X‐Ray Spectral and Timing Observations of Cygnus X‐2

Abstract

We report on a joint BeppoSAX/RXT E observation of the Z-type low-mass X-ray binary Cyg X-2. The source was in the so-called high overall intensity state and in less than 24 hr went through all three branches of the Z track. The continuum X-ray spectrum could be described by the absorbed sum of a soft thermal component, modeled as either a blackbody or a multicolor disk blackbody, and a Com-ptonized component. The timing power spectrum showed several components, including quasi-periodic oscillations in the range 28È50 Hz while the source was on the horizontal branch (horizontal branch oscillation ; HBO). We found that the HBO frequency was well correlated with the parameters of the soft thermal component in the X-ray spectrum. We discuss implications of this correlation for models of the HBO.