The X-ray spectrum of the bursting atoll source 4U 1728-34 observed with
INTEGRAL

Falanga, M.; Götz, D.; Goldoni, P.; Farinelli, R.; Goldwurm, A.; Mereghetti, S.; Bazzano, A.; Stella, L.

doi:10.1051/0004-6361:20065297

Cited by 37 publications

(53 citation statements)

References 51 publications

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“…Merging their results with those of Falanga et al (2006) for the atoll source 4U 1728-34 (GX 354-0), P06 identified four main spectral states for NS LMXBs (see Fig. 4 in P06): a low/hard state (GX 354-0), a hard/PL state (H 1750-440 and H 1608-55), an intermediate state (where the hard X-ray tail appears, e.g., Cyg X-2, Sco X-1, GX 5-1, GX 17+2), and a soft state (e.g., GX 3+1, GX 9+1, GX 9+9).…”

Section: Introductionmentioning

confidence: 78%

The X-ray spectral evolution of Cygnus X-2 in the framework of bulk Comptonization

Farinelli¹,

Paizis

Landi

et al. 2009

A&A

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Context. Strong theoretical and observational support exists that the spectral evolution of neutron-star LMXBs, including transient hard X-ray tails, can be explained by the interplay between thermal and bulk motion Comptonization. The introduction of a new XSPEC Comptonization model, Comptb, including thermal and bulk Comptonization, has provided additional support to this interpretation. Aims. We used Comptb to investigate the spectral evolution of the neutron-star LMXB Cyg X-2 along its Z track. We selected a single source to trace in a quantitative way the evolution of the physical parameters of the model. Methods. We analyzed archival broad-band BeppoSAX spectra of Cyg X-2. Five broad-band spectra were newly extracted by using information about the source position in the Z track described in the colour-colour and colour-intensity diagrams. Results. We fitted the spectra of the source with two Comptb components. The first one, with a bulk parameter δ = 0, represents the dominant component of the overall source broad-band spectrum and its origin is related to thermal upscattering (Comptonization) of cold seed photons by warm electrons in a high opacity enviroment. We attribute the origin of these seed photons to the section of the disk that illuminates the outer coronal region (transition layer) located between the accretion disk itself and the neutron-star surface. The physical properties of this thermal component are roughly constant with both time and inferred mass accretion rate. The second Comptb model describes the overall Comptonization (thermal plus bulk, δ > 0) of hotter seed photons that originate in both the inner transition layer and at the neutron-star surface. This component is more significant in the horizontal branch of the colour-colour or hardness-intensity diagram and progressively disappears towards the normal branch, where a pure blackbody spectrum is observed. Conclusions. The spectral evolution of Cyg X-2 is studied and interpreted in terms of changes in the innermost environmental conditions of the system, leading to a variable thermal-bulk Comptonization efficiency.

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Section: Introductionmentioning

confidence: 78%

The X-ray spectral evolution of Cygnus X-2 in the framework of bulk Comptonization

Farinelli¹,

Paizis

Landi

et al. 2009

A&A

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“…) show that the soft state spectrum of the source can be fitted either by a two-BB model or by a single TC model (Comptb, F08) with α ∼ 1. Note also that Falanga et al (2006) fitted the soft state spectrum of GX 354-0 with a multi-color disk BB (DBB, Mitsuda et al 1984) plus Comptt model, from which the inferred value is again α ∼ 1. Actually, when looking at the BeppoSAX results obtained over years of observations of NS LMXB sources, it is evident that the soft state spectra of these systems, rather than two BBs, need to be described by the sum of a BB component plus an unsaturated TC spectrum with cut-off energy below 10 keV.…”

Section: Spectral Index Evolution In Neutron Star Systemsmentioning

confidence: 96%

“…Most variable sources do exhibit pure TC spectra, with the electron temperature kT e progressively decreasing and the optical depth τ 0 increasing, respectively, when they move from the hard to the soft state. Examples for this are GX 354-0 (Falanga et al 2006) or 4U 1608-52 (Gierliński & Done 2002). On the other hand, bright LMXBs of the GX class such as the classical six known Z sources (Sco X-1, GX 17+2, Cyg X-2, GX 340+0, GX 5-1 and GX 349+2) and, more recently, GX 13+1, show only a small evolution of their persistent X-ray continuum (dominated by the strong TC bump with kT e ∼ 3-5 keV and τ 0 1).…”

Section: Spectral Index Evolution In Neutron Star Systemsmentioning

confidence: 99%

“…Moreover the spectra of NS sources both in the soft and hard state can be adequately fitted by single-temperature Comptonization models (e.g., Falanga et al 2006;Paizis et al 2006;Farinelli et al 2007;Cocchi et al 2010). This observational fact demonstrates that an assumption of isothermal plasma in the TL can be applicable to X-ray data analysis from NS binaries.…”

Section: Energy Release In the Neutron Star Transition Layermentioning

confidence: 99%

“…Using long-term average spectra and including former INTEGRAL results on GX 354-0 (Falanga et al 2006), P06 classified NS LMXBs into four main states: the hard/PL, low/hard, intermediate and soft. The high-energy (>20 keV) spectra in different sources were interpreted by P06 as the result of the interplay between thermal and bulk Comptonization processes, whose relative efficiency is ultimately dictated by the mass-accretion rate.…”

Section: Introductionmentioning

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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Modelling the behaviour of accretion flows in X-ray binaries

Done

Gierliński

Kubota

2007

Astron Astrophys Rev

1,228

1,289

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(abridged) We review how the recent increase in X-ray and radio data from black hole and neutron star binaries can be merged together with theoretical advances to give a coherent picture of the physics of the accretion flow in strong gravity. Both long term X-ray light curves, X-ray spectra, the rapid X-ray variability and the radio jet behaviour are consistent with a model where a standard outer accretion disc is truncated at low luminosities, being replaced by a hot, inner flow which also acts as the launching site of the jet. Decreasing the disc truncation radius leads to softer spectra, as well as higher frequencies (including QPO's) in the power spectra, and a faster jet. The collapse of the hot flow when the disc reaches the last stable orbit triggers the dramatic decrease in radio flux, as well as giving a qualitative (and often quantitative) explanation for the major hard--soft spectral transition seen in black holes and neutron stars. After collapse of the hot inner flow, the spectrum in black hole systems can be dominated by the disc emission. Its behaviour is consistent with the existence of a last stable orbit, and such data can be used to estimate the black hole spin. These systems can also show very different spectra at these high luminosities, in which the disc spectrum is strongly distorted by Comptonization. The structure of the accretion flow becomes increasingly uncertain as the luminosity approaches (and exceeds) the Eddington luminosity, though there is growing evidence that winds play an important role. We stress that these high Eddington fraction flows are key to understanding many disparate and currently very active fields such as ULX, Narrow Line Seyfert 1's, and the growth of the first black holes in the Early Universe.Comment: 76 pages, accepted for publication in Astronomy and Astrophysics Review

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The X-ray spectrum of the bursting atoll source 4U 1728-34 observed with INTEGRAL

Cited by 37 publications

References 51 publications

The X-ray spectral evolution of Cygnus X-2 in the framework of bulk Comptonization

The X-ray spectral evolution of Cygnus X-2 in the framework of bulk Comptonization

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

Modelling the behaviour of accretion flows in X-ray binaries

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