A ionized reflecting skin above the accretion disk of GX 349+2

Abstract

The broad emission features in the Fe-Kalpha region of X-ray binary spectra represent an invaluable probe to constrain the geometry and the physics of these systems. Several Low Mass X-ray binary systems (LMXBs) containing a neutron star (NS) show broad emission features between 6 and 7 keV and most of them are nowi nterpreted as reflection features from the inner part of an accretion disk in analogy to those observed in the spectra of X-ray binary systems containing a Black Hole candidate. The NS LMXB GX 349+2 was observed by the XMM-Newton satellite which allows, thanks to its high effective area and good spectral resolution between 6 and 7 keV, a detailed spectroscopic study of the Fe-Kalpha region. We study the XMM data in the 0.7-10 keV energy band. The continuum emission is modelled by a blackbody component plus a multicolored disk blackbody. A very intense emission line at 1 keV, three broad emission features at 2.63, 3.32, 3.9 keV and a broader emission feature in the Fe-Kalpha region are present in the residuals. The broad emission features above 2 keV can be equivalently well fitted with Gaussian profiles or relativistic smeared lines (diskline in XSPEC). The Fe-Kalpha feature is better fitted using a diskline component at 6.76 keV or two diskline components at 6.7 and 6.97 keV, respectively. The emission features are interpreted as resonant transitions of S xvi, Ar xviii, Ca xix, and highly ionized iron. Modelling the line profiles with relativistic smeared lines, we find that the reflecting plasma is located at less than 40 km from the NS, a value compatible with the inner radius of the accretion disk inferred from the multicolored disk blackbody component ($24 \pm 7$ km). The inclination angle of GX 349+2 is between 40 and 47 deg.