2003A&A...407..335M

We use both time-domain and Fourier techniques to study the correlated optical/X-rays variability in the black hole X-ray nova XTE J1118+480. Its X-ray timing properties such as the shape of the X-ray power spectrum and cross-correlation functions (CCFs) between X-ray energy bands, the slight decrease of rms variability from 30% in the 2-5.9keV to 19% in the 15.5-44.4keV band, as well as the X-ray hardness/flux anti-correlation are very similar to what is found in other black hole binaries in the hard state. The optical/X-ray CCF is virtually independent of the X-ray energies. The optical flux appears to be correlated not only with the X-ray flux but also with the X-ray spectral variability. Both the coherence function and the lags between optical and the X-rays are Fourier frequency dependent. The optical/X-ray coherence function reaches its maximum (∼0.3) in the 0.1-1Hz range and the time-lags decrease with frequency approximatively like f^–0.8. The correlation between X-ray and optical light curves appears to have time-scale-invariant properties. The X-ray/optical CCF maintains a similar but rescaled shape on time-scales ranging at least from 0.1s to few 10s. Using the event superposition method we show that the correlation is not triggered by a single type of event (dip or flare) in the light curves. Instead, optical and X-ray fluctuations of very different shapes, amplitudes and time-scales are correlated in a similar mode where the optical light curve is seemingly related to the time derivative of the X-rays.