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

Abstract

Full 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\(\vartheta \) between the field and the wave-vector. For essentially non-relativistic temperatures the fine structure can be observed in a wide angle range,\(\cos ^2 \vartheta \gg T_\parallel /mc^2 \), if\(\sqrt {2T_\parallel mc^2 } \cos \vartheta \underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}{< } T_ \bot \), i.e., for distributions with strong transverse anisotropyT⊥≫T‖ The transverse anisotropy may also lead to the maser amplification of the cyclotron radiation in the narrow frequency ranges corresponding to the fine structure peaks in the emissivity spectra. This occurs for sufficiently high fieldsB≥Bc(T‖/T⊥), and angles\(\vartheta \) not too close to 0 or π/2. These effects can be observed in X-ray and gamma-ray radiation of the objects associated with strongly magnetized neutron stars (particularly of the gamma-ray bursters).