Reflection component in the Bright Atoll Source GX 9+9
Résumé
$Context.$ 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 $Beppo$SAX 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 2.3 × 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$_{{−0.07}^{+0.10}}$ keV and 2.80$_{{−0.04}^{+0.09}}$ keV, respectively, while the optical depth of the Comptonizing corona is 8.9 $\pm$ 0.4. The color temperature of the inner accretion disk is 0.86$_{{−0.02}^{+0.08}}$ keV and 0.82 $\pm$ 0.02 keV for the $Beppo$SAX and $XMM-Newton$ spectrum, respectively. In the Western Model scenario, instead, we find that the seed-photon temperature is 0.87 $\pm$ 0.07 keV and 1.01 $\pm$ 0.08 keV for the $Beppo$SAX and $XMM-Newton$ spectrum, respectively. The electron temperature of the Comptonized component is 2.9 $\pm$ 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 $Beppo$SAX 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 $\pm$ 0.04 and 0.21 $\pm$ 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.Key words: stars: neutron / stars: individual: GX 9+9 / X-rays: binaries / accretion, accretion disks
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