Kouichi Hirotani

Pair Plasma Production in a Force-free Magnetosphere around a Supermassive Black Hole

We quantitatively solve the problem of plasma supply to the stationary, axisymmetric, force-free magnetosphere of a rotating black hole residing in an active galactic nucleus. At the plasma source from which both inflowing and outflowing charge-separated plasmas originate, the shortage of charge will lead to the emergence of a strong electric field along the magnetic field line. The parallel electric field accelerates migratory electrons and/or positrons to ultrarelativistic energies. These relativistic electrons/positrons scatter background photons to produce high-energy γ-rays that can materialize as pairs by colliding with background photons. The produced pairs replenish the inflowing and outflowing charges and are accelerated to result in a stationary pair production cascade. It is demonstrated that a sufficient amount of plasma can be supplied for the Blandford-Znajek process to work effectively.

Particle Accelerator in Pulsar Magnetospheres: Super Goldreich-Julian Current with Ion Emission from the Neutron Star Surface

We investigate the self-consistent electrodynamic structure of a particle accelerator in the Crab pulsar magnetosphere on the two-dimensional poloidal plane, solving the Poisson equation for the electrostatic potential together with the Boltzmann equations for electrons, positrons, and γ-rays. If the transfield thickness of the gap is thin, the created current density becomes sub-Goldreich-Julian, giving the traditional outer-gap solution but with negligible γ-ray luminosity. As the thickness increases, the created current increases to become super-Goldreich-Julian, giving a new gap solution with substantially screened acceleration electric field in the inner part. In this case, the gap extends toward the neutron star with a small-amplitude positive acceleration field, extracting ions from the stellar surface as a space-charge-limited flow. The acceleration field is highly unscreened in the outer magnetosphere, resulting in a γ-ray spectral shape that is consistent with the observations.

A two-dimensional electrodynamical outer gap model for γ-ray pulsars: γ-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. We investigate in detail relation between the spectrum and total current, which is carried by the particles produced in the gap and/or injected at the boundaries of the gap. We demonstrate that the hardness of the spectrum is strongly controlled by the current carriers. Especially, the spectrum sharply softens if we assume a larger particle injection at the outer boundary of the outer gap. This is because the mean-free path of the pair creation of the inwardly propagating γ-ray photons is much shorter than the light radius so that the many pairs are produced in the gap to quench the outer gap. 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 2 J. Takata, S. Shibata, K. Hirotani & H.-K. Chang 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 αinc >65 ◦. The present model predicts the outer gap starting from near the conventional null charge surface for the Vela pulsar.

Pair Plasma Dominance in the Parsec-Scale Relativistic Jet of 3C 345

We investigate whether a parsec-scale jet of 3C 345 is dominated by a normal plasma or an electron-positron plasma. We present a general condition that a jet component becomes optically thick for synchrotron self-absorption by extending the method originally developed by Reynolds and coworkers. The general condition gives a lower limit of the electron number density, with the aid of the surface brightness condition, which enables us to compute the magnetic field density. Comparing the lower limit with another independent constraint for the electron density that is deduced from the kinetic luminosity, we can distinguish the matter content. We apply the procedure to the five components of 3C 345 (C2, C3, C4, C5, and C7) of which angular diameters and radio fluxes at the peak frequencies were obtainable from literature. Evaluating the representative values of Doppler beaming factors by their equipartition values, we find that all the five components are likely dominated by an electron-positron plasma. The conclusion does not depend on the lower cutoff energy of the power-law distribution of radiating particles.

Outer-Gap versus Slot-Gap Models for Pulsar High-Energy Emissions: The Case of the Crab Pulsar

We analytically examine the capabilities of rotation-powered pulsars as the sources of gamma rays and show that their phase-averaged gamma-ray flux is proportional to the product of the spin-down flux and the gap transfield thickness cubed irrespective of the emission models. Applying the scheme to the Crab pulsar, we demonstrate that the outer-gap model reproduces the observed GeV fluxes and that the slot-gap model reproduces at most 20% of the observed fluxes because of the small transfield thickness. An implication for the relationship between the gamma-ray and the spin-down fluxes is discussed.

Accretion in a Kerr black hole magnetosphere: Energy and angular momentum transport between the magnetic field and the matter

A magnetobydrodynamic (MHD) interaction between accreting matter and magnetic field in a stationary and axisymmetric magnetospbere surrounding a Kerr black bole which will exist in active galactic nuclei is studied. The critical condition that the MHD ingoing flows must pass through the fast magnetosonic point is analyzed in detail under the assumption of magnetically dominated limit. It is found that this condition restricts the shape of the magnetic field lines threading the event horizon. For example, cylindrical field lines must be bent into radial or paraboloidal ones

High-Energy Emission from Pulsar Outer Magnetospheres

We investigate particle accelerators in rotating neutron-star magnetospheres by simultaneously solving the Poisson equation for the electrostatic potential together with the Boltzmann equations for electrons, positrons, and photons on the poloidal plane. Applying the scheme to the three pulsars, Crab, Vela, and PSR B1951+32, we demonstrate that the observed phase-averaged spectra are basically reproduced from infrared to very high energies. It is found that the Velas spectrum in 10-50 GeV is sensitive to the three-dimensional magnetic field configuration near the light cylinder; thus, a careful argument is required to discriminate the inner-gap and outer-gap emissions using a gamma-ray telescope like GLAST. It is also found that PSR B1951+32 has a large inverse-Compton flux in TeV energies, which can be detected by ground-based air Cerenkov telescopes as a pulsed emission.

Electrodynamic Structure of an Outer Gap Accelerator: Location of the Gap and the Gamma-Ray Emission from the Crab Pulsar

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. If no current is injected at either of the boundaries of the accelerator, the gap is located around the conventional null surface, where the local Goldreich-Julian charge density vanishes. However, we first find that the gap position shifts outward (or inward) 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 conventional null surface so that the observed spectrum of pulsed GeV fluxes may be emitted via a curvature process. Some implications of the existence of a solution for a super Goldreich-Julian current are discussed.

One-dimensional electric field structure of an outer gap accelerator — III. Location of the gap and the gamma-ray spectrum

We investigate a stationary particle acceleration zone 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. As a result of this pair-production cascade, the replenished charges partially screen the electric field, which is self-consistently solved together with the distribution 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 find that the gap position shifts outwards (or inwards) when particles are injected at the inner (or outer) boundary. We apply the theory to the nine pulsars of which X-ray fields are known from observations. We show that the gap should be located near to or outside of the null surface for the Vela pulsar and PSR B1951+32, so that their expected GeV spectrum may be consistent with observations. We then demonstrate that the intrinsically large TeV flux from the outer gap of PSR B0540-69 is absorbed by the magnetospheric infrared photons to be undetectable. We also point out that the electrodynamic structure and the resultant GeV emission properties of millisecond pulsars are similar to young pulsars.

HIGH-ENERGY EMISSION FROM PULSAR MAGNETOSPHERES

A synthesis of the present knowledge on gamma-ray emission from the magnetosphere of a rapidly rotating neutron star is presented, focusing on the electrodynamics of particle accelerators. The combined curvature, synchrotron, and inverse-Compton emission from ultra-relativistic positrons and electrons, which are created by two-photon and/or one-photon pair creation processes, or emitted from the neutron-star surface, provide us with essential information on the properties of the accelerator — electric potential drop along the magnetic field lines. A new accelerator model, which is a mixture of traditional inner gap and outer gap models, is also proposed, by solving the Poisson equation for the electrostatic potential together with the Boltzmann equations for particles and gamma-rays in the two-dimensional configuration and two-dimensional momentum spaces.