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-consis tently solved in the case of a spherically symmetric, ''cold,'' pure s cattering flow, including general relativity. A sequence of models, ea ch characterized by the value of the total observed luminosity, was ob tained assuming that the spectrum at the star surface is blackbody in shape. It is found that when the luminosity approaches the Eddington l imit dynamical effects become important shifting the spectrum to the b lue and producing a power-law, high-energy tail. The relevance of thes e results in connection with the observed spectral properties of LMXBs , and of Cyg X-2 in particular, are discussed.