The rms-flux relations in different branches in Cyg X-2

TL;DR

This study analyzes the rms-flux relations in different branches of Cyg X-2 using RXTE data, revealing positive, negative, and 'arch'-like correlations, and discusses potential explanations for these phenomena.

Contribution

It identifies and characterizes distinct rms-flux relations in different branches of Cyg X-2, providing new observational insights into their behaviors.

Findings

Positive rms-flux correlation in the normal branch

Anti-correlation in the vertical horizontal branch

'Arch'-shaped relation in the horizontal branch

Abstract

In this paper, the rms-flux (root mean square-flux) relation along the Z-track of the bright Z-Source Cyg X-2 is analyzed using the observational data of Rossi X-ray Timing Explorer (RXTE). Three types of rms-flux relations, i.e. positive, negative, and 'arch'-like correlations are found in different branches. The rms is positively correlated with flux in normal branch (NB), but anti-correlated in the vertical horizontal branch (VHB). The rms-flux relation shows an 'arch'-like shape in the horizontal branch (HB). We also try to explain this phenomenon using existing models.