Distinguishing between a white dwarf and a neutron star in an X-ray binary

It is proposed that the single most useful clue to the nature of the accreting compact star in an X-ray binary is its ratio of X-ray to visual luminosity L_x(2-10 keV)/L_v. Present data indicate that typical L_x/L_v ratios are ~4 for white dwarfs, and ~5000 for neutron stars. These numbers are straightforward consequences of the energetics of accretion, and are approximately independent of the distance, stellar mass, and mass transfer rate. The data suggest that the ratios are also not very sensitive to any of the other system parameters, except the radius of the compact star.