observed distribution. The gamma-ray luminosity of a general pulsar resides between these two extreme cases, reflecting the envelope composition and the magnetic inclination angle with respect to the rotation axis. The cutoff energy of the primary curvature emission is regulated below several GeV even for young pulsars, because the gap thickness, and hence the acceleration electric field is suppressed by the polarization of the produced pairs.
Pulsar Outer-gap Electrodynamics: Hardening of Spectral Shape in the Trailing Peak in Gamma-ray Light Curve
Kouichi Hirotani
Physics , 2011, DOI: 10.1088/2041-8205/733/2/L49
Abstract: The spectral characteristics of the pulsed gamma-ray emission from outer-magnetospheric particle accelerators are investigated. Either positrons or electrons are accelerated outwards by the magnetic-field-aligned electric field to emit gamma-rays via curvature process. Since the particles move along relatively straight paths in the trailing side of a rotating magnetosphere, they attain higher Lorentz factors to emit more energetic gamma-rays than those in the leading side. It is first demonstrated that the cutoff energy of the curvature radiation evolves with the rotation phase owing to the variation of the curvature radii of the particle paths and maximizes at a slightly later phase of the trailing peak in the gamma-ray light curve.
Electrodynamic Structure of an Outer-Gap Accelerator: Implausibility of Super Goldreich-Julian Current
Kouichi Hirotani,Shinpei Shibata
Physics , 2001,
Abstract: We investigate a stationary pair production cascade in the outer magnetosphere of a spinning neutron star. The charge depletion due to global flows of charged particles, causes a large electric field along the magnetic field lines. Migratory electrons and/or positrons are accelerated by this field to radiate curvature gamma-rays, some of which collide with the X-rays to materialize as pairs in the gap. The replenished charges partially screen the electric field, which is self-consistently solved, together with the distribution functions of particles and gamma-rays. By solving the Vlasov equations describing this pair production cascade, we demonstrate the existence of a stationary gap in the outer magnetosphere of PSR B1055-52 for a wide range of current densities flowing in the accelerator: From sub to super Goldreich-Julian values. However, we find that the expected GeV spectrum becomes very soft if the current density exceeds the Goldreich-Julian value. We also demonstrate that the GeV spectrum softens with decreasing magnetic inclination and with increasing distance to this pulsar. We thus conclude that a sub-Goldreich-Julian current, a large magnetic inclination, and a small distance (500 pc, say) are plausible to account for EGRET observations. Furthermore, it is found that the TeV flux due to inverse Compton scatterings of infrared photons whose spectrum is inferred from the Rayleigh-Jeans side of the soft blackbody spectrum is much less than the observational upper limit.
Electrodynamic Structure of an Outer-Gap Accelerator: Gamma-Ray Emission from the Crab Pulsar
Kouichi Hirotani,Shinpei Shibata
Physics , 2000,
Abstract: We investigate a stationary pair production cascade in the outer magnetosphere of a spinning neutron star. The charge depletion due to a global current, causes a large electric field along the magnetic field lines. Migratory electrons and/or positrons are accelerated by this field to radiate curvature gamma-rays, some of which collide with the X-rays to materialize as pairs in the gap. The replenished charges partially screen the electric field, which is self-consistently solved together with the distribution functions of particles and gamma-rays. If no current is injected at neither of the boundaries of the accelerator, the gap is located around the so-called null surface, where the local Goldreich-Julian charge density vanishes. However, we first find that the gap position shifts outwards (or inwards) when particles are injected at the inner (or outer) boundary. Applying the theory to the Crab pulsar, we demonstrate that the pulsed TeV flux does not exceed the observational upper limit for moderate infrared photon density and that the gap should be located near to or outside of the null surface so that the observed spectrum of pulsed GeV fluxes may be emitted via curvature process.
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