Ya. N. Istomin

Absorption of gamma-ray photons in a vacuum neutron star magnetosphere: I. Electron-positron pair production

The production of electron-positron pairs in a vacuum neutron star magnetosphere is investigated for both low (compared to the Schwinger one) and high magnetic fields. The case of a strong longitudinal electric field where the produced electrons and positrons acquire a stationary Lorentz factor in a short time is considered. The source of electron-positron pairs has been calculated with allowance made for the pair production by curvature and synchrotron photons. Synchrotron photons are shown to make a major contribution to the total pair production rate in a weak magnetic field. At the same time, the contribution from bremsstrahlung photons may be neglected. The existence of a time delay due to the finiteness of the electron and positron acceleration time leads to a great reduction in the electron-positron plasma generation rate compared to the case of a zero time delay. The effective local source of electron-positron pairs has been constructed. It can be used in the hydrodynamic equations that describe the development of a cascade after the absorption of a photon from the cosmic gamma-ray background in a neutron star magnetosphere.

The filling of neutron star magnetospheres with plasma: Dynamics of the motion of electrons and positrons

We consider the motion of charged particles in the vacuum magnetospheres of rotating neutron stars with a strong surface magnetic field, B ≳ 1012 G. The electrons and positrons falling into the magnetosphere or produced in it are shown to be captured by the force-free surface E · B = 0. Using the Dirac-Lorentz equation, we investigate the dynamics of particle capture and subsequent motion near the force-free surface. The particle energy far from the force-free surface has been found to be determined by the balance between the power of the forces of an accelerating electric field and the intensity of curvature radiation. When captured, the particles perform adiabatic oscillations along the magnetic field lines and simultaneously move along the force-free surface. We have found the oscillation parameters and trajectories of the captured particles. We have calculated the characteristic capture times and energy losses of the particles through the emission of both bremsstrahlung and curvature photons by them. The capture of particles is shown to lead to a monotonic increase in the thickness of the layer of charged plasma accumulating near the force-free surface. The time it takes for a vacuum magnetosphere to be filled with plasma has been estimated.

Absorption of gamma-ray photons in a vacuum neutron star magnetosphere: II. The formation of “lightnings”

The absorption of a high-energy photon from the external cosmic gamma-ray background in the inner neutron star magnetosphere triggers the generation of a secondary electron-positron plasma and gives rise to a lightning—a lengthening and simultaneously expanding plasma tube. It propagates along magnetic fields lines with a velocity close to the speed of light. The high electron-positron plasma generation rate leads to dynamical screening of the longitudinal electric field that is provided not by charge separation but by electric current growth in the lightning. The lightning radius is comparable to the polar cap radius of a radio pulsar. The number of electron-positron pairs produced in the lightning in its lifetime reaches 1028. The density of the forming plasma is comparable to or even higher than that in the polar cap regions of ordinary pulsars. This suggests that the radio emission from individual lightnings can be observed. Since the formation time of the radio emission is limited by the lightning lifetime, the possible single short radio bursts may be associated with rotating radio transients (RRATs).

The appearance of a radio-pulsar magnetosphere from a vacuum with a strong magnetic field. Motion of charged particles

The motion of electrons and positrons in the vacuum magnetosphere of a neutron star with a surface magnetic field of B ≈ 1012 G is considered. Particles created in the magnetosphere or falling into it from outside are virtually instantaneously accelerated to Lorentz factors γ ≈ 108. After crossing the force-free surface, where the projection of the electric field onto the magnetic field vanishes, a particle begins to undergo ultra-relativistic oscillations. The particle experiences a regular drift along the force-free surface simultaneous with this oscillatory motion.

The observed effect of the evolution of the inclination angle of a radio pulsar

It is shown that small glitches in the rotation period of the pulsar B1822-09 can be explained by changes in the shape of the neutron star when the shape becomes inconsistent with the rotation axis, i.e., when the symmetry axis does not coincide with the instantaneous rotation axis. Due to variations of the angle between the rotation axis and the instantaneous dipole axis due to the decreasing momentum, the angle α between the rotation axis and the symmetry axis differs from zero. As a result of mechanical stress that develops in the neutron-star crust, this angle reaches its maximum value α ≈ 2 × 10−4, then returns to zero. This change in the shape of the neutron star is observed as a slow glitch in the frequency of the pulsar’s rotation.

Formation of “lightnings” in a neutron star magnetosphere and the nature of RRATs

The connection between the radio emission from “lightnings” produced by the absorption of high-energy photons from the cosmic gamma-ray background in a neutron star magnetosphere and radio bursts from rotating ratio transients (RRATs) is investigated. The lightning length reaches 1000 km; the lightning radius is 100 m and is comparable to the polar cap radius. If a closed magnetosphere is filled with a dense plasma, then lightnings are efficiently formed only in the region of open magnetic field lines. For the radio emission from a separate lightning to be observed, the polar cap of the neutron star must be directed toward the observer and, at the same time, the lightning must be formed. The maximum burst rate is related to the time of the plasma outflow from the polar cap region. The typical interval between two consecutive bursts is ∼100 s. The width of a single radio burst can be determined both by the width of the emission cone formed by the lightning emitting regions at some height above the neutron star surface and by a finite lightning lifetime. The width of the phase distribution for radio bursts from RRATs, along with the integrated pulse width, is determined by the width of the bundle of open magnetic field lines at the formation height of the radio emission. The results obtained are consistent with the currently available data and are indicative of a close connection between RRATs, intermittent pulsars, and extreme nullers.

The Appearance of a Radio-Pulsar Magnetosphere from a Vacuum with a Strong Magnetic Field. Accumulation of Particles

The accumulation of electrons and positrons in the vacuum magnetosphere of a neutron star with a surface magnetic field of B ∼ 1012 G is considered. It is shown that particles created in the magnetosphere or falling into the magnetosphere from outside undergo ultra-relativistic oscillations with a frequency of 10–100 MHz. These oscillations decay due to energy losses to curvature radiation and bremsstrahlung, with their frequencies reaching 1–10 GHz. Simultaneously, the particles undergo regular motion along the force-free surface along closed trajectories. This leads to the gradual accumulation of particles at the force-free surface and the formation of a fully charge-separated plasma layer with a density of the order of the Goldreich-Julian density. The presence of a constant source of electron-positron pairs in the magnetosphere due to the absorption of energetic cosmic gamma-rays leads to the growth of this layer, bringing about a rapid filling of the pulsar magnetosphere with electron-positron plasma if the paircreation multiplication coefficient is sufficiently high.

Structure of the Magnetic Field of the Jovian Magnetosphere

We exactly solved the problem of the interaction between the rotating magnetic field of Jupiter and the equatorial plasma disk formed by the gases flowing from the Jovian satellite Io. The disk is shown to expel the Jovian magnetic field in both directions, inward, toward Jupiter, compressing its dipole magnetic field, and outward. Jupiter spins up the disk up to velocities that correspond to nearly constant angular rotation, but with an angular frequency lower than the angular frequency of Jupiter itself. The radial velocity of the plasma in the disk approaches its azimuthal velocity. We determined the power of Jupiter’s rotational energy losses. Part of this energy is transferred to the disk, and the other part goes into heating the Jovian ionosphere. We show that the Pedersen surface conductivity of the Jovian ionosphere must have a lower limit to maintain the electric current that arises in the disk-rotating magnetic field system. This current in the Jovian magnetosphere flows only along the preferential magnetic surfaces that connect the inner and outer edges of the disk to the ionosphere.

Anomalous frequency dependence of scattering of the radio emission from the Crab pulsar

The frequency dependence of scattering of the radio emission from the Crab pulsar at the low frequencies 111, 63, and 44 MHz has been measured and analyzed during sporadic enhancements of scattering and dispersion measure in October–December 2006 and December 2008. The frequency dependence of the scattering differs from the generally accepted dependence, τsc(ν) ∝ νγ, where γ = −4.0 for Gaussian and γ = −4.4 for power-law Kolmogorov distributions of inhomogeneities of the scattering medium. In intervals of enhancement, the exponent of the frequency dependence γ decreased to −3.2(0.2) at the above frequencies. A model is proposed in which this is due to the presence of a dense plasma structure in the nebula in the line of sight toward the pulsar, in which scattering of the radio emission on turbulence differs from scattering in the interstellar medium. It is shown that the frequency dependence of scattering of the radio emission can be weaker in a dense plasma than in the rarefied interstellar medium.

Gamma-ray bursts as a result of an interaction of a supernova shock with the star-companion in a binary system

Interaction of a fast shock wave generated during a supernova explosion with a magnetized star-companion of the supernova precursor produces a current sheet. We consider an evolution of this current sheet and show that a singularity (shock) is formed in finite time within the ideal magnetohydrodynamics framework. Charged particles (electrons) are accelerated in the vicinity of the singularity, and their distribution function has a plateau up to the energies of the order of 104mc2. These fast particles radiate in the γ-range in the strong magnetic field of the current sheet (B ≃ 106 G). Radiation is concentrated within a narrow angle around the current sheet, Δθ ≃ 3 × 10−4, and its spectrum has the maximum at several hundreds of keV. Presented calculations confirm the model of cosmological GRBs proposed earlier by Istomin & Komberg.