BOUND-BOUND TRANSITIONS IN STRONGLY MAGNETIZED HYDROGEN PLASMA

Bound-bound absorption of polarized photons by hydrogen atoms is inves tigated for conditions expected in atmospheres of strongly magnetized neutron stars. Thermal motion of atoms across the strong magnetic fiel d causes striking changes of the binding energies and oscillator stren gths; these changes then lead to a new type of broadening of spectral lines. This magnetic broadening exceeds the collisional and Doppler br oadenings by orders of magnitude, smearing the lines over wide, overla pping 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 held caused by the motion across-the magne tic field. The absorption spectra depend significantly on the polariza tion and direction of radiation with respect to the magnetic field. Th e bound-bound transitions are expected to determine the spectra and th e light curves of the soft X-ray/UV thermal-like radiation from middle -aged cooling neutron stars.