Radiation-Driven Warping in Cygnus X-2/V1341 Cygni

The long-term period (78-82 days) recently discovered in Cygnus X-2/V1341 Cygni with the Rossi X-ray Timing Explorer (Wijnands, Kuulkers & Smale 1996) can be explained in terms of a disk warp maintained by the influence of the X-ray emission on the structure of the disk. The radiation-driven warping model has been successfully applied to the X-ray pulsar systems Her X-1 and LMC X-4. Here we compare simultaneous HST and ASCA data as well as archival IUE data of Cygnus X-2 with models that predict ultraviolet and optical continuum emission from the X-ray heated disk and star. The model accurately reproduces observed optical magnitudes and ultraviolet variations over both orbital and long-term periods. The simultaneous HST and ASCA observations occurred during the low state of the 82-day cycle. The required mass accretion rates are within the range previously determined for the system: 0.5x 10(-8) M_sun yr(-1) to 1.6x 10(-8) M_sun yr(-1) .