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.

💡 Analysis

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.

📄 Content

arXiv:0906.3716v1 [astro-ph.HE] 19 Jun 2009 Astronomy & Astrophysics manuscript no. iaria˙manuscript˙v3˙PRINTER c⃝ESO 2021 October 20, 2021 A ionized reflecting skin above the accretion disk of GX 349+2 R. Iaria1, A. D’A´ı1, T. Di Salvo1, N. R. Robba1, A. Riggio2, A. Papitto3,4, L. Burderi2 1 Dipartimento di Scienze Fisiche ed Astronomiche, Universita di Palermo, via Archirafi36 - 90123 Palermo, Italy 2 Dipertimento di Fisica, Universita degli Studi di Cagliari, SP Monserrato-Sestu, KM 0.7, Monserrato, 09042 Italy 3 Dipartimento di Fisica, Universit`a degli Studi di Roma ’Tor Vergata’, via della Ricerca Scientifica 1,00133 Roma,Italy 4 INAF Osservatorio Astronomico di Roma, via Frascati 33, Monteporzio Catone, 00040, Italy 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 Hole 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. Key words. line: identification – line: formation – stars: individual (GX 349+2) — X-rays: binaries — X-rays: general

1. Introduction Low-mass X-ray binaries (LMXBs) consist of a low-mass star (M < 1M⊙) and a neutron star (NS), which generally has a rel- atively weak magnetic field (B < 1010G). In these systems, the X-ray source is powered by accretion of mass overflowing the Roche lobe of the companion star and forming an accretion disk around the neutron star. LMXBs containing a neutron star (NS LMXBs) are generally divided into Z and Atoll sources, accord- ing to the path they describe in an X-ray Color-Color Diagram (CD) or hardness-intensity diagram (Hasinger & van der Klis

1989. assembled by using the source count rate and colors cal- culated over a typical (usually 2-20 keV) X-ray energy range. Atoll sources are usually characterized by relatively low lu- minosities (0.01-0.2 LEdd ) and often show transient behavior, while the Z sources in the Galaxy are among the most lumi- nous LMXBs, persistently accreting close to the Eddington limit (LEdd) for a 1.4 M⊙NS. The position of an individual source in the CD, which determines most of the observed spectral and temporal properties of the source, is thought to be an indicator of the instantaneous mass accretion rate (e.g. Hasinger et al. 1990; van der Klis 1995, for a review). It has been suggested that the mass accretion rate (but not necessarily the X-ray luminosity) of individual sources increases along the track from the top left to the bottom right, i.e. from the islands to the banana branch Send offprint requests to: R. Iaria, e-mail: iaria@fisica.unipa.it in atoll sources and from the horizontal branch (hereafter HB) to the normal branch (NB) and to the flaring branch (FB) in Z sources. GX 349+2, also known as Sco X-2, was called an odd-ball among the Z sources (Kuulkers & van der Klis 1998). Similar to the case of Sco X-1, GX 349+2 shows a short and under- developed HB (if at all). The source variabili

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