X-ray observations of VY Scl type nova-like binaries in the high and low state

Four VY Scl-type nova-like systems were observed in X-rays during both the low and the high optical states. We examined Chandra, ROSAT, Swift and Suzaku archival observations of BZ Cam, MV Lyr, TT Ari, and V794 Aql. The X-ray flux of BZ Cam is higher during the low state, but there is no supersoft X-ray source (SSS) as hypothesized in previous articles. No SSS was detected in the low state of the any of the other systems, with the X-ray flux decreasing by a factor between 2 and 50. The best fit to the Swift X-ray spectra is obtained with a multi-component model of plasma in collisional ionization equilibrium. The high state high resolution spectra of TT Ari taken with Chandra ACIS-S and the HETG gratings show a rich emission line spectrum, with prominent lines of in Mg, Si, Ne, and S. The complexity of this spectrum seems to have origin in more than one region, or more than one single physical mechanism. While several emission lines are consistent with a cooling flow in an accretion stream, there is at least an additional component. We discuss the origin of this component, which is probably arising in a wind from the system. We also examine the possibility that the VY Scl systems may be intermediate polars, and that while the boundary layer of the accretion disk emits only in the extreme ultraviolet, part of the X-ray flux may be due to magnetically driven accretion.Comment: 13 pages, 8 figures, 7 tables. Accepted for publication by MNRA

Optical studies of the X-ray transient XTE J2123-058 -I. Photometry

We present optical photometry of the X-ray transient XTE J2123-058, obtained in July-October 1998. The light curves are strongly modulated on the 5.95hrs orbital period, and exhibit dramatic changes in amplitude and form during the decline. We used synthetic models which include the effect of partial eclipses and X-ray heating effects, to estimate the system parameters, and we constrain the binary inclination to be i=73+-4 degrees. The model is successful in reproducing the light curves at different stages of the decay by requiring the accretion disc to become smaller and thinner by 30% as the system fades by 1.7 mags in the optical. From Aug 26 the system reaches quiescence with a mean magnitude of R=21.7+-0.1 and our data are consistent with the optical variability being dominated by the companion's ellipsoidal modulation.Comment: 6 pages, 6 figure