BOUND-BOUND TRANSITIONS IN STRONGLY MAGNETIZED HYDROGEN PLASMA

Bound-bound absorption of polarized photons by hydrogen atoms is investigated for conditions expected in atmospheres of strongly magnetized neutron stars. Thermal motion of atoms across the strong magnetic field causes striking changes of the binding energies and oscillator strengths; these changes then lead to a new type of broadening of spectral lines. This magnetic broadening exceeds the collisional and Doppler broadenings by orders of magnitude, smearing the lines over wide, overlapping frequency bands. Low-energy components of the resulting spectra are due to strongly decentered atoms whose electrons are shifted from the protons by the electric field caused by the motion across-the magnetic field. The absorption spectra depend significantly on the polarization and direction of radiation with respect to the magnetic field. The bound-bound transitions are expected to determine the spectra and the light curves of the soft X-ray/UV thermal-like radiation from middle-aged cooling neutron stars.