Reflection component in the Bright Atoll Source GX 9+9

Abstract

Context. GX9 +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 GX9 +1, GX3 +1, and GX13 +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 GX9 +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 GX9 +9 was observed in a soft state and its luminosity is 2.3 x 10(37) erg s(-1) 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(-00.07)(+0.1) keV and 2.80(-0.04)(+0.09) keV, 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)(+0.08) keV and 0.82 +/- 0.02 keV for the BeppoSAX and XMM-Newton spectrum, respectively. In theWestern Model scenario, instead, we find that the seed-photon temperature is 0.87 +/- 0.07 keV and 1.01 +/- 0.08 keV for the BeppoSAX and XMM-Newton spectrum, 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)(+0.09) keV and 1.85(-0.15)(+0.07) keV 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)(+6) deg and 51(-2)(+9) deg for the Eastern and Western Model, respectively. The value of the reflection fraction Omega/2 pi 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.