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, Hα emission should arise as the accelerated electrons thermalize in the optically emitting disc, perhaps leading to correlated variability between X-rays, Hα 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 Hα. We find power on time-scales ≳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.