Simultaneous optical and X-ray flickering observations of GX 339-4: correlations on sub-second timescales

We present a fast timing analysis of VLT (optical) and RXTE (X-ray) observations of the Galactic black hole GX 339-4 in the low/hard rapid-flickering state, soon after emerging from outburst. Both optical and X-ray power density spectra are qualitatively similar with a break or QPO-like feature at timescales of similar to 10-20 s. In a cross-correlation analysis on the fastest time resolution in the light curves (similar to 50-100 ms), a significant (> 10 sigma) correlation peak is detected, with the optical lagging X-rays by similar to 150 ms. The correlation exhibits a complex, but stable pattern with a shallow rise, a steep decline, and anti-correlation dips on timescales of several seconds. The optical radiation is unlikely to be a result of reprocessed X-rays. We discuss the results in the context of cyclo-synchrotron emission. The complex variability pattern resembles that seen in another important X-ray binary XTE J1118+480, and is likely to be the result of several interacting components in the inner accreting regions. We emphasize the need for further rapid multi-wavelength observations of this fascinating source.