Atomic ionization in magnetic neutron star atmospheres: Transverse motion effects

At high pulsar magnetic fields, and typical temperatures characteristic of the photospheric surface layers of isolated radio pulsars, magnetic hydrogen atoms are found to be strongly deformed due to transverse motion effects. Using a Tübingen University code for these atomic states, we investigate the resulting modification on the atomic ionization processes. While mainly concentrating here on photoionization, we draw attention to the interesting competing mechanism of motion ionization, whereby atoms ionize principally due to thermal motion at the prevailing photospheric temperatures. This is a direct result of the motion-induced Lorentz forces on the electron and the proton, which tend to pull the atom apart. These same forces are also responsible for lowering the atomic ionization threshold energies, which therefore depend on the atomic transverse velocity. On the photoionization cross sections, the modifications obtained arise primarily due to atomic deformation, and the concomitant breaking of cylindrical symmetry. Certain "forbidden" photoionization channels involving a change in the l_z orbital angular momentum are thus found to be greatly enhanced by transverse motion. For photons polarized perpendicular to the magnetic field, the cross section is further found to be strongly enhanced by the transverse motion. This result may be central in predicting the correct spectra and anisotropy in the thermal emission from magnetic neutron stars. Our results should thus influence models of the thermal X-ray emission from isolated neutron stars, and the interpretation of recent and future satellite observations.