Simultaneous ASCA and Hubble Space Telescope/GHRS Observations of Cygnus X‐2/V1341 Cygni11Based partially on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5‐26555.

Abstract

ABSTRACTWe present results from ultraviolet and X‐ray observations of the low‐mass X‐ray binary Cygnus X‐2. The simultaneous Hubble Space Telescope/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 N v (λ1238.8; λ1242.8) and He ii (λ1640.5). The low‐resolution (GHRS/G140L) observations captured Si iv (λ1393.8; λ1402.8), N iv (λ1486.5), and C iv (λ 1548); absorption lines detected in the spectra are interstellar. 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 lack of double peaks suggests that most of the line emission is from the surface of the companion and 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.