The Bursting Behavior of 4U 1728-34: The Parameters of a Neutron Star and the Geometry of a Neutron Star-Disk System

We analyze a set of type I X-ray bursts from the low-mass X-ray binary 4U 1728-34, observed with the Rossi X-Ray Timing Explorer. We infer the dependence of the neutron star (NS) mass and radius with respect to the assumed distance to the system using an analytical model of X-ray burst spectral formation. The model behavior clearly indicates that the burster atmosphere is dominated by helium. Our results strongly favor the soft equation of state of the NS for 4U 1728-34. We find that the distance to the source should be within the 4.5-5.0 kpc range. We obtain rather narrow constrains for the NS radius in the 8.7-9.7 km range and in the 1.2-1.6 Msolar interval for the NS mass for this particular distance range. We uncover a temporal behavior of redshift-corrected burst flux for the radial expansion episodes, and we put forth a dynamical evolution scenario for the NS-accretion disk geometry during which an expanded envelope affects the accretion disk and increases the area of the NS exposed to the Earth observer. In the framework of this scenario, we provide a new method for the estimation of the inclination angle that leads to the value of ~50° for 4U 1728-34