Hot Atmospheres Around Accreting Neutron Stars: A Possible Source For Hard X--ray Emission

The structure of static atmospheres around unmagnetized neutron stars undergoing steady, spherical accretion is discussed. We focus on the ``hot'' configurations presented by Turolla et al. (1994) and calculate the radiation spectrum using a characteristics method. In particular, it is found that e$^+$--e$^-$ pair production may affect significantly the external atmospheric layers, where positron and proton number densities become of the same order. The consequent increase of the scattering opacity lowers the Eddington limit and this, in turn, may drive a dynamical instability if the accretion luminosity is large enough, ultimately producing a rapid expulsion of the envelope. If ``hot'' states are indeed accessible, this mechanism could give rise to transient phenomena in hard X--rays of potential great astrophysical interest.