Mass and Radius Determination for the Neutron Star in X-ray Burst Source 4U/MXB 1728-34

We analyzed archival X-ray spectra of MXB 1728-34 obtained in 1996-99 by the Proportional Counter Array on board of the RXTE satellite. X-ray spectra were fitted to our extensive grids of model atmosphere spectra to determine the effective temperature Teff on the neutron star surface, logarithm of surface gravity log g, and simultaneously the gravitational redshift z. We chose fitting of numerical model spectra plus broad Gaussian line, modified by interstellar absorption and the absorption on dust. We arbitrarily assumed either hydrogen-helium chemical composition of a model atmosphere, or H-He-Fe mixture in solar proportion. The statistically best values of log g and z were subsequently used to determine mass and radius of the neutron star. We obtained the best values of parameters for the neutron star in X-ray burst source MXB1728-34: mass either M=0.40 Msolar or 0.63 Msolar (for H-He or H-He-Fe models, respectively), radius R=4.6 km or 5.3 km, log g=14.6 or 14.6 and the gravitational redshift z=0.14 or 0.22. All the above parameters have very wide 1σ confidence limits. Their values strongly support the equation of state for strange matter in MXB 1728-34.