Mass, radius, surface gravity and gravitational redshift determination for the neutron star in the X-ray burst source MXB 1728-34

Agnieszka Majczyna

Warsaw University Observatory (Warsaw, Poland)


We analyse archival X-ray spectra of MXB 1728-34 obtained in 1996-99 by the instrument PCA 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 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 assume either hydrogen-helium chemical composition of a model atmosphere, or H-He-Fe mixture in solar proportion. The 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 the parameters for the neutron star in X-ray burst source MXB 1728-34: mass either M=0.52 or 0.64 Msol (for H-He or H-He-Fe models, respectively), radius and R=5.7 or 5.8 km, log(g)=14.4 or 14.5 and the gravitational redshift z=0.17 or 0.22. Confidence limits are rather large, however, they strongly support the equation of state for strange matter.


Go back to the program page