High time resolution optical/X-ray cross-correlations for X-ray binaries: anti-correlations and rapid variability

TL;DR

This study analyzes high time-resolution X-ray and optical data from four X-ray binaries, revealing dynamic, anti-correlated variability patterns that challenge reprocessing models and suggest complex, non-linear interactions.

Contribution

First comprehensive dynamical cross-correlation analysis of simultaneous optical and X-ray data in multiple X-ray binaries, revealing time-varying anti-correlations and complex variability patterns.

Findings

Anti-correlation signals precede positive correlations in all sources.

Cross-correlation functions vary significantly over time.

Anti-correlations are more prominent in hard X-ray spectra.

Abstract

Using simultaneous observations in X-rays and optical, we have performed a homogeneous analysis of the cross-correlation behaviours of four X-ray binaries: SWIFT J1753.5-0127, GX 339-4, Sco X-1, and Cyg X-2. With high time-resolution observations using ULTRACAM and RXTE, we concentrate on the short time-scale, dt<20 s, variability in these sources. Here we present our database of observations, with three simultaneous energy bands in both the optical and the X-ray, and multiple epochs of observation for each source, all with ~second or better time resolution. For the first time, we include a dynamical cross-correlation analysis, i.e., an investigation of how the cross-correlation function changes within an observation. We describe a number of trends which emerge. We include the full dataset of results, and pick a few striking relationships from among them for further discussion. We find, that the surprising form of X-ray/optical cross-correlation functions, a positive correlation signal preceded by an anti-correlation signal, is seen in all the sources at least some of the time. Such behaviour suggests a mechanism other than reprocessing as being the dominant driver of the short-term variability in the optical emission. This behaviour appears more pronounced when the X-ray spectrum is hard. Furthermore, we find that the cross-correlation relationships themselves are not stable in time, but vary significantly in strength and form. This all hints at dynamic interactions between the emitting components which could be modelled through non-linear or differential relationships.