Long-term studies of Z sources with hexte/rxte

Using the High Energy X-Ray Timing Experiment (HEXTE) on-board the Rossi XRay Timing Explorer (RXTE) we perform a spectral study of the Z sources, with emphasis on the production of hard X-ray tails (HXT). Data from the Proportional Counter Array (PCA) were used to determine the position of the source in the Z diagram. We present current results of this ongoing study. Previous results show that, at least for Sco X-1, there is no correlation between the presence of a HXT and the position on the Z. For GX 5-1, in 1 out of 12 observations, the presence of a HXT is compelling, although contamination by the nearby hard source GRS 1758-258 may be possible. We took advantage of the HEXTE background field measurements to avoid possible contamination by the diffuse Galactic Ridge X-ray emission. No other HXT were found for GX 349+2 (in 10 observations), GX 17+2 (13 observations), GX 340+0 (14 observations) and Cyg X-2 (13 observations). Based on the HXT detected by HEXTE/RXTE and BeppoSAX in Z sources, we argue that the x hard X- ray emission in such sources is a process triggered when the brightness of the thermal component exceeds a certain threshold level.