The Bursting Behavior of 4U 1728-34: Parameters of a Neutron Star and Geometry of a NS-disk system

Abstract

We analyze a set of Type I X-ray bursts from the low mass X-ray binary 4U 1728 -34, observed with Rossi X-ray Timing Explorer (RXTE). and we implement an analytical model of X-ray spectral formation in the neutron star (NS) atmosphere during a burst. We infer the dependence of the neutron star 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 helium-dominated. Our results strongly favor the soft equation of state (EOS) of NS for 4U 1728-34. We find that distance to the source should be within 4.5-5.0 kpc range. We obtain rather narrow constrains for the NS radius in 8.7-9.7 km range and interval 1.2-1.6 solar masses for NS mass for this particular distance range. We uncover a temporal behavior of red-shift 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 neutron star exposed to the Earth observer. Our scenario enables us to explain the timing characteristics and peak flux variation observed during the burst expansion stage, which is now believed to be common phenomenon. In the framework of this scenario we provide a new method for the estimation of the inclination angle which leads to the value of 50 degrees for 4U 1728-34 .Comment: 8 pages, 3 figures and one table, accepted for publication in the Astrophysical Journal Letter