The discovery of a hard X-ray component in the horizontal branch spectrum of the Z source GX 17+2
Abstract
We report on a long BeppoSAX (0.1 - 200 keV energy range) observation of the Z-source GX 17+2. The source was on Horizontal and Normal branches of the hardness-intensity diagram. Energy spectra were selected based on the source position in this diagram. The spectral continuum is well described by the sum of a ~0.6 keV blackbody and a Comptonized component, resulting from upscattering of ~1 keV seed photons by electrons at a temperature of ~3 keV and optical depth of ~10. Iron K-line and edge were also present at energies ~6.7 and ~8.5 keV, respectively, corresponding to high ionization stages (Fe XXIII-XXV). In the spectra of the Horizontal branch a hard tail was clearly detected at energies above ~30 keV. It can be fit to a power law of photon index ~2.7, contributing ~8% of the source flux. This component gradually disappears as the source moved towards the Normal branch. We discuss the similarities with spectra of accreting black holes and implications on spectral models.
- Publication:
-
Exploring the Gamma-Ray Universe
- Pub Date:
- September 2001
- Bibcode:
- 2001ESASP.459..341D
- Keywords:
-
- X-RAY SPECTRUM;
- NEUTRON STARS