Abstract
Recent observations of Cen X-3 show a dip in its light curve around phase 0·6. This is interpreted as being due to electron scattering in a dense wake, via which the secondary accretes material from a primary stellar wind. The phase and width of the dip yield estimates of the wind velocity and temperature. The optical depth in the wake is a function of the accretion rate to secondary mass ratio, enabling this quantity to be determined. The absorption optical depth in the stellar wind is derived and is also a function of the accretion rate and secondary mass. Assuming that the observed transition from a low to a high intensity state is caused by a fall in the wind strength, the accretion rate and secondary mass are found separately. The neutron star mass so derived is in the range ¼ to ½ M. The accretion rate gives the intrinsic luminosity of the source, which yields a distance ~2.4 Kpc.

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