Links Between Optical and X-ray Light in Cygnus X-2

TL;DR

This study investigates the relationship between optical and X-ray emissions in Cygnus X-2, revealing correlations and anti-correlations linked to the system's state on the Z track, and models the spectral data to understand the underlying physics.

Contribution

It provides new insights into the optical-X-ray connection in Cygnus X-2, demonstrating state-dependent correlations and modeling the accretion disc corona to interpret the physical parameters.

Findings

Near-zero lag peaks suggest reprocessing in the flaring branch.

Optical flux correlates with Z track position more than with physical parameters.

Correlations are strongest in higher intensity regions and vary across Z track states.

Abstract

We observed the low mass X-ray binary Cyg X-2 for a total of 18 nights over two observing runs in July and September of 2006, using the Otto Struve Telescope at McDonald Observatory and the Rossi X-ray Timing Explorer. Using discrete cross correlations, we found peaks occurring at near-zero lags in the flaring branch of the colour-colour diagram, which could signify reprocessing, in addition to an anti-correlation within the normal branch. When comparing optical flux to the system's placement on the Z track, two distinct behaviors were seen: (1) a state with no correlation, and (2) a multi-valued (horizontal and normal branches)/correlated (flaring branch) state. The correlation was the result of direct steps and more gradual falls to and from the flaring branch respectively. Finally, we modeled timed spectra with 64 second bins with an extended accretion disc corona model. We found that correlations occurred between the optical and the various fitted parameters, particularly the blackbody normalization (and blackbody radius by extension) in higher intensity regions. Despite this, the Z track location was found to be a far better predictor of physical parameters than the optical flux, with clean correlations seen in every branch of the Z track. Where optical correlations are found, the location on the Z track was a better predictor of optical flux than any individual physical parameter.