Correlations between X-Ray Spectral and Timing Characteristics in Cygnus X-2

Correlations between the quasi-periodic oscillations (QPOs) and the spectral power-law index have been reported for a number of black hole candidate sources and for four neutron star (NS) sources, 4U 0614+09, 4U 1608-52, 4U 1728-34, and Sco X-1. An examination of QPO frequencies and index relationship in Cyg X-2 is reported herein. The RXTE spectrum of Cyg X-2 can be adequately represented by a simple two-component model of Compton upscattering with a soft photon electron temperature of about 0.7 keV and an iron K line. Inferred spectral power-law index shows correlation with the low QPO frequencies. We find that the Thomson optical depth of the Compton cloud (CC) τ, in framework of spherical geometry, is in the range of ~4-6, which is consistent with the surface of the neutron star (NS) being obscured. The NS high-frequency pulsations are presumably suppressed as a result of photon scattering off CC electrons because of such high values of τ. We also point out a number of similarities in terms of timing (presence of low- and high-frequency QPOs) and spectral (high CC optical depth and low CC plasma temperature) appearances between Cyg X-2 and Sco X-1.