Simultaneous optical and X-ray high-speed photometry of Cyg X-2

The X-ray emission from X-ray binaries may originate in flares occurring when magnetic loops anchored in the disc reconnect. In analogy with our Sun, H emission should arise as the accelerated electrons thermalize in the optically emitting disc, perhaps leading to correlated variability between X-rays, H and the optical continuum. We present simultaneous X-ray and optical high-speed photometry of the neutron star low-mass X-ray binary Cyg X-2 to search for such correlations. The highest time resolution achieved is 5 ms in white light and 100 ms with a 3-nm filter centred on H. We find power on time-scales greater than or similar to 100 s (flickering) in optical with a total rms of a few per cent, about an order of magnitude less than that seen in X-rays. We do not find significant correlations between the X-ray and optical fluxes on short time-scales, and hence cannot conclude whether magnetic flares contribute significantly to the optical emission.