1,721,053 research outputs found
Formation of X-ray spectra in close binary systems - Reflection effects
No full textThe reflection of the X-rays emitted by a compact source from the atmosphere of a normal star is considered. An equation that describes Compton scattering by electrons is derived. Analytic expressions are obtained for the differential, integrated, and spherical albedos, and also for the spectrum of the reflected radiation. The influence of the corona of the normal star on the spectrum of the reflected radiation is investigated
Effects of Resonance in Quasi-periodic Oscillators of Neutron Star Binaries
No full textUsing a large quantity of Rossi X-Ray Timing Explorer data presented in the literature, I offer a detailed investigation into the accuracy of the quasi-periodic oscillation (QPO) frequency determination. The QPO phenomenon seen in X-ray binaries is possibly a result of the resonance of the intrinsic (eigen) oscillations and harmonic driving forces of the system. I show that the resonances, in the presence of the damping of oscillations, occur at frequencies that are systematically and randomly shifted with respect to the eigenfrequencies of the system. The shift value strongly depends on the damping rate that is measured by the half-width of the QPO feature. Taking into account this effect, I analyze the QPO data for four Z sources, Scorpius X-1, GX 340+0, GX 5-1, and GX 17+2, and two atoll sources, 4U 1728-34 and 4U 0614+09. The transition-layer model (TLM) predicts the existence of the invariant quantity δ, an inclination angle of the magnetospheric axis with respect to the normal to the disk. I calculate δ and the error bars of δ using the resonance shift, and I find that the inferred δ-values are consistent with constants for these four Z sources, in which horizontal-branch oscillation and kilohertz frequencies have been detected and correctly identified. It is shown that the inferred δ are in the range between 5.5d and 6.5d. I conclude that the TLM seems to be compatible with the data
Rayleigh-Taylor Gravity Waves and Quasi-periodic Oscillation Phenomena in X-Ray Binaries
No full textAccretion onto compact objects in X-ray binaries (black hole, neutron star [NS], white dwarf) is characterized by nonuniform flow density profiles. Such an effect of heterogeneity in the presence of gravitational forces and pressure gradients causes Rayleigh-Taylor gravity waves. They should be seen as quasi-periodic wave oscillations (QPOs) of the accretion flow in the transition (boundary) layer between the Keplerian disk and the central object. In this paper I show that the main QPO frequency, which is very close to the Keplerian frequency, is split into separate frequencies (hybrid and low branch) under the influence of the gravitational forces in the rotational frame of reference. The observed low and high QPO frequencies are an intrinsic signature of Rossby gravity waves (large-scale fluid motion in the rotational frame of reference). I elaborate the conditions for the density profile for which the gravity-wave oscillations are stable. A comparison of the inferred QPO frequencies with QPO observations is presented. I find that hectohertz frequencies detected from NS binaries can be identified as the low-branch frequencies of the gravity-wave oscillations. I also predict that an observer can see the double NS spin frequency (some sort of the Rossby wave phenomenon) during the NS long (super) burst events when the pressure gradients and buoyant forces are suppressed. The Coriolis force is the only force that acts in the rotational frame of reference, and its presence causes perfect coherent pulsations with a frequency twice that of the NS spin. The QPO observations of NS binaries have established that the high QPO frequencies do not go beyond a certain upper limit. I explain this observational effect as a result of the density profile inversions. In particular, I demonstrate that a particular problem of the gravity waves in the rotational frame of reference in the approximation of very small pressure gradients is reduced to the problem of the classical oscillator in the rotational frame of reference that was previously introduced and applied for the interpretation of the kHz QPO observations in our previous publications. The Rossby type of Rayleigh-Taylor gravity waves must be present, and the related QPOs should be detected in any system where the gravity, buoyancy, and Coriolis force effects cannot be excluded (even in the Earth and solar environments)
On the specta of X-ray bursters: Expansion and contraction stages
No full textThe theory of spectral formation during the explosion and contraction stages of X-ray bursters, which include the effects of Computonization and free-free absorption and emission, is described. Analytical expressions are provided for color ratios, and the spectral shape is given as a function of input parameters, elemental abundance, neutron star mass and radius, and Eddington ratio. An Eulerian calculation is used to determine the photospheric evolution accurately during the Eddington luminosity phase. The developed analytical theory for hydrodynamics of the expansion takes into account the dependence of Compton scattering opacity on electron temperature. An analytical expression is derived from the sonic point position and the value of the sonic velcoity. Using this value as a boundary condition at the sonic point, the velocity, density, and temperature profile are calculated throughout the whole photosphere. It is shown that the atmosphere radiates spectra having a low-energy power-law shape and blackbody-like hard tail. In the expansion stage the spectra depend strongly on the temperature of the helium-burning zone at the neutron star surface. The X-ray photosheric radius increases to approximately 100 km or more, depending on the condition of the nuclear burning on the surface of the neutron star in the course of the expansion
Generalized Comptonization models and application to the recent high-energy observations
No full textThe theory of spectral formation in thermal X-ray sources, where the effects of Comptonization and Klein-Nishina corrections are important, is presented. Analytical expressions are obtained for the produced spectrum as a function of such input parameters as the plasma temperature, the optical depth of the plasma cloud and the injected soft photon spectrum. The analytical theory developed here takes into account the dependence of the scattering opacity on the photon energy. It is shown that the plasma temperature as well as the asymptotic rate of photon escape from the plasma cloud determine the shape of the upscattered hard tail in the emergent spectra, even in the case of very small optical depths. The escape distributions of photons are given for any optical depth of the plasma cloud and their asymptotic dependence for very small and large optical depths are examined. It is shown that this new generalized approach can fit spectra for a large variety of hard X-ray sources and determine the plasma temperature in the region of main energy release in Cyg X-1 and the Seyfert galaxy NGC 4151
δ-Invariant for Quasi-periodic Oscillations and Physical Parameters of the 4U 0614+09 Binary
No full textThe recently formulated two-oscillator (TO) model interprets the lowest of the kilohertz frequencies of the twin-peak quasi-periodic oscillations in X-ray binaries as the Keplerian frequency νK. The high twin frequency νh in this model holds the upper hybrid frequency relation to the rotational frequency of the neutron star's magnetosphere Ω: ν2h=ν2K+4(Ω/ 2π)2. The vector Ω is assumed to have an angle δ with the normal to the disk. The first oscillator in the TO model allows one to interpret the horizontal branch observed below 100 Hz as the lower mode of the Keplerian oscillator under the influence of the Coriolis force, with frequency νL being dependent on νh, νK, and δ. For some stars such as 4U 0614+09, Scorpius X-1, and 4U 1702-42, νh, νK, and νL have been observed simultaneously, thus providing the opportunity to check the central prediction of the TO model, i.e., the constancy of δ for a particular source. Given the considerable variation of each of these three frequencies, the existence of an observational invariant with a clear physical interpretation as a global parameter of the neutron star magnetosphere is an important test of the TO model. Using the results of recent observations of 4U 0614+09, we verify the existence of this invariant and determine the angle δ=15.6d+/-0.5d for this star. The second oscillator in the model deals with both a radial (presumably sound) oscillation and a diffuse process in the viscous layer surrounding the neutron star. Our analysis of the viscous oscillation frequency νV and the break frequency νb of the diffusion shows that the spin value of the inner boundary of the transition layer for 4U 0614+09 is at least 2 times more than the values for 4U 1728-34 and Sco X-1
Outflows near an Accreting Black Hole: Ionization and Temperature Structures
No full texte calculate the temperature and ionization balance in an outflow from an accreting black hole under illumination by hard radiation from the central object. Electron scattering of the Fe Kα photons within the highly ionized expanding flow leads to a decrease of their energy (redshift), which is of first order in v/c, when v is the outflow velocity and v is much less than the speed of light c. This photon redshift is an intrinsic property of any outflow for which divergence is positive. We also find that the equivalent widths of red-skewed Fe Kα that originated in the wind is on the order of keV. We conclude that redshifted lines are intrinsic properties of the powerful outflows that are observed in many compact objects. Downscattering of the primary line photons generated in the outflow (a more natural and probable mechanism than the general relativistic effects in the innermost part of the accretion flow) leads to the formation of red-skewed lines
On the Nature of the Flux Variability during an Expansion Stage of a Type I X-Ray Burst: Constraints on Neutron Star Parameters for 4U 1820-30
No full textWe investigate closely the flux profile during the burst expansion stage observed from 4U 1820-30 with the Rossi X- Ray Timing Explorer on 1997 May 2. We are able to uncover the behavior of a photospheric radius and to simulate the evolution of the neutron star (NS) accretion disk system. We argue that although the bolometric luminosity is always the Eddington value L_Edd, the photon flux at the bottom of the expanded envelope can decrease during the expansion stage. In fact, at the initial moment of explosion when the bottom burning temperature is ~2×10^9 K, the bottom flux Lbot is a few times the Eddington limit, because the electron cross section is a few times less than the Thomson cross section at such high temperatures. The surplus of energy flux with respect to the Eddington, L_bot-L_Edd, goes into the potential energy of the expanded envelope. As cooling of the burning zone starts the surplus decreases, and thus the envelope shrinks while the emergent photon flux stays the same, L=L_Edd. At a certain moment the NS low hemisphere, previously screened by the disk, becomes visible to the observer. Consequently, the flux detected by the observer increases. We estimate the anisotropy due to geometry and find that the system should have a high inclination angle. Finally, we apply an analytical model of X-ray spectral formation in the NS atmosphere during the burst decay stage to infer the NS mass-radius relation
A Method for Black Hole Mass Determination in Accretion-powered X-Ray Sources
No full textWe describe a method for the determination of black hole masses based on information inferred from high-energy spectra. It is required that the spectral energy distribution consist of thermal and Comptonized components. One can then, in principle, infer the depth of the gravitational potential well for sources of known distance. The thermal component is inferred by the integration of a blackbody spectral form over the disk. We assume that the color temperature distribution in the disk has a specific shape given by the Shakura-Sunyaev disk model that goes to zero at the inner disk radius and at infinity and has a maximum at 4.2 R_S. In this formulation there is only one parameter, the so-called color correction factor, relating the apparent temperature to effective temperature, which characterizes the thermal emission component. We have made use of improved Galactic black hole binary dynamical mass determinations to derive, in effect, an empirical calibration of this factor. We then present our analysis of observational data for representative objects of several classes: Galactic black hole X-ray binaries, narrow-line Seyfert galaxies (NLS1s), and ``ultraluminous'' extragalactic X-ray sources (ULXs). We then apply our mass determination calculation and present our results. We argue that this approach can potentially fill a void in the current knowledge of NLS1 and ULX properties and discuss how a deeper understanding of both classes has relevance to the broader issues of how cosmic black holes, beyond the stellar-mass realm, are formed and what is their overall mass distribution
Determination of Black Hole Masses in Galactic Black Hole Binaries Using Scaling of Spectral and Variability Characteristics
No full textWe present a study of correlations between X-ray spectral and timing properties observed from a number of Galactic black hole (BH) binaries during hard-soft state spectral evolution. We analyze 17 transition episodes from eight BH sources observed with Rossi X-Ray Timing Explorer. Our scaling technique for BH mass determination uses a correlation between the spectral index and quasi-periodic oscillation (QPO) frequency. In addition, we use a correlation between the index and the normalization of the disk "seed" component to cross-check the BH mass determination and estimate the distance to the source. While the index-QPO correlations for two given sources contain information on the ratio of the BH masses in those sources, the index-normalization correlations depend on the ratio of the BH masses and the distance square ratio. In fact, the index-normalization correlation also discloses the index-mass accretion rate saturation effect given that the normalization of disk "seed" photon supply is proportional to the disk mass accretion rate. We present arguments that this observationally established index saturation effect is a signature of the bulk motion (converging) flow onto a BH, which was early predicted by the dynamical Comptonization theory. We use GRO J1655 - 40 as a primary reference source for which the BH mass, distance, and inclination angle are evaluated by dynamical measurements with unprecedented precision among other Galactic BH sources. We apply our scaling technique to determine BH masses and distances for Cygnus X-1, GX 339 - 4, 4U 1543 - 47, XTE J1550 - 564, XTE J1650 - 500, H 1743 - 322, and XTE J1859 - 226. A good agreement of our results for sources with known values of BH masses and distance provides independent verification for our scaling technique
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