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 lowmass X-ray binary GX 17+2. The source was on the so-called Horizontal andNormal branches. Energy spectra were selected based on the source position inthe X-ray hardness-intensity diagram. The continuum could be fairly welldescribed by the sum of a ~0.6 keV blackbody, contributing ~10% of the observed0.1-200 keV flux, and a Comptonized component, resulting from upscattering of\~1 keV seed photons by an electron cloud with temperature of ~3 keV andoptical depth of ~10. Iron K-line and edge were also present at energies 6.7and 8.5 keV, respectively. In the spectra of the Horizontal branch a hard tailwas clearly detected at energies above ~30 keV. It could be fit by a power lawof photon index ~2.7, contributing ~8% of the source flux. This componentgradually faded as the source moved towards the Normal branch, where it was nolonger detectable. We discuss the possible origin of this component and thesimilarities with the spectra of Atoll sources and black hole X-ray binaries.Comment: 11 pages, including 2 figures. Accepted for publication in ApJ Letter