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 BeppoSAX and 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 × 1037 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+0.10-0.07 keV and 2.80+0.09-0.04 keV, respectively, while the optical depth of the Comptonizing corona is 8.90.4. The color temperature of the inner accretion disk is 0.86+0.08-0.02 keV and 0.82 0.02 keV for the BeppoSAX and XMM-Newton spectrum, respectively. In the Western Model scenario, instead, we find that the seed-photon temperature is 0.87 0.07 keV and 1.01 0.08 keV for the BeppoSAX and XMM-Newton spectrum, respectively. (Abridged abstract)