Reflection component in the Bright Atoll Source GX 9+9

Abstract

GX 9+9 (4U 1728-16) is a low mass X-ray binary (LMXB) 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. 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 {\it BeppoSAX} and \textit{XMM-Newton} spectra covering the 0.3-40 keV energy band. 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. The spectrum of GX 9+9 was observed in a soft state and its luminosity is $2.3\times 10^{37}$ 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 {\it BeppoSAX} and \textit{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 {\it BeppoSAX} and \textit{XMM-Newton} spectrum, respectively. (Abridged abstract)

arXiv comment10 pages, 8 figures, accepted by Astronomy & Astrophysics

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