Fast Transitions of X-ray Variability in the Neutron Star Low Mass X-ray Binary Cygnus X-2

TL;DR

This study analyzes rapid spectral and timing changes in Cygnus X-2, revealing how different quasi-periodic oscillations correlate with spectral parameters and suggesting boundary layer dynamics as a key factor.

Contribution

It provides a detailed spectral-timing analysis of rapid QPO transitions in Cygnus X-2, highlighting changes in Comptonization optical depth and boundary layer behavior.

Findings

Optical depth decreases significantly during the transition.

Spectral parameters of the disc component remain stable.

Hard rms spectra indicate boundary layer driving variability.

Abstract

We present a spectral-timing analysis of two NICER observations of the weakly magnetized neutron star low-mass X-ray binary Cygnus X-2. During these observations, we detect a rapid transition from a narrow 50-Hz horizontal-branch oscillation to a broad 5-Hz normal-branch oscillation, accompanied by an increase in source flux and a decrease in spectral hardness. Thanks to the large effective area of NICER, we are able to conduct a detailed comparison of the spectra associated with different types of quasi-periodic oscillations (QPOs) on short timescales. By fitting the spectra with a model that includes a disc and Comptonization components plus two emission lines, we find that the parameters of the disc component do not change significantly during the transition. However, assuming a fixed electron temperature, the optical depth of the Comptonization component decreases significantly. This drop in optical depth may be attributed to the expansion of the boundary layer or spreading layer.In addition, we find that the rms spectra for both the HBO and NBO are hard, suggesting that the boundary layer or spreading layer is driving the variability. We discuss the potential physical origin of the different types of QPOs.