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

Abstract

Context: The broad emission features in the Fe-Kα 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 now interpreted 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 bole candidate. Aims: 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-Kα region. Methods: 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-Kα 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-Kα feature is better fitted using a diskline component at 6.76 keV or two diskline components at 6.7 and 6.97 keV, respectively Results: 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 ± 7 km). The inclination angle of GX 349+2 is between 40° and 47°, the emissivity index of the primary emission is between -2.4 and -2, and the reflecting plasma extends up to (2-8) × 108 cm. Conclusions: We compare our results with the twin source Sco X-1 and with the other NS LMXBs showing broad relativistic lines in their spectra. We conclude that the blackbody component in the spectrum is the primary emission that hits the inner accretion disk producing the emission lines broadened by relativistic and Doppler effects dominant around the neutron star.