Long term studies of Z sources with HEXTE/RXTE

We have analyzed the long pointed observations of the Z sources in the Rossi X-Ray Timing Explorer (RXTE) public archive to study the high energy emission in those sources. Our analysis is concentrated on the High Energy X-Ray Timing Experiment (HEXTE) waveband, since we are primarily interested in studying the hard X-ray (i.e., E>20 keV) production in those sources. We give here the preliminary results of this ongoing study. We have found no hard X-ray tails (besides ScoX-1) in our database from any of the Z sources, i.e., GX 349+2 (<7.9 × 10 ^-5 photons cm ^-2 s ^-1, 3 σ, 50-150 keV), Cyg X-2 (<8.4 × 10 ^-5 photons cm ^-2 s ^-1, 3 σ, 50-150 keV), GX 17+2 (<4.2 × 10 ^-5 photons cm ^-2 s ^-1, 3 σ, 50-150 keV), GX5-1 (<2.1 × 10 ^-5 photons cm ^-2 s ^-1, 3 σ, 50-150 keV), and GX 340+0 (<6.0 × 10 ^-5 photons cm ^-2 s ^-1 3 σ, 50-150 keV). From the point of view of HEXTE/ RXTE observations shown here, the production of hard X-ray tails in Z sources is a process triggered when special conditions are fulfilled. One of these conditions, as derived from our analysis, is a threshold of ∼4 × 10 ³⁶ erg s ^-1 for the luminosity of the source's thermal component.