Dynamically Comptonized Spectra from Near Critical Accretion onto Neutron Stars

We investigate the effects of dynamical Comptonization on the emergent radiation spectrum produced by near critical accretion onto a neutron star. The flow dynamics and the transfer of radiation are self--consistently solved in the case of a spherically symmetric, ``cold'', pure scattering flow, including general relativity. A sequence of models, each characterized by the value of the total observed luminosity, was obtained assuming that the spectrum at the star surface is black body in shape. It is found that when the luminosity approaches the Eddington limit dynamical effects become important shifting the spectrum to the blue and producing a power--law, high--energy tail. The relevance of these results in connection with the observed spectral properties of LMXBs, and of Cyg X--2 in particular, are discussed.