Detection of a Hard Tail in the X‐Ray Spectrum of the Z Source GX 349+2

Salvo, T. Di; Robba, N. R.; Iaria, R.; Stella, L.; Burderi, L.; Israel, G. L.

doi:10.1086/321353

Cited by 110 publications

(121 citation statements)

References 37 publications

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“…For GX 349+2 (Di Salvo et al 2000a) and GX 354-0 (Di Salvo et al 2000b) we derived the value of the spectral index α with the equation for the non-relativistic regime (see Eq. (22) in TL95):…”

Section: Observational Resultsmentioning

confidence: 99%

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On the stability of the thermal Comptonization index in neutron star low-mass X-ray binaries in their different spectral states

Farinelli¹,

Titarchuk²

2010

A&A

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Context. Most of the spectra of neutron star low-mass X-ray binaries (NS LMXBs), whether they are persistent or transient, are characterized by the presence of a strong thermal Comptonization bump, which is thought to originate in the transition layer (TL) between the accretion disk and the NS surface. The observable quantities that characterize this component, which is dominating the emission below 30 keV, are the spectral index α and the rollover energy, both related to the electron temperature and optical depth of the plasma. Aims. Starting from observational results on a sample of NS LMXBs in different spectral states, we formulate the problem of X-ray spectral formation in the TL of these sources. We predict a stability of the thermal Comptonization spectral index in different spectral states if the energy release in the TL is much higher than the intercepted flux coming from the accretion disk. Methods. We use an equation for the energy balance and the radiative transfer diffusion equation for a slab geometry in the TL to derive a formula for the thermal Comptonization index α. We show that in this approximation the TL electron temperature kT e and optical depth τ 0 can be written as a function of the energy flux from the disk intercepted by the corona (TL) and that in the corona itself, Q disk /Q cor . Because the spectral index α depends on kT e and τ 0 , this in turn leads to a relation α = f (Q disk /Q cor ), with α ∼ 1 when Q disk /Q cor 1. Results. We show that the observed spectral index α for the sample of sources here considered lies in a belt around 1 ± 0.2 apart for the case of GX 354-0. Comparing our theoretical predictions with observations, we claim that this result, which is consistent with the condition Q disk /Q cor 1, can give us constraints on the accretion geometry of these systems, an issue that seems difficult to be solved with only the spectral analysis method.

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Section: Observational Resultsmentioning

confidence: 99%

“…Moreover, in addition to the persistent X-ray emission, BeppoSAX (e.g., Di Salvo et al 2000aSalvo et al , 2002) RXTE (D'Amico et al 2001;Di Salvo et al 2006) and later also INTEGRAL (Paizis et al 2006, hereafter P06) allowed the possibility to discover a transient powerlaw (PL) X-ray emission above 30 keV in bright NS LMXBs.…”

Section: Introductionmentioning

confidence: 99%

On the stability of the thermal Comptonization index in neutron star low-mass X-ray binaries in their different spectral states

Farinelli¹,

Titarchuk²

2010

A&A

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“…In these sources such a transient feature has as yet never been observed, even with the past missions. In GX 5-1, a transient hard X-ray tail was instead previously detected by Ginga (Asai et al 1994) and INTEGRAL (Paizis et al 2005), in GX 13+1 it was detected for the first time by INTEGRAL (P06), while in GX 349+2, the feature was observed by BeppoSAX (Di Salvo et al 2001) when the source was out-of-flare. Also taking into account previous results on other Z sources (GX 17+2, Sco X−1, GX 340+0, see e.g.…”

Section: The Absence Of the Hard Tail In Most Bright Atoll "Gx" Sourcesmentioning

confidence: 93%

Spectral evolution of bright NS LMXBs withINTEGRAL: an application of the thermal plus bulk Comptonization model

Mainardi¹,

Paizis²,

Farinelli

et al. 2010

A&A

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Aims. The aim of this work is to investigate in a physical and quantitative way the spectral evolution of bright neutron star low-mass X-ray binaries (NS LMXBs) with special regard to the transient hard X-ray tails. Methods. We analyzed INTEGRAL data for five sources (GX 5-1, GX 349+2, GX 13+1, GX 3+1, GX 9+1) and built broad-band X-ray spectra from JEM-X1 and IBIS/ISGRI data. For each source, X-ray spectra from different states were fitted with the recently proposed model compTB. Results. The spectra have been fit with a two-compTB model. In all cases the first compTB describes the dominant part of the spectrum that we interpret as thermal Comptonization of soft seed photons (<1 keV), likely from the accretion disk, by a 3−5 keV corona. In all cases, this component does not evolve much in terms of Comptonization efficiency, with the system converging to thermal equilibrium for an increasing accretion rate. The second compTB varies more dramatically, spanning from bulk plus thermal Comptonization of blackbody seed photons to the blackbody emission alone. These seed photons (R < 12 km, kT s > 1 keV), likely from the neutron star and the innermost part of the system, the transition layer, are Comptonized by matter in a converging flow. The presence and nature of this second compTB component (whether a pure blackbody or Comptonized) are related to the inner local accretion rate which can influence the transient behavior of the hard tail: high values of accretion rates correspond to an efficient bulk Comptonization process (bulk parameter δ 0), while even higher values of accretion rates suppress the Comptonization, resulting in simple blackbody emission (δ = 0). Conclusions. The spectral evolution of the sources has been successfully studied in terms of thermal and bulk Comptonization efficiency in relation to the physical conditions in the transition layer.

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“…The iron lines in EXOSAT and BeppoSAX have been analyzed in the past (e.g., Gottwald et al 1995;Di Salvo et al 2001;Oosterbroek et al 2001;Iaria et al 2004), however, generally only a Gaussian line profile has been considered. An exception to this is the case of 4U 1705−44, where both Piraino et al (2007) and Lin et al (2010) have fitted a relativistic diskline model to BeppoSAX data, finding that it is a better fit than a simple Gaussian.…”

Section: Asymmetric Line Profilesmentioning

confidence: 99%

“…Detailed analysis of these BeppoSAX observations have previously been presented by Di Salvo et al (2001) and Iaria et al (2004), and both those papers carefully looked at variability in the Fe line. The variability we see is consistent with their work.…”

Section: Sourcementioning

confidence: 99%

A comparison of broad iron emission lines in archival data of neutron star LMXBs

Cackett

Miller

Reis

et al. 2012

AIP Conference Proceedings

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Relativistic X-ray disk-lines have been found in multiple neutron star low-mass X-ray binaries, in close analogy with black holes across the mass-scale. These lines have tremendous diagnostic power and have been used to constrain stellar radii and magnetic fields, often finding values that are consistent with independent timing techniques. Here, we compare CCD-based data from Suzaku with Fe K line profiles from archival data taken with gas-based spectrometers. In general, we find good consistency between the gas-based line profiles from EXOSAT, BeppoSAX and RXTE and the CCD data from Suzaku, demonstrating that the broad profiles seen are intrinsic to the line and not broad due to instrumental issues. However, we do find that when fitting with a Gaussian line profile, the width of the Gaussian can depend on the continuum model in instruments with low spectral resolution, though when the different models fit equally well the line widths generally agree. We also demonstrate that three BeppoSAX observations show evidence for asymmetric lines, with a relativistic disk-line model providing a significantly better fit than a Gaussian. We test this by using the posterior predictive p-value method, and bootstrapping of the spectra to show that such deviations from a Gaussian are unlikely to be observed by chance.

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Detection of a Hard Tail in the X‐Ray Spectrum of the Z Source GX 349+2

Cited by 110 publications

References 37 publications

On the stability of the thermal Comptonization index in neutron star low-mass X-ray binaries in their different spectral states

On the stability of the thermal Comptonization index in neutron star low-mass X-ray binaries in their different spectral states

Spectral evolution of bright NS LMXBs withINTEGRAL: an application of the thermal plus bulk Comptonization model

A comparison of broad iron emission lines in archival data of neutron star LMXBs

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