Spectra of radiation from a strongly magnetized plasma

Radiation 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 density N, temperature T and magnetic field B 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 at E_B »E_a (E_B=11.6 B₁₂ keV, E_a≃0.1 N₂₂^1/2T₁^-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 region E_a ≲E ≲E_B. At E ≲T the spectrum of the energy flux is almost flat in the region {Mathematical expression}. For homogeneous plasma without Comptonization the cyclotron line at E≃=E_B appears in emission, though in many other cases it may appear in absorption. The vacuum polarization may produce the 'vacuum feature' at E≃E_W≃13 N₂₂^1/2B₁₂^-1 keV, which, as a rule, appears in absorption. The intensity spectra vary noticeably with the direction of radiation, in particular, at some directions near B, the spectra become harder than in other directions. Quantization of the magnetic field (E_B>T) strongly increases the plasma luminosity (∝E_B/T for homogeneous plasma). The results obtained explain a number of basic features in the observed X-ray pulsar spectra.