X-ray pulsars: Their masses, radii and equations of state

The semi-empirical mass - radius relations of X-ray pulsars, which depend on the 'exterior' behavior of X-ray pulsars, are calculated for four X-ray pulsars, Cen X-3, 4U 1626-67, Her X-1 and EXO 2030+375, based on the observed data and our modified accretion torque model. Comparison between the calculated and the theoretical mass - radius relations shows that three of them, Cen X-3, 4U 1626-67 and EXO 2030+375, are consistent with the stiff or intermediate stiff equations of state, yielding the masses and radii around the typical values of neutron stars. Her X-1 seems not to be well described by the standard equations of state, though it may be reconciled with the soft equation of state.