Spectra of radiation from a strongly magnetized plasma

Abstract

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 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\(\sqrt {E_a E_B } \lesssim E \lesssim E_B \). 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.