Correlations between X-ray Spectral and Timing Characteristics in Cyg X-2

TL;DR

This study investigates the relationship between X-ray spectral and timing features in Cyg X-2, revealing correlations between QPO frequencies and spectral index, and suggesting high optical depth in the Compton cloud affects pulsation visibility.

Contribution

It provides the first detailed analysis of QPO and spectral index correlations in Cyg X-2, highlighting similarities with Sco X-1 and estimating the Compton cloud's optical depth.

Findings

Spectral power-law index correlates with low QPO frequencies.

Thomson optical depth of the Compton cloud is estimated to be 4-6.

High optical depth likely suppresses neutron star pulsations.

Abstract

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 up-scattering 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) tau, in framework of spherical geometry, is in the range of ~4-6, which is consistent with the neutron star's surface 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 tau. We also point out a number of similarities in terms 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.