Hot Atmospheres around Accreting Neutron Stars: A Possible Source for Hard X‐Ray Emission

The structure of static atmospheres around unmagnetized neutron starsundergoing steady, spherical accretion is discussed. We focus on the ``hot''configurations presented by Turolla et al. (1994) and calculate the radiationspectrum using a characteristics method. In particular, it is found thate$^+$--e$^-$ pair production may affect significantly the external atmosphericlayers, where positron and proton number densities become of the same order.The consequent increase of the scattering opacity lowers the Eddington limitand this, in turn, may drive a dynamical instability if the accretionluminosity is large enough, ultimately producing a rapid expulsion of theenvelope. If ``hot'' states are indeed accessible, this mechanism could giverise to transient phenomena in hard X--rays of potential great astrophysicalinterest.Comment: 17 pages plus 3 postscript figures, to be published in Ap