Mass and radius determination for the neutron star in X-ray burst source 4U/MXB 1728-34

Abstract

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 T_eff on the neutron star surface, logarithm of surface gravity log(g), and the gravitational redshift z simultaneously. We have chosen fitting by 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 subsequently were used to determine mass and radius of the neutron star. We obtained the best values of the parameters for the neutron star in X-ray burst source MXB 1728-34: mass either M=0.40 or 0.63 M_sol (for H-He or H-He-Fe models, respectively), radius R=4.6 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 1-sigma confidence limits. Their values strongly support the equation of state for strange matter in MXB 1728-34.