Three Type I X-ray bursts from Cyg X-2: Application of Analytical Models for Neutron Star Mass and Radius Determination

Abstract

We study the spectral and temporal properties of three Type I X-ray bursts observed from Cyg X-2 with the Rossi X-ray Timing Explorer. Despite the short time durations (~ 5 sec) these bursts show radial expansion of order of several neutron star (NS) radii. We apply the analytical models of spectral formation during the expansion and contraction stages to derive physical conditions for the matter in the burning zone close to the surface of the NS as well as the NS mass-radius relation. Our results, combined with statistical errors, show that the central object is a compact star with mass ~1.4 solar masses and radius ~9 km. Our results favor the softer equation of state for neutron star matter.