One-photon and two-photon annihilation lines in gamma-bursts,I

This paper and subsequent Paper II are an investigation of the annihilation line formation in gamma-ray bursts based on the assumption of positron production in a strong magnetic field (because of one-photon absorption of hard gamma-quanta radiated in the neutron star hot polar spot). We discuss a two-photon annihilation line in this paper. It is shown that if the star magnetic field is greater than 3×10¹² G, the relative flux in the line depends solely on the hardness of the continuum and is, as a rule, less than or about 10–20% of the total flux. This is consistent with the spectral data recorded by ‘Venera-11’ and ‘Venera-12’ space probes. The annihilation region formation above the hot polar spot is discussed, and positron density and annihilation region dimensions are estimated.     

Cyclotron lines in the spectra of solar flares and solar active regions

It was shown by Zheleznyakov and Zlotnik (1980a, b) that in complex configurations of solar magnetic fields (in hot loops above the active centres, in neutral current sheets in the preflare phase, in hot X-ray kernels in the initial flare phase) a system of cyclotron lines in the spectrum of microwave radiation is likely to be formed. Such a line was obtained by Willson (1985) in the VLA observations at harmonics of the electron gyrofrequency. This communication interprets these observations on the basis of an active region model in which thermal cyclotron radiation is produced by hot plasma filling the magnetic tube in the corona above a group of spots. In this model the frequency of the recorded 1658 MHz line corresponds to the third harmonic of electron gyrofrequency, which yields the magnetic field (196 ± 4) G along the magnetic tube axis. The linewidth f/f 0.1 is determined by the 10% inhomogeneity of the magnetic field over the cross-section of the tube; the line profile indicates the kinetic temperature distribution of electrons over the tube cross-section with the maximum value 4 × 10⁶ K. Analysis shows that study of cyclotron lines can serve as an efficient tool for diagnostics of magnetic fields and plasma in the solar active regions and flares.     

Microwave radiation from magnetic stars

Cyclotron microwave emission from magnetic stars is considered, assuming that they have coronae with the temperatureT10⁷ K and the emission measureEM10⁵⁴ cm^–3. It has been shown that the cyclotron radiation from a star with a dipole magnetic field has a specific spectrum with a maximum in the frequency rangesv _o/2 >v >sv _o/2 (s being the number of cyclotron harmonic, andv _o the gyrofrequency corresponding to the polar magnetic field) and radiation flux decreasing towards lower frequencies asv _4/3. The frequency of the spectrum maximum depends on the angle between the line-of-sight and the magnetic axis of the star. The observed radiation from a rotating magnetic star can be modulated with a modulation depth of about 0.2 at frequencies near maximum. The radiation is partially circularly-polarized in the sense of an extraordinary mode. The degree of polarization is almost constant at frequenciesv >sv _o/2 and increases with frequency atv >sv _o/2. The estimation of cyclotron radio fluxes of the nearest magnetic stars shows that they are observable in microwaves by means of modern radio astronomy.     

Cyclotron wave instability in the corona and origin of solar radio emission with fine structure

The longitudinal waves (Bernstein modes and plasma waves near the hybrid frequency) in a mixture of equilibrium coronal plasma and a small group of energetic electrons are investigated. The energetic electrons have a nonequilibrium momentum distribution inherent in trapped particles. The frequency dependence of the cyclotron instability increments is studied. Attention is paid to a significant role of the relativistic effects for the cyclotron instability of longitudinal waves. For sufficiently large velocity of nonequilibrium electrons the increments are shown to increase when the hybrid frequency coincides with one of the gyrofrequency harmonics (double plasma resonance). The results obtained are used in Parts II and III to explain tadpoles and zebra-pattern in solar radio bursts.     

On the theory of X-ray pulsar radiation

The origin of hard X-ray spectrum (continuum and cyclotron lines) of pulsars in binary systems is discussed. A model of the polar region of a neutron star consisting of a hot spot in a dense plasma atmosphere with a quasi-homogeneous magnetic field and an extended accreting column in an inhomogeneous dipolar field is investigated. In the hot spot bremsstrahlung and Thomson scattering form continuum radiation, while bremsstrahlung and cyclotron scattering produce the absorption cyclotron lines. By the observed continuum intensity one can estimate the maximum distances to pulsars. Cyclotron scattering in gyro-resonant layers localized in the accreting column leads to a general attenuation of the radiation of a hot spot, but is unable to ensure the formation of cyclotron lines. For strong accretion the hot spot radiation becomes insignificant, the lines disappear and the pulsating component of an X-ray pulsar is produced by the accreting column bremsstrahlung transformed by Thomson scattering.     

Interaction of hot plasma with radiation of magnetic white dwarfs

The influence of radiation on the electron velocity distribution in a hot nonrelativistic plasma localized near the surface of magnetic white dwarfs is investigated. The part played by the plasma in the formation of cyclotron features in the optical spectrum of these stars is studied. The region of parameters where the transverse temperature of plasma is defined by the brightness temperature of extraordinary radiation at the gyrofrequency is found. When escaping from the plasma in a homogeneous magnetic field, this component forms a cyclotron line in absorption. The ordinary radiation at the gyrofrequency and both modes at higher cyclotron harmonics are in emission or absorption depending on the magnetic field strength and hot plasma density. Possible interpretation of the observed spectral features of magnetic white dwarfs in terms of the developed theory is discussed.     

Transfer of polarization of radiation in a magnetoactive cosmic plasma

The variation in the polarization of radiation propagating in a magnetoactive plasma due to the Faraday effect and differential absorption of ordinary and extraordinary waves is considered. This problem is especially important for polarization studies of the distributed cosmic emission, the radiation of discrete sources, etc. An Equation (1.10) describing the variation of the polarization tensor (1.2) (or (1.2a)) along the direction of propagation is formulated. This equation correctly accounts for the effect of absorption in distinction to the corresponding equation ofKawabata (1964). Equation (1.10), which was obtained for a homogeneous medium, is also true for an inhomogeneous plasma when the geometrical optics approximation is valid for the radiation, the difference between the refractions of ordinary and extraordinary waves is negligible, and inequalities (1.13) are satisfied. In this case, however, the tensorsS _iq ,R _iqlm , andK _iqlm in (1.10) will depend on the coordinate.The case of quasi-longitudinal propagation for circularly polarized ordinary and extraordinary waves is treated in detail by means of (1.10). In this case, which is frequently realized in a cosmic plasma, the equations of transfer written in terms of the Stokes parameters (1.3) take the form of (2.3). Their solution for the case of a uniform plasma is obtained as (2.8)–(2.10). From the analysis of these solutions it follows that, if absorption is neglected, the orientation of the polarization ellipse of the radiation emitted in a layer of thicknessz of a magnetoactive plasma varies according to (2.20), i.e. twice as slowly as the angle of radiation incident on the layer (see (2.15)). In the presence of absorption the polarization ellipse ceases to rotate at a distance from the beginning of the layer (K _{e, 0} is the amplitude of the absorption coefficient of the extraordinary wave). If the Faraday effect is not important (see (2.24)), the angle is close to the ellipse orientation of sources in the plasma ^S . For a strong Faraday effect (2.24a) the angle is displaced relative to ^S by ±/4.The character of polarization of radiation in a plasma changes abruptly if the conditions for negative re-absorption are satisfied (K _{e, o}<0). For strong amplification within a source of dimensionsL and a marked difference in re-absorption of ordinary and extraordinary waves , the radiation emitted by the source belongs entirely to one type of wave; the polarization of this radiation is completely defined by the polarization of waves of this type in a cosmic plasma and does not depend directly on the polarization of radiation emitted by individual electrons of this source. The latter circumstance is of great importance for a treatment of the polarization characteristics of radio emission from cosmic sources with negative re-absorption.Translated from the Russian by Dean F. Smith.     

Coherent mechanisms of radio emission and magnetic models of pulsars

This paper is primarily concerned with the questions of models and the mechanisms of radio emission for pulsars, the polarization of this radiation and related topic. For convenience and to provide a more complete picture of the problems involved, a short summary of the data on pulsars is also given. Besides the introduction, the paper contains the following sections:

1. Some Facts about Pulsars.
2. The Astrophysical Nature of Pulsars.
3. Coherent Mechanisms of Radio Emission from Pulsars.
4. Models of Pulsars: Magnetic, Pulsating White Dwarfs and Neutron Stars.
5. The Polarization of the Radio Emission from Pulsars.
6. A Synthesized Model of Pulsars — Magnetic, Pulsating and Rotating Neutron Stars.
7. Concluding Remarks.

     

On the theory of cyclotron lines in the spectra of magnetic white dwarfs

The radiation transfer at the gyrofrequency in the coronae of magnetic white dwarfs is considered. The electron distribution over Landau levels, taking both radiative and collisional transitions into account, is obtained. The emissivity and absorption coefficients of extraordinary radiation at the gyrofrequency are calculated. The ranges of parameters where cyclotron lines are observed in emission or absorption are found. The upper limit on coronal plasma density (2×10¹¹ cm^–3) for isolated magnetic white dwarfs with absorption lines in the spectrum is specified.     

On a Theory of the Origin of Quasi-Harmonic Bursts on the Crab Pulsar

Astronomy Letters - The propagation and conditions for the escape of microwave radio emission from the local source of quasi-harmonic bursts that is a current sheet filled with an...