Abstract. We consider the interaction of a slowly rotating unmagnetized neutron star with a hot (ion supported, ADAF) accretion flow. The virialized protons of the ADAF penetrate into the neutron star atmosphere, heating a surface layer. Detailed calculations are presented of the equilibrium between heating by the protons, electron thermal conduction, bremsstrahlung and multiple Compton scattering in this layer. Its temperature is of the order 40-70 keV. Its optical depth increases with the incident proton energy flux, and is of the order unity for accretion at 10 −2 -10 −1 of the Eddington rate. At these rates, the X-ray spectrum produced by the layer has a hard tail extending to 100 keV, and is similar to the observed spectra of accreting neutron stars in their hard states. The steep gradient at the base of the heated layer gives rise to an excess of photons at the soft end of the spectrum (compared to a blackbody) through an "inverse photosphere effect". The differences with respect to previous studies of similar problems are discussed, they are due mostly to a more accurate treatment of the proton penetration process and the vertical structure of the heated layer.

Abstract. We show that the inner regions of a cool accretion disk in an X-ray binary can transform into an advective, ion supported accretion flow (an optically thin ADAF, here called ISAF), through events involving only the known properties of the Coulomb interaction in a two-temperature plasma, standard radiation processes, and viscous heating. The optically thin inner edge of the disk is heated to a few 100 keV by the strong flux of hot ions from the surrounding hot ISAF. We show that he resident ions in this "warm" disk are thermally unstable due to internal viscous heating, and heat up to their virial temperature. The innermost disk regions thus evaporate and feed the ISAF. These processes are demonstrated with time dependent calculations of a two-temperature plasma in vertical hydrostatic equilibrium, including heating by external ions, internal proton-electron energy exchange, and viscous heating. The process complements the "coronal" evaporation mechanism which operates at larger distances from the central object.

Abstract. The X-ray spectrum from a cool accretion disk heated by virialized protons is computed. The cool disk is either embedded in a magnetically heated accretion disk corona or partly extends into an ion supported torus (or ADAF). We calculate the stationary equilibrium between proton heating, electron thermal conduction and the radiative losses by bremsstrahlung and Compton scattering. A heated surface layer on top of the accretion disk is produced with temperatures between 60-90 keV above a cool layer with temperatures of 0.01 keV (AGN) and 1 keV (galactic black hole candidates). The spectra produced by the surface layer are reminiscent of hard state spectra, but a bit too steep, especially for AGN's. Near the inner edge of the disk, where the optical depth of the disk τ . 1, we find that the cool component of the disk disappears. Instead, the hot protons from the corona/ADAF heat the disk, on a dynamical time-scale, to temperatures of several 100 keV, limited by pair production. This region, here called a "warm disk", could contribute significantly to the hard X-ray spectra and could be important for feeding material into an ADAF.

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