摘要：Context.GX 9+9 (4U 1728−16) is a low mass X-ray binary source harboring a neutron star. Although it belongs to the subclass of the bright Atoll sources together with GX 9+1, GX 3+1, and GX 13+1, its broadband spectrum is poorly studied and apparently does not show reflection features in the spectrum.Aims.To constrain the continuum well and verify whether a relativistic smeared reflection component is present, we analyze the broadband spectrum of GX 9+9 usingBeppoSAX andXMM-Newtonspectra covering the 0.3−40 keV energy band.Methods.We fit the spectrum adopting a model composed of a disk-blackbody plus a Comptonized component whose seed photons have a blackbody spectrum (Eastern Model). A statistically equivalent model is composed of a Comptonized component whose seed photons have a disk-blackbody distribution plus a blackbody that mimics a saturated Comptonization likely associated with a boundary layer (Western model). Other trials did not return a good fit.Results.The spectrum of GX 9+9 was observed in a soft state and its luminosity is 2.3 × 1037erg s−1assuming a distance to the source of 5 kpc. In the Eastern Model scenario, we find the seed-photon temperature and electron temperature of the Comptonized component to be 1.14−0.07+0.10keV and 2.80−0.04+0.09keV, respectively, while the optical depth of the Comptonizing corona is 8.9 ± 0.4. The color temperature of the inner accretion disk is 0.86−0.02+0.08keV and 0.82 ± 0.02 keV for theBeppoSAX andXMM-Newtonspectrum, respectively. In the Western Model scenario, instead, we find that the seed-photon temperature is 0.87 ± 0.07 keV and 1.01 ± 0.08 keV for theBeppoSAX andXMM-Newtonspectrum, respectively. The electron temperature of the Comptonized component is 2.9 ± 0.2 keV, while the optical depth is 9.4−1.1+1.5. The blackbody temperature is 1.79−0.18+0.09keV and 1.85−0.15+0.07keV for theBeppoSAX andXMM-Newtonspectrum, respectively. The addition of a relativistic smeared reflection component improved the fit in both the scenarios, giving compatible values of the parameters, even though a significant broad emission line in the Fe-K region is not observed.Conclusions.From the reflection component we estimated an inclination angle of about 43−4+6deg and 51−2+9deg for the Eastern and Western Model, respectively. The value of the reflection fraction Ω/2πis 0.18 ± 0.04 and 0.21 ± 0.03 for the Eastern and Western Model, respectively, suggesting that the Comptonized corona should be compact and close to the innermost region of the system.
