Three Type I X-Ray Bursts from Cygnus 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 Cygnus X-2 with the Rossi X-Ray Timing Explorer. Despite the short time durations (~5 s), these bursts show a radial expansion on the 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 to derive the NS's mass-radius relation. Our results, combined with statistical errors, show that the central object is a compact star with a mass of ~1.4 M☉ and a radius of ~9 km. Our results favor the softer equation of state for NS matter.