PHOTOIONIZATION OF HYDROGEN IN ATMOSPHERES OF MAGNETIC NEUTRON-STARS

The strong magnetic fields (B similar to 10(12)-10(13) G) characterist ic of neutron stars make all the properties of an atom strongly depend ent on the transverse component K-perpendicular to of its generalized momentum. In particular, the photoionization process is modified subst antially: (1) threshold energies are decreased as compared with those for an atom at rest, (2) cross section values are changed significantl y, and (3) selection rules valid for atoms at rest are violated by the motion so that new photoionization channels become allowed. To calcul ate the photoionization cross sections, we employ, for the first time, exact numerical treatment of both initial and final atomic states. Th is enables us to take into account the quasi-bound (autoionizing) atom ic states as well as coupling of different ionization channels. We ext end the previous consideration, restricted to the so-called centered s tates corresponding to relatively small values of K-perpendicular to, to arbitrary states of atomic motion. We fold the cross sections with the thermal distribution of atoms over K. For typical temperatures of neutron star atmospheres, the averaged cross sections differ substanti ally from those of atoms at rest. In particular, the photoionization e dges are strongly broadened by the thermal motion of atoms; this ''mag netic broadening'' exceeds the usual Doppler broadening by orders of m agnitude. The decentered states of the atoms give rise to the low-ener gy component of the photoionization cross section. This new component grows significantly with increasing temperature above 10(5.5) K and de creasing density below 1 g cm(-3), i.e., for the conditions expected i n atmospheres of middle-aged neutron stars.