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

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, δt < 20 s, variability in these sources. Here we present our data base 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 data set 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 anticorrelation 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.