The discovery of a state-dependent hard tail in the X-ray spectrum of the luminous Z source GX 17+2

We report results of a BeppoSAX (0.1-200 keV) observation of the Z-type low -mass X-ray binary GX 17 + 2. The source was on the so-called horizontal an d normal branches. Energy spectra were selected based on the source positio n in the X-ray hardness-intensity diagram. The continuum could be fairly we ll described by the sum of a similar to0.6 keV blackbody, contributing simi lar to 10% of the observed 0.1-200 keV flux, and a Comptonized component, r esulting from upscattering of similar to1 keV seed photons by an electron c loud with temperature of similar to3 keV and optical depth of similar to 10 . Iron K line and edge were also present at energies of similar to6.7 and s imilar to8.5 keV, respectively. In the spectra of the horizontal branch, a hard tail was clearly detected at energies above similar to 30 keV. It coul d be fit by a power law of photon index similar to2.7, contributing similar to8% of the source flux. This component gradually faded as the source move d toward the normal branch, where it was no longer detectable. We discuss t he possible origin of this component and the similarities with the spectra of atoll sources and black hole X-ray binaries.