G. G. Pavlov

Quantum effects in cyclotron plasma absorption

It is shown that X-ray radiation of neutron stars with magnetic fieldsB=1011–1013 G near cyclotron resonancesω=sωB(s=1,2,...) is deeply affected by such quantum effects as electron-positron vacuum polarization (significant at V=3×1028ne−1(B/BC4)≳1, whereBC=4.4×1013G), the quantizing character of the magnetic field (significant atV=3 x 1028ne−1(B/Bc)4≳1 whereBc=4.4 x 1013G), the non-harmonic character of the Landau levels, and the quantum recoil of electrons. The latter two factors shift the resonances by the frequency −s2ωB(B/2Bc)sin2ϑ, ϑ being the angle between the direction of radiation propagation and the magnetic field. IfV≪V0 (for ϑ∼1,V0∼β−1=(mc2/2T)1/2), the normal mode (NM) polarizations, as well as the absorption coefficientk1 of the extraordinary NM in the Doppler core of the first resonance (|ω−ω| ≲ωBβ cos ϑ), is only slightly affected by varyingb and/orV, whereas for the ordinary NM (at ϑ∼1)k2∼k1β2[b + (3 + tan2ϑ−2V)2]≪k1. For sufficiently largeb and/orV the quantum effects amplify resonant absorption of the ordinary NM atω∼ωB, with spin-flip transitions playing a major role atb≳1+V2. IfV∼V0, the coefficientsk1 andk2 in the Doppler core of the resonance are of the same order and acquire some peculiar features (shifts, intersections, etc.), with the NM polarizations depending sharply on ω and being strongly non-orthogonal. AtV≫V0,k2=k1(cos2 ϑ+B/2BC) and the polarizations are almost linear. Near high resonances (s≥2), as a rule,k1,2α(1 + b)s−1β2s−3 i.e., absorption increases withb due to replacement of the thermal energy of the transverse motion of electron,T, by the magnetic energyħωB. The above effects should be taken into account for an interpretation of observational data on X-ray pulsars (e.g., Her X-1) and other X-ray sources associated with neutron stars.

Radiation from a strongly-magnetized plasma: The case of predominant scattering

On the basis of diffusion approach for normal modes, solutions of the radiative transfer problem are obtained and analysed for an optically thick tenuous plasma with a strong magnetic field. The case is considered when the scattering processes without change of photon frequency ω are dominant. The radiative transfer coefficients as well as spectra, angular dependences and polarization of the outgoing radiation are investigated in detail for a ‘cold’ plasma,kTe≩mc2, |ω−sωB|≫ωkTe/mc2 )1/2|cosϑ|, whereTe is the electron plasma temperature,ωB=eB/mc the electron cyclotron frequency,s=1,2,... the number of cyclotron harmonic and ϑ the angle between the magnetic field and wave vector. The effects of electronpositron vacuum polarization are taken into account and shown to be very significant. Simple analytic solutions are obtained for various limiting cases (small and large vacuum polarization; high, low and close to the cyclotron resonance radiation frequencies; different orientations of the magnetic field, etc). The results obtained are necessary for analysing X-ray and gamma-ray radiation from strongly magnetized neutron stars.

The radiation of a hot magnetized plasma accreted by degenerate stars with a strong magnetic field

The absorption coefficients for extraordinary and ordinary electromagnetic modes are found for a tenuous hot magnetized plasma, taking into account the collisions between plasma particles and the scattering of photons. An approach is suggested which generalizes ‘collisionless’ and ‘cold-plasma’ approximations. The simple formulae obtained are valid both near, and at a distance from, the cyclotron harmonics. In particular, the ordinary mode is shown to have resonance at the cyclotron frequency. The number of noticeable reasonances of absorption coefficient at cyclotron harmonics is estimated for both modes.Using the coefficients obtained, the intensity, Stokes parameters and polarization of radiation of a homogeneous plasma slab are calculated for conditions which may be realized in the heated regions of accreted plasma in an AM Herculis-type system. The large difference between the absorption coefficient of extra-ordinary and ordinary modes near the cyclotron harmonics may result in the emission of the broad polarized continuum together with the narrow cyclotron lines. The polarization of these lines has a complicated spectral dependence.The results obtained are shown to be useful for explaining the main properties of AM Herculistype objects.

Spectra of radiation from a strongly magnetized plasma

AbstractRadiation from an optically thick, tenuous, isothermal and magnetized plasma is considered under conditions typical for X-ray pulsars, in the approximation of coupled diffusion of normal modes. The spectra are calculated of the fluxes and specific intensities of outgoing radiation, their dependences on the plasma densityN, temperatureT and magnetic fieldB are analysed with due regard to the vacuum polarization by a strong magnetic field. Simple analytical expressions are obtained in the limiting cases for the fluxes and intensities. It is shown that atEB »Ea (EB=11.6B12 keV,Ea≃0.1N221/2T1−3/4 keV,B12=B/1012 G,N22=N/1022 cm−3,T1=T/10 keV) the magnetic field strongly intensifies the flux and changes its spectrum in the regionEa ≲E ≲EB. AtE ≲T the spectrum of the energy flux is almost flat in the region

Quantum relativistic theory of the cyclotron radiation in strongly-magnetized plasmas : fine structure of the spectra and maser effect

\sqrt {E_a E_B } \lesssim E \lesssim E_B

Annihilation radiation from thermal electron-positron plasma on the ground Landau level: The case of low magnetic fields

. For homogeneous plasma without Comptonization the cyclotron line atE≃=EB appears in emission, though in many other cases it may appear in absorption. The vacuum polarization may produce the ‘vacuum feature’ atE≃EW≃13N221/2B12−1 keV, which, as a rule, appears in absorption. The intensity spectra vary noticeably with the direction of radiation, in particular, at some directions nearB, the spectra become harder than in other directions. Quantization of the magnetic field (EB>T) strongly increases the plasma luminosity (∝EB/T for homogeneous plasma). The results obtained explain a number of basic features in the observed X-ray pulsar spectra.

Annihilation radiation from a power-law distributed electron-positron plasma on the ground Landau level: the case of low magnetic fields

Full quantum relativistic treatment of the cyclotron/synchrotron emission and absorption in tenuous plasmas with superstrong magnetic field is developed for the case when the radiation wave-vector is parallel to the magnetic field. The emissivities and absorption coefficients for a plasma with arbitrary distribution function of particles are presented in terms of simple sums over the Landau levels. On the basis of these expressions, the negative absorption (maser amplification) is shown to be impossible for the longitudinal propagation in a tenuous plasma. The summation over the Landau levels is performed analytically, and the quantum effects are analysed in detail, for the thermal distribution of plasma particles. A new type of quantum relativistic oscillations is predicted in the emission and absorption spectra for a plasma with anisotropic temperature. The results obtained are useful for an interpretation of the X-ray and gamma-ray observations of the objects associated with strongly magnetized neutron stars (particularly of the gamma-ray bursters).

Annihilation Radiation in Strong Magnetic Fields and Gamma-Ray Burst Spectra

AbstractFull quantum relativistic treatment of the cyclotron/synchrotron emission and absorption in tenuous plasma with superstrong magnetic fields is developed for the case when the radiation modes are linearly polarized. Spectra of emission, absorption and polarization are investigated both analytically and numerically for the thermal distribution of the radiating particles and for distribution with anisotropic temperature. Quantum relativistic effects lead to a fine structure of the cyclotron harmonics with typical spacing (atB ≪Bc = 4.41 × 1013 G) ∼vΩB(B/Bc), where ν=1,2, ... is the harmonic number, and Ωc is the cyclotron frequency. The fine structure is the most developed for subrelativistic temperatures and magnetic fields at not too small angles