In December 1995 we observed the low mass X-ray binary Cyg X-2 simultaneously in the UV and in X-rays with HST and ASCA. The source is observed to be in the upper bend of the Z-shape where the Normal Branch (NB) and the Horizontal Branch (HB) join. The UV continuum fluxes are consistent with the NB and HB levels observed by Vrtilek et al. (1990). The Goddard High Resolution Spectometer (GHRS) time resolution of 50 ms allows us to search for the 5-10 Hz quasi-periodic oscillations found on the NB and so far seen only in X-rays. The GHRS measurements of N V (lambda1238 .8; lambda1242 .8) and He II (lambda 1640.5) show radial velocities of -100 km/s with a FWHM of about 250 km/s. The radial velocities are consistent with those observed by Cowley, Crampton, & Hutchings (1979) for the orbital phase of our observations (0.7). The low-resolution observations captured Si IV (lambda 1393.8; lambda1402 .8), N IV (lambda 1486.5), and C IV (lambda 1548); the absorption lines have both circumstellar and interstellar components. The line fluxes are also consistent with those expected during the NB and HB branches. The lines provide a new and promising probe into the structure and dynamics of the accretion flow, and also enable us to test models of the UV emission and constrain properties of the secondary star.
American Astronomical Society Meeting Abstracts #188
- Pub Date:
- May 1996