Simultaneous ASCA and HST/GHRS observations of Cygnus X-2/V1341 Cygni

Abstract

We present results from ultraviolet and X-ray observations of the low mass X-ray binary Cygnus X-2. The simultaneous HST/GHRS and ASCA observations took place during the low state of an 82-day cycle. We compare our observations as well as archival IUE and RXTE data with models that predict ultraviolet and optical continuum emission from an X-ray heated disk and a Roche-lobe-filling star. The model predictions are consistent with observed optical, ultraviolet, and X-ray variations over both orbital and long-term periods. The X-ray spectral state, the luminosities implied by fits to the X-ray data, the ultraviolet continuum and line fluxes, and the mass accretion rates obtained from fits to the ultraviolet continuum are consistent with location of our observations on the normal and horizontal branches of the Z-shaped X-ray color-color diagram. A combination of changes to mass accretion rate and obstruction by a warped disk can be invoked as a possible explanation for the motion of the ``Z'' in the color-color plane. The GHRS/G160M measurements concentrated on NV and HeII. The low-resolution (GHRS/G140L) observations captured SiIV, NIV, and CIV. Although the relative line fluxes are consistent with emission from an X-ray heated accretion disk corona, predictions from models of line emission from simple disks do not fit the observed emission line profiles.The radial velocities (80-130 km s$^{-1}$) are consistent with emission from the optical star at the orbital phase (0.70-0.74) of our observations.