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Search Results: 1 - 10 of 191057 matches for " Lev G. Titarchuk "
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Spectral Properties of Accretion Disks Around Galactic and Extragalactic Black Holes
Sandip K. Chakrabarti,Lev G. Titarchuk
Physics , 1995, DOI: 10.1086/176610
Abstract: We study the spectral properties of a very general class of accretion disks which can be decomposed into three distinct components apart from a shock at $r=r_s$: (1) An optically thick Keplerian disk on the equatorial plane ($r>r_s$), (2) A sub-Keplerian optically thin halo above and below this Keplerian disk $r>r_s$ and (3) A hot, optically slim, $\tau\sim 1$ postshock region $r
The Duration -- Photon Energy Relation of Gamma Ray Bursts and Its Interpretations
Demosthenes Kazanas,Lev G. Titarchuk,Xin-Min Hua
Physics , 1997, DOI: 10.1086/305131
Abstract: Fenimore et al. (1995) have recently presented a very tight correlation between the spectral and the temporal structure in Gamma Ray Bursts (hereafter GRBs). In particular, they discovered that the durations of the constituent subpulses which make up the time profile of a given GRB have a well defined power-law dependence, of index $\simeq 0.45$, on the energy $E$ of the observed photons. In this note we present two simple models which can account in a straightforward fashion for the observed correlation. These models involve: (a) The impulsive injection of a population of relativistic electrons and their subsequent cooling by synchrotron radiation. (b) The impulsive injection of monoenergetic high energy photons in a medium of Thompson depth $\tau_T \sim 5$ and their subsequent downgrading in energy due to electron scattering. We present arguments for distinguishing between these two models from the existing data.
Soft Phase Lags of Pulsed Emission from the Millisecond X-ray Pulsar SAX J1808.4-3658
Wei Cui,Edward H. Morgan,Lev G. Titarchuk
Physics , 1998, DOI: 10.1086/311569
Abstract: We report the discovery of phase shifts between X-ray pulses at different energies in the newly discovered millisecond (ms) X-ray pulsar SAX J1808.4-3658. The results show that low-energy pulses lag high-energy pulses by as much as $\sim$0.2 ms (or $\sim$8% of the pulse period). The measurements were made in two different ways: (1) computing cross power spectra between different energy bands, and (2) cross-correlating the folded pulse profiles in different energy bands; consistent results were obtained. We speculate that the observed soft lags might be related to the lateral expansion and subsequent cooling of a ``hot spot'' on the neutron star surface in which the pulsed X-ray emission originates. Also presented is the possibility of producing soft lags via Compton down scattering of hard X-ray photons from the hot spot in the cool surrounding atmosphere. We will discuss possible X-ray production mechanisms for SAX J1808.4-3658 and constraints on the emission environment, based on the observed soft lags, pulse profiles, and energy spectrum.
On the Spectral Slopes of Hard X-ray Emission from Black Hole Candidates
Ken Ebisawa,Lev G. Titarchuk,Sandip K. Chakrabarti
Physics , 1995, DOI: 10.1093/pasj/48.1.59
Abstract: Most black hole candidates exhibit characteristic power-law like hard X-ray emission above $\sim$ 10 keV. In the {\em high state}, in which 2 -- 10 keV luminosity is relatively high, the energy index of the hard X-ray emission is usually greater than 1 --- typically $\sim 1.5$. On the other hand, in the {\em low state}, the hard X-ray energy index is 0.3 -- 0.9. In this paper, we suggest that this difference of the hard X-ray spectral slopes may be due to two different Comptonization mechanisms. We propose that, in the high state, the hard component is governed by the Comptonization due to the bulk motion of the almost freely falling (convergent accretion) flow close to the black hole, rather than thermal Comptonization. The spectral slope of the hard component is insensitive to the disk accretion rate governing the soft component, hence is nearly invariant in spite of the soft component variations. The power-law component due to the bulk motion Comptonization has a sharp cut-off at around the electron rest mass energy, which is consistent with high energy observations of the high state. In the low state, the spectrum is formed due to thermal Comptonization of the low-frequency disk radiation by a sub-Keplerian component (possibly undergoing a centrifugally-supported shock) which is originated from the Keplerian disk. In the limit of low disk accretion rate, the power law index is uniquely determined by the mass accretion rate of the sub-Keplerian component.
Effects of Resonance in Quasiperiodic Oscillators of Neutron Star Binaries
Lev Titarchuk
Physics , 2002, DOI: 10.1086/344499
Abstract: Using a large quantity of Rossi X-ray Timing Explorer data presented in the literature I offer a detailed investigation into the accuracy of quasiperiodic oscillations (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 the frequencies which are systematically and randomly shifted with respect to the eigenfrequencies of the system. The shift value strongly depends on the damping rate which is measured by the halfwidth of the QPO feature. Taking into account this effect I analyze the QPO data for four Z-sources: Sco X-1, GX 340+0, GX 5-1, GX 17+2 and two atoll sources: 4U 1728-34, 4U 0614+09. The transition layer model (TLM) predicts the existence of the invariant quantity: delta, an inclination angle of the magnetospheric axis with respect to the normal to the disk. I calculate delta and the error bars of delta using the resonance shift and I find that the inferred delta-values are consistent with constants for these four Z-sources, where horizontal branch oscillation and kilohertz frequencies have been detected and correctly identified. It is shown that the inferred delta are in the range between 5.5 and 6.5 degrees. I conclude that the TLM seems to be compatible with data.
Rayleigh - Taylor Gravity Waves and Quasiperiodic Oscillation Phenomenon in X-ray Binaries
Lev Titarchuk
Physics , 2002, DOI: 10.1086/367630
Abstract: Accretion onto compact objects in X-ray binaries [black hole, neutron star (NS), white dwarf] is characterized by non-uniform flow density profiles. Such an effect of heterogeneity in presence of gravitational forces and pressure gradients exhibits Raylegh-Taylor gravity waves (RTGW). They should be seen as quasioperiodic wave oscillations (QPO). 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 the RTGW. I elaborate the conditions for the density profile when the RTGW 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 RTGW low branch frequencies. I also predict that an observer can see the double NS spin frequency during the NS long (super) burst events when the pressure gradients and buoyant forces are suppressed. The Coriolis force is the only force which acts in the rotational frame of reference and its presence causes perfect coherent pulsations with a frequency twice of the NS spin.
Determination of Black Hole Masses in Galactic Black Hole Binaries using Scaling of Spectral and Variability Characteristics
Nickolai Shaposhnikov,Lev Titarchuk
Physics , 2009, DOI: 10.1088/0004-637X/699/1/453
Abstract: We 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 8 BH sources observed with RXTE. Our scaling technique for BH mass determination uses a correlation between spectral index and quasi-periodic oscillation (QPO) frequency. In addition, we use a correlation between 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 black hole 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 forCygnus X-1, GX 339-4, 4U 1543-47, XTE J1550-564, XTE J1650-500, H 1743-322 and XTE J1859-226. Good agreement of our results for sources with known values of BH masses and distance provides an independent verification for our scaling technique.
Correlations between kHz QPO and Low Frequency Features Attributed to Radial Oscillations and Diffusive Propagation in the Viscous Boundary Layer Around a Neutron Star
Lev Titarchuk,Vladimir Osherovich
Physics , 1999, DOI: 10.1086/312083
Abstract: We present a dimensional analysis of two characteristic time scales in the boundary layer where the disk adjusts to the rotating neutron star (NS). The boundary layer is treated as a transition region between the NS surface and the first Keplerian orbit. The radial transport of the angular momentum in this layer is controlled by a viscous force defined by the Reynolds number, which in turn is related to the mass accretion rate. We show that the observed low-Lorentzian frequency is associated with radial oscillations in the boundary layer, where the observed break frequency is determined by the characteristic diffusion time of the inward motion of the matter in the accretion flow. Predictions of our model regarding relations between those two frequencies and frequencies of kHz QPO's compare favorably with recent observations for the source 4U 1728-34. This Letter contains a theoretical classification of kHz QPO's in NS binaries and the related low frequency features. Thus, results concerning the relationship of the low-Lorentzian frequency of viscous oscillations and the break frequency are presented in the framework of our model of kHz QPO's viewed as Keplerian oscillations in a rotating frame of reference.
A Method for Black Hole Mass Determination in Accretion Powered X-Ray Sources
Chris Shrader,Lev Titarchuk
Physics , 2003, DOI: 10.1086/378801
Abstract: We 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 (1973) disk model which goes to zero at the inner disk radius and at infinity and has a maximum at 4.2 Schwarzchild radii. 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 (NLS1), and ``ultra-luminous'' extragalactic X-ray sources (ULX). 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.
On the Nature of the Compact Object in SS~433. Observational Evidence of X-ray Photon Index Saturation
Elena Seifina,Lev Titarchuk
Physics , 2010, DOI: 10.1088/0004-637X/722/1/586
Abstract: We present an analysis of the X-ray spectral properties observed from black hole candidate (BHC) binary SS~433. We have analyzed RXTE data from this source, coordinated with Green Bank Interferometer/RATAN-600. We show that SS~433 undergoes a X-ray spectral transition from the low hard state (LHS) to the intermediate state (IS). We show that the X-ray broad-band energy spectra during all spectral states are well fit by a sum of so called ``Bulk Motion Comptonization (BMC) component'' and by two (broad and narrow) Gaussians for the continuum and line emissions respectively. In addition to these spectral model components we also find a strong feature that we identify as a "blackbody-like (BB)" component which color temperature is in the range of 4-5 keV in 24 IS spectra during the radio outburst decay in SS~433. Our observational results on the "high temperature BB" bump leads us to suggest the presence of gravitationally redshifted annihilation line emission in this source. I\ We have also established the photon index saturation at about 2.3 in index vs mass accretion correlation. This index-mass accretion correlation allows us to evaluate the low limit of black hole (BH) mass of compact object in SS~433, M_{bh}> 2 solar masses, using the scaling method using BHC GX 339-4 as a reference source. Our estimate of the BH mass in SS 433 is consistent with recent BH mass measurement using the radial-velocity measurements of the binary system by Hillwig & Gies who find that M_{x}=(4.3+/-0.8 solar masses. This is the smallest BH mass found up to now among all BH sources. Moreover, the index saturation effect versus mass accretion rate revealed in SS~433, like in a number of other BH candidates, is the strong observational evidence for the presence of a BH in SS~433.
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