Spherical Accretion onto Neutron Stars Revisited: Are Hot Solutions Possible ?

Stationary, spherical accretion onto an unmagnetized neutron star is here reconsidered on the wake of the seminal paper by Zel’dovich & Shakura (1969). It is found that new hot'' solutions may exist for a wide range of luminosities. These solutions are characterized by a high temperature, $10^{9}\div 10^{11}$ K, and arise from a stationary equilibrium model where the dominant radiative mechanisms are multiple Compton scattering and bremsstrahlung emission. For low luminosities, $\mincir 10^{-2} \ L_{E}$, only the cold’’ (`a la Zel’dovich and Shakura) solution is present.