2020A&A...635A.209I

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 using BeppoSAX and XMM-Newton spectra 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 2.3x10³⁷erg/s assuming 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±0.02keV for the BeppoSAX and XMM-Newton spectrum, respectively. In the Western Model scenario, instead, we find that the seed-photon temperature is 0.87±0.07keV and 1.01±0.08keV for the BeppoSAX and XMM-Newton spectrum, respectively. The electron temperature of the Comptonized component is 2.9±0.2keV, 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 the BeppoSAX and XMM-Newton spectrum, 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/2π is 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.