The Discovery of a State-Dependent Hard Tail in the X-Ray Spectrum of the Luminous Z Source GX 17+2

Abstract

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 and normal branches. Energy spectra were selected based on the source position in the X-ray hardness-intensity diagram. The continuum could be fairly well described by the sum of a ~0.6 keV blackbody, contributing ~10% of the observed 0.1-200 keV flux, and a Comptonized component, resulting from upscattering of ~1 keV seed photons by an electron cloud with temperature of ~3 keV and optical depth of ~10. Iron K line and edge were also present at energies of ~6.7 and ~8.5 keV, respectively. In the spectra of the horizontal branch, a hard tail was clearly detected at energies above ~30 keV. It could be fit by a power law of photon index ~2.7, contributing ~8% of the source flux. This component gradually faded as the source moved toward the normal branch, where it was no longer detectable. We discuss the possible origin of this component and the similarities with the spectra of atoll sources and black hole X-ray binaries.