1,721,071 research outputs found
BeppoSAX view of the NS-LMXB GS 1826-238
No full textContext. The spectroscopic characteristics of GS 1826-238 a neutron star in a low-mass X-ray binary system, have already been studied by sensitive, wide band X-ray telescopes (e.g. BeppoSAX, RXTE, INTEGRAL). Up to now, the source has always been observed in a low-hard spectral state, with two spectral components typically detected. The persistent high-energy (>10 keV) emission is effectively explained by thermal Comptonisation by a hot electron cloud (kT e ∼ 20 keV); a further low energy component, modelled either by pure blackbody emission or by Compton-modified blackbody radiation by a few keV electron plasma, is generally needed to yield acceptable fits in the soft X-ray band. Aims. The aim of the present work is to investigate the origin and the nature of the low energy emission of GS 1826-238 further, along with its contribution to the bolometric output of the source, dominated by the high-temperature thermally Comptonised radiation. Methods. This kind of investigation needs sensitive data in the widest available energy band. Simultaneous covering of both the soft X-rays (below 1 keV) and the hard X-rays (up to hundreds of keV) is crucial for an unbiased characterisation of the two spectral components, so we searched the whole BeppoSAX-NFI archive for all the available GS 1826-238observations. We analysed a total of six data sets, collected from 1997 to 2000; data analysis of two of them was still unpublished. In this study we applied both a well-established (comptt) and a more recent, updated Comptonisation model (comptb), in order to get the widest quantitative information about the physical parameters at work. Results. Our results confirm that the 0.1-200 keV emission of GS 1826-238 needs two components to be explained. In particular, two populations of soft seed photons, with different colour temperatures, are observed. One population is Comptonised to high energies by a hot electron cloud (temperatures in the range 19-24 keV, anticorrelated with the source luminosity), while the other is directly observed and can be modelled by a pure blackbody. We also propose an alternative model in which both the seed photon populations are Compton-modified by the electron plasma. This model explains the observed emission of GS 1826-238 as accurately as the traditional one and, moreover, fits well in a wider evolutionary scenario able to describe the state transitions observed in neutron-star low-mass X-ray binaries. The use of comptb also indicates that, in the case of GS 1826-238, the seed photons populations are not distributed as a pure blackbody. © 2011 ESO
The Hard X-Ray Tails in Neutron Star Low-Mass X-Ray Binaries: BeppoSAX Observations and Possible Theoretical Explanation of the Case of GX 17+2
No full textWe report results of a new spectral analysis of two BeppoSAX observations of the Z source GX 17+2. In one of the observations, the source exhibited a power-law-like hard (>30 keV) X-ray tail, which was described in a previous work with a hybrid Comptonization model. Recent high-energy observations with INTEGRAL of a sample of low-mass X-ray binaries including both Z and atoll classes have shown that dynamical (bulk) Comptonization of soft photons is a possible alternative mechanism for producing hard X-ray tails in such systems. We start from the INTEGRAL results and exploit the broadband capability of BeppoSAX to better investigate the physical processes at work. We use GX 17+2 as a representative case. Moreover, we suggest that weakening (or disappearance) of the hard X-ray tail can be explained by increasing radiation pressure that originates at the surface of the neutron star (NS). As a result, the high radiation pressure stops the bulk inflow, and consequently, this radiation feedback from the NS surface leads to quenching of the dynamical (bulk) Comptonization
On the stability of the thermal Comptonization index in neutron star low-mass X-ray binaries in their different spectral states
No full textContext. 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 kTe and optical depth
τ0 can be written as a function of the energy flux from the
disk intercepted by the corona (TL) and that in the corona itself,
Qdisk/Qcor.
Because the spectral index α depends on
kTe and τ0,
this in turn leads to a relation
α = f(Qdisk/Qcor),
with α ~1 when
Qdisk/Qcor ≪ 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
Qdisk/Qcor ≪ 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
An Upscattering Spectral Formation Model for the Prompt Emission of Gamma-Ray Bursts
No full textWe propose a model for the spectral formation of gamma-ray burst (GRB) prompt emission, where the phenomenological Band function is usually applied to describe this emission. We suggest that the GRB prompt emission is mainly a result of two upscattering processes. The first process is the Comptonization of relatively cold soft photons of the star off electrons of a hot shell of plasma of temperature T e of the order of 109K (or kT e 100keV) that moves subrelativistically with the bulk velocity V b substantially less than the speed of light c. In this phase, the Comptonization parameter Y is high and the interaction between a blackbody-like soft seed photon population and hot electrons leads to formation of a saturated Comptonization spectrum modified by the subrelativistic bulk outflow. The second process is an upscattering of the previously Comptonized spectrum by the plasma outflow once it becomes relativistic. This process gives rise to the high-energy power-law (PL) component above the peak in the EF(E) diagram where F(E) is the energy flux. The latter process can be described by a convolution of the Comptonized spectrum with a broken-PL Green function. Possible physical scenarios for this second upscattering process are discussed. In the framework of our model, we give an interpretation of the Amati relation between the intrinsic spectral peak photon energy and radiated energy or luminosity, and we propose a possible explanation of the GRB temporal variability. © 2012. The American Astronomical Society. All rights reserved.
A New Comptonization Model for Weakly Magnetized, Accreting Neutron Stars in Low-Mass X-Ray Binaries
No full textWe have developed a new model for the X-ray spectral fitting package XSPEC that takes into account the effects of both thermal and dynamical (i.e., bulk) Comptonization. The model consists of two components: one is the direct blackbody-like emission due to seed photons that are not subjected to effective Compton scattering, while the other is a convolution of the Green's function of the energy operator with a blackbody-like seed photon spectrum. When combined thermal and bulk effects are considered, the analytical form of the Green's function may be obtained as a solution of the diffusion equation describing Comptonization. Using data from the BeppoSAX, INTEGRAL, and RXTE satellites, we test our model on the spectra of a sample of six bright neutron star low-mass X-ray binaries with low magnetic fields, covering three different spectral states. Particular attention is given to the transient power-law-like hard X-ray (>~30 keV) tails, which we interpret in the framework of the bulk motion Comptonization process. We show that the values of the best-fit δ-parameter, which represents the importance of bulk with respect to thermal Comptonization, can be physically meaningful and can at least qualitatively describe the physical conditions of the environment in the innermost part of the system. Moreover, we show that in fitting the thermal Comptonization spectra to the X-ray spectra of these systems, the best-fit parameters of our model are in excellent agreement with those from compTT, a broadly used and well-established XSPEC model
Mass-radius relation for neutron stars inf(R)gravity
Full text linkWe discuss the mass-radius diagram for static neutron star models obtained by the numerical solution of modified Tolman-Oppenheimer-Volkoff equations in f (R ) gravity where the Lagrangians f (R )=R +α R2(1 +γ R ) and f (R )=R1 +∊ are adopted. Unlike the case of the perturbative approach previously reported, the solutions are constrained by the presence of an extra degree of freedom, coming from the trace of the field equations. In particular, the stiffness of the equation of state determines an upper limit on the central density ρc above which the positivity condition of energy-matter tensor trace Tm=ρ -3 p holds. In the case of quadratic f (R ) gravity, we find higher masses and radii at lower central densities with an inversion of the behavior around a pivoting ρc which depends on the choice of the equation of state. When considering the cubic corrections, we find solutions converging to the required asymptotic behavior of the flat metric only for γ 1 +∊ considering ∊ as the leading parameter. We work strictly in the Jordan frame in order to consider matter minimally coupled with respect to geometry. This fact allows us to avoid ambiguities that could emerge in adopting the Einstein frame. <P /
Supergiant Fast X-Ray Transients: Swift Results and Theoretical Progress
No full textWe present an overview of our Supergiant Fast X-ray Transients (SFXT) project, by highlighting the unique observational contribution Swift is giving to this exciting new field. Since 2007 Swift has been detecting outbursts from these fast transients with the BAT and following them intensively for days with the XRT so that we now have a firm estimate of the time SFXTs spend in each intensity phase. We summarize some preliminary results on our XSPEC model that numerically solves the bulk-motion radiative transfer equation with a strong magnetic field, in the Fokker-Planck approximation
The X-ray spectral evolution of Cygnus X-2 in the framework of bulk Comptonization
No full textContext. 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, Compt
Comptonization signatures in the prompt emission of Gamma Ray Bursts
No full textWe report results of a systematic study of the broad band (2–2000 keV) time–resolved prompt emission spectra of a sample of Gamma-Ray Bursts (GRBs) detected with both theWide Field Cameras (WFCs) aboard the BeppoSAX satellite and the BATSE experiment aboard CGRO. The main goal of the paper is to test spectral models of the GRB prompt emission that have recently been proposed.
In particular, we test the photosperic model proposed by Ryde and Pe’er (2009), i.e., blackbody plus power–law, the addition of a blackbody emission to the Band function in the cases in which this function does not fit the data, and the Comptonization model developed by Titarchuk et al. (2012). By considering the few spectra for which the simple Band function does not provide a fully acceptable fit to the data(Frontera et al. 2012), only in one case we find a statistically significant better fit by adding a blackbody to this function. We confirm the results found by Ryde and Pe’er (2009) using the BATSE spectra alone. Instead when the BATSE GRB spectra are joined to those obtained with WFCs (2–28 keV), their model becomes unacceptable in most of time intervals in which we subdivide the GRB light curves. We find instead that the Comptonization model is fully consistent with the spectral data, even in the few cases in which the Band function is not acceptable. We discuss the implications of these results
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