Search

The outburst activity in the MAXI~J1348--630 has sparked a great deal of controversy, whether the source contains a black hole (BH). Here, we present the results of our analysis of the outburst of MAXI J348--630 using Swift/XRT data. We find that energy spectra in all spectral states can be modeled using a combination of Comptonization and Gaussian iron-line components. In addition, we show that the X-ray photon index, Gamma, is correlated with the mass accretion rate, Mdot. We find that Gamma increases monotonically with Mdot from the LHS to the HSS, and then saturated at Gamma~3. This index behavior is similar to that exhibited by a number of other BH candidates. This result represents observational evidence of the presence of a BH in MAXI~J1348--630. We also show that the value of Gamma is correlated with the QPO frequency, \nu_{L}. Based on this correlation, we applied a scaling method to estimate a BH mass of 14.8 +/- 0.9 M_{\odot}, using the BH binary XTE~J1550--564 as a reference source. The recent discovery of a giant dust scattering ring around MAXI~J1348--630 by SRG/eROSITA has refined distance estimates to this X-ray source. With this distance, we were able to estimate the disk inclination i = (65+/- 7)^0 using the scaling technique for the correlation between Gamma and normalization proportional to Mdot. An initial decrease in kT_s occurred simultaneously with an increase in the illumination fraction, f. At the start of the outburst, the Compton cloud (or "corona") is very extended and, thus, the seed photons injected to the corona from the relatively far-away disk region, where kT_s is about 0.2--0.4 keV. While Mdot increases (or luminosity increases), the corona contracts, thus increasing the seed photon temperature, kT_s., Comment: 17 pages, 8 figures (A&A)

In this paper we test if the ∼0.3–15 keV XMM-Newton EPIC pn spectral continuum of IPs can be described by the thermal Comptonization compTT model. We used publicly observations of 12 IPs (AE Aqr, EX Hya, V1025 Cen, V2731 Oph, RX J2133.7+5107, PQ Gem, NY Lup, V2400 Oph, IGR J00234+6141, IGR J17195-4100, V1223 Sgr, and XY Ari). We find that our modeling is capable of fitting well the average spectral continuum of these sources. In this framework, UV/soft X-ray seed photons (with 〈kT s〉 of 0.096 ± 0.013 keV) coming presumably from the star surface are scattered off by electrons present in an optically thick plasma (with 〈kT e〉 of 3.05 ± 0.16 keV and optical depth 〈τ〉 of 9.5 ± 0.6 for plane geometry) located nearby (on top) to the more central seed photon emission regions. A soft blackbody (bbody) component is observed in 5 out of the 13 observations analyzed, with a mean temperature 〈kT bb 〉 of 0.095 ± 0.004 keV. We observed that the spectra of IPs show in general two photon indices Γ, which are driven by the source luminosity and optical depth. Low luminosity IPs show 〈Γ〉 of 1.83 ± 0.19, whereas high luminosity IPs show lower 〈Γ〉 of 1.34 ± 0.02. Moreover, the good spectral fits of PQ Gem and V2400 Oph indicate that the polar subclass of CVs may be successfully described by the thermal Comptonization as well.

X-ray spectra of nonmagnetic cataclysmic variables (nmCVs) in the ~ 0.3$-$15 keV energy band have been described either by one or several optically thin thermal plasma components, or by cooling flow models. We tested if the spectral continuum in nmCVs could be successfully described by Comptonization of soft photons off hot electrons presented in a cloud surrounding the source [the transition layer, (TL)]. We used publicly XMM-Newton Epic-pn, Chandra HETG/ACIS and LETG/HRC, and RXTE PCA and HEXTE observations of four Dwarf Novae (U~Gem, SS~Cyg, VW~Hyi and SS~Aur) observed in the quiescence and outburst states. In total, we analyzed 18 observations, including a simultaneous 0.4$-$150 keV Chandra/RXTE spectrum of SS~Cyg in quiescence. We fitted the spectral continuum with up to two thermal Comptonization components (compTT or compTB models in XSPEC), using only one thermal plasma temperature and one optical depth. In this framework the two seed photon components are coming presumably from the innermost and outer parts of the TL (or innermost part of the disk). We obtained that the thermal Comptonization can successfully describe the spectral continuum of these nmCV in the ~ 0.4$-$150 keV energy band. Moreover, we present the first principal radiative transfer model which explains the quasi-constancy of the spectral photon index observed around 1.8, which strongly supports the Comptonization framework in nmCVs., Comment: 34 pages, 8 figures

Context. Broad, asymmetric, and red-skewed Fe Kα emission lines have been observed in the spectra of low-mass X-ray binaries hosting neutron stars (NSs) as a compact object. Because more than one model is able to describe these features, the explanation of where and how the red-skewed Fe lines are produced is still a matter of discussion. It is broadly accepted that the shape of the Fe Kα line is strongly determined by the special and general relativistic effects occurring in the innermost part of the accretion disk. In this relativistic framework, the Fe fluorescent lines are produced in the innermost part of the accretion disk by reflection of hard X-ray photons coming from the central source (corona and/or NS surface). We developed an alternative and nonrelativistic model, called the WINDLINE model, that is capable to describe the Fe line features. In this nonrelativistic framework, the line photons are produced at the bottom of a partly ionized outflow (wind) shell as a result of illumination by the continuum photons coming from the central source. In this model the red-skewness of the line profile is explained by repeated electron scattering of the photons in a diverging outflow. Aims. Examining the asymmetry of the fluorescent Fe K emission line evident in the XMM-Newton EPIC-pn spectra of the NS source GX 13+1, we aim to distinguish between the two line models. Because GX 13+1 is a well-known disk-wind source, it is a perfect target for testing the WINDLINE model and compare the spectral fits between the relativistic and nonrelativistic line models. Methods. We used two XMM-Newton EPIC-pn observations in which the Fe line profiles were previously reported in the literature. These observations are not strongly affected by pile-up, and the Fe emission lines appear very strong and red-skewed. In order to access the goodness of the fit and distinguish between the two line models, we used the run-test statistical method in addition to the canonical χ2 statistical method. Results. The DISKLINE and WINDLINE models both fit the asymmetric Fe line well that is present in the XMM-Newton energy spectra of GX 13+1. From a statistical point of view, for the two observations we analyzed, the run-test was not able to distinguish between the two Fe line models, at 5% significance level.

We consider a Compton cloud (CC) surrounding a black hole (BH) in an accreting black hole system, where electrons propagate with thermal and bulk velocities.In that cloud, soft (disk) photons may be upscattered off these energetic electrons and attain several MeV energies. They could then create pairs due to these photon-photon interactions. In this paper, we study the formation of the 511 keV annihilation line due to this photon-photon interaction, which results in the creation of electron-positron pairs, followed by the annihilation of the created positrons with the CC electrons. The appropriate conditions of annihilation line generation take place very close to a BH horizon within (10^3-10^4)m cm from it, where m is a BH hole mass in solar units. As a result, the created annihilation line should be seen by the Earth observer as a blackbody bump, or so called {reflection} bump at energies around (511/20) (20/z) keV where z~20 is a typical gravitational red-shift experienced by the created annihilation line photons when they emerge. This transient feature should occur in any accreting black hole systems, Galactic or extragalactic. Observational evidences for this feature in several galactic black hole systems is detailed in an accompanying paper (II). An extended hard tail of the spectrum up to 1 MeV may be also formed due to X-ray photon up-scattering off created pairs., Comment: 10 Pages, 14 figures accepted by ApJ, April 22, 2018

We present our analysis of X-ray spectral properties observed from the Seyfrert 1 galactic nucleus NGC~7469 using the RXTE and ASCA observations. We demonstrate strong observational evidence that NGC~7469 undergoes spectral transitions from the low hard state (LHS) to the intermediate state (IS) during these observations. The RXTE observations (1996--2009) show that the source was in the IS ~ 75 % of the time only, ~ 25 % of the time in the LHS. The spectra of NGC~7469 are well fitted by the so-called bulk motion Comptonization (BMC) model for all spectral states. We have established the photon index saturation level, Gamma_{sat}+2.1+/-0.1, in the Gamma versus mass accretion rate, Mdot correlation. This Gamma- Mdot correlation allows us to estimate the black hole (BH) mass in NGC~7469 to be M__BH> 3 x 10^6 solar masses assuming the distance to NGC~7469 of 70 Mpc. For this BH mass estimate, we use the scaling method taking Galactic BHs, GRO~J1655--40, Cyg~ X--1 and an extragalactic BH source, NGC~4051 as reference sources. The Gamma versus Mdot correlation revealed in NGC~7469 is similar to those in a number of Galactic and extragalactic BHs and it clearly shows the correlation along with the strong Gamma saturation at ~2.1. This is robust observational evidence for the presence of a BH in NGC~7469. We also find that the seed photon temperatures are quite low, of the order of 140-200 eV, which are consistent with a high BH mass in NGC~7469 that is more than 3x10^6 solar masses., 15 pages, 10 figures, accepted for publication in Astronomy & Astrophysics

We present a discovery of the correlation between the X-ray spectral (photon) index and mass accretion rate observed in AGN NGC 4051. We analyzed spectral transition episodes observed in NGC 4051 using XMM/Newton, Suzaku and RXTE. We applied a scaling technique for a black hole (BH) mass evaluation which uses a correlation between the photon index and normalization of the seed (disk) component, which is proportional to a mass accretion rate. We developed an analytical model that shows the spectral (photon) index of the BH emergent spectrum undergoes an evolution from lower to higher values depending on a mass accretion rate in the accretion disk. We considered Cygnus X-1 and GRO~J1550-564 as reference sources for which distances, inclination angles and the BH masses are evaluated by dynamical measurements. Application of the scaling technique for the photon index-mass accretion rate correlation provides an estimate of the black hole mass in NGC 4051 to be more than 6x10^5 solar masses., Comment: 11 pages, 6 Figures and 8 Tables. arXiv admin note: text overlap with arXiv:0902.2852

In our recent work, we demonstrated that a novel X-ray scaling method, originally introduced for Galactic black holes, could be reliably extended to estimate the mass of supermassive black holes accreting at moderate to high level. Here, we apply this X-ray scaling method to ultraluminous X-ray sources (ULXs) to constrain their \mbh. Using 49 ULXs with multiple \xmm\ observations, we infer that ULXs host both stellar mass BHs and intermediate mass BHs. The majority of the sources of our sample seem to be consistent with the hypothesis of highly accreting massive stellar BHs with $M_{\rm BH}\sim100\,M_{\odot}$. Our results are in general agreement with the \mbh\ values obtained with alternative methods, including model-independent variability methods. This suggests that the X-ray scaling method is an actual scale-independent method that can be applied to all BH systems accreting at moderate-high rate., 14 pages, 10 figures

A tidal disruption event (TDE) is an astronomical phenomenon in which a previously dormant black hole (BH) destroys a star passing too close to its central part. We analyzed the flaring episode detected from the TDE sources, Swift~J1644+57 and Swift J2058+05 using RXTE, Swift and Suzaku data. The spectra are well fitted by the so called Bulk Motion Comptonization model for which the best-fit photon index Gamma varies from 1.1 to 1.8. We have firmly established the saturation of Gamma versus mass accretion rate at Gamma_{sat} about 1.7 -- 1.8. The saturation of Gamma is usually identified as a signature of a BH now established in Swift~J1644+57 and Swift J2058+05. In Swift~J1644+57 we found the relatively low Gamma_{sat} values which indicate a high electron (plasma) temperature, kT_e ~ 30 -- 40 keV. This is also consistent with high cutoff energies, E_{cut} ~ 60 -- 80 keV found using best fits of the RXTE spectra. Swift~J2058+05 shows a lower electron temperature, kT_e ~ 4-10 keV than that for Swift~J1644+57. For the BH mass estimate we used the scaling technique taking the Galactic BHs, GRO J1655--40, GX~339--4, Cyg~X--1 and 4U~1543--47 as reference sources and found that the BH mass in Swift~J1644+57 is M_{BH}> 7x10^6 solar masses assuming the distance to this of 1.5 Gpc. For Swift J2058+05 we obtain M_{BH}> 2x 10^7 solar masses assuming the distance to this source of 3.7 Gpc., 23 pages, 8 Tables, 11 figures

We analyze the X-ray spectra of the atoll 4U~1705-44 when the source undergoes the island-banana state transition. We use the Rossi X-ray Timing Explorer (RXTE) and BeppoSAX observations for this analysis. We demonstrate that the broad-band energy spectral distributions for all evolutinary states can be fitted by a model, consisting two Comptonized components. One arises from the seed photons coming from a neutron star (NS) atmosphere at a temperature kT_{s1}80 keV. This phase is similar to that was previously found in the Z-source Sco X-1. We interpret the decreasing index phase using a model in which a super-Eddington radiation pressure from the neutron star causes an expansion of the Compton cloud similar to that found previously in Sco~X-1 during the Flaring branch., Comment: 19 pages, 11 figures, 5 Tables, Accepted for publication in The Astrophysical Journal

We present the results of a comprehensive investigation on the evolution of spectral and timing properties of the Galactic black hole candidate 4U 1630--47 during its spectral transitions. In particular, we show how a scaling of the correlation of the photon index of the Comptonized spectral component Gamma with low frequency of quasi-periodic oscillations (QPO), nu_L and mass accretion rate, $\dot M$ can be applied to the black hole mass and the inclination angle estimates. We analyze the transition episodes observed with the {\it Rossi} X-ray Timing Explorer (RXTE) and BeppoSAX satellites. We find that the broadband X-ray energy spectra of 4U~1630--47 during all spectral states can be modeled by a combination of a thermal component, a Comptonized component and a red-skewed iron line component. We also establish that Gamma monotonically increases during transition from the low-hard state to the high-soft state and then saturates for high mass accretion rates. The index saturation levels vary for different transition episodes. Correlations of Gamma versus nu_L also shows saturation at Gamma ~ 3. Gamma-Mdot and Gamma-nu_L correlations with their index saturation revealed in 4U~1630--47 are similar to those established in a number of other BHCs and can be considered as an observational evidence for the presence of a black hole in these sources. The scaling technique, which relies on XTE J1550--564, GRO 1655-40 and H 1743-322 as reference sources, allows us to evaluate a black hole mass in 4U~1630--47 yielding M_{BH}~10+/- 0.1 solar masses, and to constrain the inclination angle of i< 70 degrees., 33 pages, 13 Figures. The paper is accepted and scheduled for publication in The Astrophysical Journal, July 1, 2014, v 789, 1 issue

We 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.

Predicting the emerging X-ray spectra in several astrophysical objects is of great importance, in particular when the observational data are compared with theoretical models. To this aim, we have developed an algorithm solving the radiative transfer equation in the Fokker-Planck approximation when both thermal and bulk Comptonization take place. The algorithm is essentially a relaxation method, where stable solutions are obtained when the system has reached its steady-state equilibrium. We obtained the solution of the radiative transfer equation in the two-dimensional domain defined by the photon energy E and optical depth of the system tau using finite-differences for the partial derivatives, and imposing specific boundary conditions for the solutions. We treated the case of cylindrical accretion onto a magnetized neutron star. We considered a blackbody seed spectrum of photons with exponential distribution across the accretion column and for an accretion where the velocity reaches its maximum at the stellar surface and at the top of the accretion column, respectively. In both cases higher values of the electron temperature and of the optical depth tau produce flatter and harder spectra. Other parameters contributing to the spectral formation are the steepness of the vertical velocity profile, the albedo at the star surface, and the radius of the accretion column. The latter parameter modifies the emerging spectra in a specular way for the two assumed accretion profiles. The algorithm has been implemented in the XSPEC package for X-ray spectral fitting and is specifically dedicated to the physical framework of accretion at the polar cap of a neutron star with a high magnetic field (> 10^{12} G), which is expected to be typical of accreting systems such as X-ray pulsars and supergiant fast X-ray transients., 13 pages, 20 figures, accepted for publication in A&A

In this Paper, we present theoretical arguments that the observationally established index saturation effect vs mass accretion rate is a signature of the bulk (converging) flow onto the black hole. We demonstrate that the index saturation value depends on the plasma temperature of converging flow. We self-consistently calculate the Compton cloud (CC) plasma temperature as a function of mass accretion rate using the energy balance between energy dissipation and Compton cooling. We explain the observable phenomenon, index- mdot correlations using a Monte-Carlo simulation of radiative processes in the innermost part (CC) of a BH source and we account for the Comptonization processes in the presence of thermal and bulk motions, as basic types of plasma motion. We show that, when mdot increases, BH sources evolve to high and very soft states (HSS and VSS, respectively), in which the strong blackbody-like and steep power-law components are formed in the resulting X-ray spectrum. The simultaneous detections of these two components strongly depends on sensitivity of high energy instruments, given that the relative contribution of the hard power-law tail in the resulting VSS spectrum can be very low, which is why, to date {\it RXTE} observations of the VSS X-ray spectrum has been characterized by the presence of the strong BB-like component only. We also predict specific patterns for high-energy efold (cutoff) energy (E_{fold}) evolution with mdot for thermal and dynamical (bulk) Comptonization cases. For the former case, E_{fold} monotonically decreases with mdot, in the latter case, the E_{fold}-decrease is followed by its increase at high values of mdot. The observational evolution of E_{fold} vs mdot can be one more test for the presence of a converging flow effect in the formation of the resulting spectra in the close vicinity of BHs., 15 pages, 11 figures, accepted for the publication in the Astrophysical Journal

The bright and highly variable X-ray and radio source known as Cygnus X-3 was among the first X-ray sources discovered, yet it remains in many ways an enigma. Its known to consist of a massive, Wolf-Rayet primary in an extremely tight orbit with a compact object. Yet one of the most basic of parameters - the mass of the compact object - is not known. Nor is it even clear whether its is a neutron star or a black hole. In this Paper we present our analysis of the broad-band high-energy continua covering a substantial range in luminosity and spectral morphology. We apply these results to a recently identified scaling relationship which has been demonstrated to provide reliable estimates of the compact object mass in a number of accretion powered binaries. This analysis leads us to conclude that the compact object in Cygnus X-3 has a mass greater than $4.2M_\odot$ thus clearly indicative of a black hole and as such resolving a long-standing issue. The full range of uncertainty in our analysis and from using a range of recently published distance estimates constrains the compact object mass to lie between $4.2M_\odot$ and $14.4M_\odot$. Our favored estimate, based on a 9.0 kpc distance estimate is $\sim 10 M_\odot$ with the error margin of 3.2 solar masses. This result may thus pose challenges to shared-envelope evolutionary models of compact binaries, as well as establishing Cygnus X-3 as the first confirmed accretion-powered galactic gamma-ray source.

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., 29 pages, 14 figures. Accepted for the ApJ, 2010, October 1, v. 721 (2)

GS 1826-238 is a well-studied X-ray bursting neutron star in a low mass binary system. Thermal Comptonisation by a hot electron cloud is a widely accepted mechanism accounting for its high energy emission, while the nature of most of its soft X-ray output is not completely understood. A further low energy component is typically needed to model the observed spectra: pure blackbody and Comptonisation-modified blackbody radiation by a lower temperature (a few keV) electron plasma were suggested to explain the low energy data. We studied the steady emission of GS 1826-238 by means of broad band (X to soft Gamma-rays) measurements obtained by the INTEGRAL observatory in 2003 and 2006. The newly developed, up-to-date Comptonisation model CompTB is applied for the first time to study effectively the low-hard state variability of a low-luminosity neutron star in a low-mass X-ray binary system. We confirm that the 3-200 keV emission of \GS is characterised by Comptonisation of soft seed photons by a hot electron plasma. A single spectral component is sufficient to model the observed spectra. At lower energies, no direct blackbody emission is observed and there is no need to postulate a low temperature Compton region. Compared to the 2003 measurements, the plasma temperature decreased from 20 to 14 keV in 2006, together with the seed photons temperature. The source intensity was also found to be 30% lower in 2006, whilst the average recurrence frequency of the X-ray bursts significantly increased. Possible explanations for this apparent deviation from the typical limit-cycle behaviour of this burster are discussed., 6 pages, 2 figures. Accepted for publication in A&A

We present a study of the correlations between spectral, timing properties and mass accretion rate observed in X-rays from the Galactic Black Hole (BH) binary GRS 1915+105 during the transition between hard and soft states. We analyze all transition episodes from this source observed with RXTE, coordinated with Ryle Radio Telescope (RT) observations. We show that broad-band energy spectra of GRS 1915+105 during all these spectral states can be adequately presented by two Bulk Motion Comptonization (BMC) components: a hard component (BMC1, photon index Gamma_1=1.7-3.0) with turnover at high energies and soft thermal component (BMC2, Gamma_2=2.7-4.2) with characteristic color temperature, 33 pages, 16 figures, accepted for publication in ApJ, on December 10, 2009, v. 707

International audience; We perform the analysis of the iron Kα lines detected in three sources representing three types of accreting compact sources: cataclysmic variable (CV) GK Per, neutron star (NS) Serpens X-1, and black hole (BH) GX 339-4. We find, using data from Epic-PN Camera onboard XMM-Newton observatory, that the iron Kα emission line in GK Per has a noticeable red-skewed profile. We compare the GK Per asymmetric line with the red-skewed lines observed by XMM-Newton in Serpens X-1 and GX 339-4. The observation of the Kα emission with red-skewed features in CV GK Per cannot be related to the redshift effects of General Relativity (GR). Therefore, if the mechanism of the Kα-line formation is the same in CVs, NSs, and BHs then it is evident that the GR effects would be ruled out as a cause of red skewness of Kα line. The line reprocessing in an outflowing wind has been recently suggested as an alternative model for a broad red-shifted iron line formation. In the framework of the outflow scenario the red-skewed iron line is formed in the strong extended wind due to its illumination by the radiation emanating from the innermost part of the accreting material. The outflow is a common phenomenon for CVs, NSs, and BHs. In this paper, we demonstrate that the asymmetric shapes of the lines detected from these CV, NS, and BH sources are well described with the wind (outflow) model. Furthermore we find by analyzing Rossi X-Ray Timing Explorer(RXTE) observations that when the strong red-skewed iron line is observed in GX 339-4 the high-frequency variability is strongly suppressed. While this fact is hard to reconcile with the relativistic models, it is consistent with the outflowing gas washing out high-frequency modulations of the radiation presumably originated in the innermost part of the source.

We report on the Suzaku observation of neutron star low-mass X-ray binary Cygnus X-2 which reveals a presence of the iron K_alpha emission line. The line profile shows a significant red wing. This discovery increases the number of neutron star sources where red-skewed iron lines were observed and strongly suggests that this phenomenon is common not only in black holes but also in other types of accreting compact objects. We examine the line profile in terms of models which attribute its production to the relativistic effects due to reflection of X-ray radiation from a cold accretion disk and also as a result of the line formation in the extended wind/outflow configuration. Both models are able to adequately represent the observed line profile. We consider the results of line modeling in the context of subsecond variability. While we were unable to conclusively disqualify one of the models, we find that the wind paradigm has several advantages over the relativistic disk reflection model., accepted for publication in ApJ

We report the detection of a weak X-ray point source coincident with the nucleus of the bulgeless disk galaxy NGC 3621, recently discovered by Spitzer to display high ionization mid-infrared lines typically associated with AGN. These Chandra observations provide confirmation for the presence of an AGN in this galaxy, adding to the growing evidence that black holes do form and grow in isolated bulgeless disk galaxies. Although the low signal-to-noise ratio of the X-ray spectrum prevents us from carrying out a detailed spectral analysis of the nuclear source, the X-ray results, combined with the IR and optical spectroscopic results, suggests that NGC 3621 harbors a heavily absorbed AGN, with a supermassive black hole of relatively small mass accreting at a high rate. Chandra also reveals the presence of two bright sources straddling the nucleus located almost symmetrically at 20" from the center. Both sources have X-ray spectra that are well-fitted by an absorbed power-law model. Assuming they are at the distance of NGC 3621, these two sources have luminosities of the order of 1.e39 erg/s, which make them ULXs and suggest that they are black hole systems. Estimates of the black hole mass based on the X-ray spectral analysis and scaling laws of black hole systems suggest that the 2 bright sources might be intermediate mass black holes with M_BH of the order of a few thousand solar masses. However, higher quality X-ray data combined with multi-wavelength observations are necessary to confirm these conclusions., 12 pages, 9 figures, 1 table, accepted for publication in ApJ

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., Comment: 25 pages, 9 figures, 5 tables. Accepted and scheduled for publication in The Astrophysical Journal

A characteristic feature of the Fourier Power Density Spectrum (PDS) observed from black hole X-ray binaries in low/hard and intermediate spectral states is a broad band-limited noise, characterized by a constant below some frequency (a ``break'' frequency) and a power law above this frequency. It has been shown that the variability of this type can be produced by the inward diffusion of the local driving perturbations in a bounded configuration (accretion disk or corona). In the framework of this model, the perturbation diffusion time t_0 is related to the phenomenological break frequency, while the PDS power-law slope above the ``break'' is determined by the viscosity distribution over the configuration. he perturbation diffusion scenario explains the decay of the power of X-ray variability observed in a number of compact sources (containing black hole and neutron star) during an evolution of theses sources from low/hard to high/soft states. We compare the model predictions with the subset of data from Cyg X-1 collected by the Rossi X-ray Time Explorer (RXTE). Our extensive analysis of the Cyg X-1 PDSs demonstrates that the observed integrated power P_x decreases approximately as a square root of the characteristic frequency of the driving oscillations \nu_{dr}. The RXTE observations of Cyg X-1 allow us to infer P_{dr} and t_0 as a function of \nu_{dr}. Using the inferred dependences of the integrated power of the driving oscillations P_{dr} and t_0 on \nu_{dr} we demonstrate that the power predicted by the model also decays as P_{x,diff} proportional to \nu_{dr}^{-0.5} that is similar to the observed P_{x} behavior., Comment: 15 page, 5 figures, accepted for publication in the Astrophysical Journal

We developed a new model for the X-ray spectral fitting \xspec package which 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 which are not subjected to effective Compton scattering, while the other one 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 analytic form of the Green's function may be obtained as a solution of the diffusion Comptonization equation. Using data from the BeppoSAX, INTEGRAL and RXTE satellites, we test our model on the spectra of a sample of six persistently low magnetic field bright neutron star Low Mass X-ray Binaries, covering three different spectral states. Particular attention is given to the transient powerlaw-like hard X-ray (> 30 keV) tails that we interpret in the framework of the bulk motion Comptonization process. We show that the values of the best-fit delta-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 of COMPTT, a broadly used and well established XSPEC model., 15 pages, 8 figures, accepted for publication in ApJ

We present a model of Fourier Power Density Spectrum (PDS) formation in accretion powered X-ray binary systems derived from the first principles of the diffusion theory. Timing properties of X-ray emission are considered to be a result of diffusive propagation of the driving perturbations in a bounded medium. We prove that the integrated power P_x of the resulting PDS is only a small fraction of the integrated power P_dr of the driving oscillations, which is distributed over the disk. The resulting PDS continuum is a sum of two components, a low frequency (LF) component which presumably originates in an extended accretion disk and a high frequency (HF) component which originates in the innermost part of the source (Compton cloud or corona). The LF PDS component has a power-law shape with index of 1.0-1.5 at higher frequencies (``red'' noise) and a flat spectrum below a characteristic (break) frequency (``white'' noise). This white-red noise (WRN) continuum spectrum holds information about the physical parameters of the bounded extended medium, diffusion time scale and the dependence law of viscosity vs radius. We apply our model of the PDS to a sample of RXTE and EXOSAT timing data from Cyg X-1 and Cyg X-2 which describes adequately the spectral transitions in these sources., Comment: 45 pages, 12 figures, Accepted and scheduled for publication in the Astrophysical Journal, for April 10, 2007, v 659 1 issue

In Paper by Titarchuk & Shrader the general formulation and results for photon reprocessing (downscattering) that included recoil and Comptonization effects due to divergence of the flow were presented. Here we show the Monte Carlo (MC) simulated continuum and line spectra. We also provide an analytical description of the simulated continuum spectra using the diffusion approximation. We have simulated the propagation of monochromatic and continuum photons in a bulk outflow from a compact object. Electron scattering of the photons within the expanding flow leads to a decrease of their energy which is of first order in V/c (where V is the outflow velocity). The downscattering effect of first order in V/c in the diverging flow is explained by semi-analytical calculations and confirmed by MC simulations. We conclude that redshifted lines and downscattering bumps are intrinsic properties of the powerful outflows for which Thomson optical depth is greater than one. We fitted our model line profiles to the observations using four free parameters, \beta=V/c, optical depth of the wind \tau, the wind temperature kT_e and the original line photon energy E_0. We show how the primary spectrum emitted close to the black hole is modified by reprocessing in the warm wind. In the framework of the our wind model the fluorescent iron line K_alpha is formed in the partly ionized wind as a result of illumination by central source continuum photons. The demonstrated application of our outflow model to the XMM observations of MCG 6-30-15, and to the ASCA observations of GRO J1655-40, points out a potential powerful spectral diagnostic for probes of the outflow-central object connection in Galactic and extragalactic BH sources., Comment: Accepted for publication in the Astrophysical Journal and it is scheduled for ApJ February 20, 2007, v 656, 2 issue

We report the first clear evidence for the simultaneous presence of a low frequency break and a QPO in the fluctuation power spectrum of a well known ultraluminous X-ray source (ULX) in M82 using long XMM-Newton observations. The break occurs at a frequency of 34.2_{-3}^{+6}mHz. The QPO has a centroid at 114.3\pm1.5mHz, a coherence Q~3.5 and an amplitude (rms) of 19% in the 2-10keV band. The power spectrum is approximately flat below the break frequency and then falls off above the break frequency as a power law with the QPO superimposed. This form of the power spectrum is characteristic of the Galactic X-ray binaries (XRBs) in their high or intermediate states. M82 X-1 was likely in an intermediate state during the observation. The EPIC PN spectrum is well described by a model comprising an absorbed power-law (Gamma~2) and an iron line at ~6.6keV with a width sigma~0.2keV and an equivalent width of ~180eV. Using the well established correlations between the power and energy spectral parameters for XRBs, we estimate a black hole mass for M82 X-1 in the range of 25-520Msun including systematic errors that arise due to the uncertainty in the calibration of the photon spectral index versus QPO frequency relation., Final version, accepted for publication in ApJ Letters

Compton Gamma Ray Observatory, OSSE, SMM, TGRS, balloon and recent INTEGRAL data reveal a feature of the 0.511 MeV annihilation radiation of the Galactic Center with a flux of approximately 5 × 10 −4 0.511 MeV photons cm −2 s −1 . We argue that e + e − pairs can be generated when the X-ray radiation photons and ∼ 10 − 30 MeV photons interact with each other in the compact region in the proximity of the Galactic Center black hole. In fact, disks formed near black holes of 10 17 g mass should emit the ∼ 10 MeV temperature blackbody radiation. If positron (e + ) sources are producing about 10 42 e + s −1 near the Galactic Center they would annihilate on the way out and result in 0.511 MeV emission. We suggest that the annihilation radiation can be an observational consequence of the interaction of the accretion disk radiation of the SMall Mass Black Holes (SMMBHs) with X-ray radiation in the Galactic Center. This is probably the only way to identify and observe these SMMBHs.

We studied and compared the long-term average hard X-ray (>20keV) spectra of a sample of twelve bright low-mass X-ray binaries hosting a neutron star (NS). Our sample comprises the six well studied Galactic Z sources and six Atoll sources, four of which are bright ("GX") bulge sources while two are weaker ones in the 2-10keV range (H1750-440 and H1608-55). For all the sources of our sample, we analysed available public data and extracted average spectra from the IBIS/ISGRI detector on board INTEGRAL. We can describe all the spectral states in terms of the bulk motion Comptonisation scenario. We find evidence that bulk motion is always present, its strength is related to the accretion rate and it is suppressed only in the presence of high local luminosity. The two low-dim Atoll source spectra are dominated by photons up-scattered presumably due to dynamical and thermal Comptonisation in an optically thin, hot plasma. For the first time, we extend the detection of H1750-440 up to 150keV. The Z and bright "GX" Atoll source spectra are very similar and are dominated by Comptonised blackbody radiation of seed photons, presumably coming from the accretion disc and NS surface, in an optically thick cloud with plasma temperature in the range of 2.5-3keV. Six sources show a hard tail in their average spectrum: CygX-2 (Z), GX340+0 (Z), GX17+2 (Z), GX5-1 (Z), ScoX-1 (Z) and GX13+1 (Atoll). This is the first detection of a hard tail in the X-ray spectrum of the peculiar GX13+1. Using radio data from the literature we find, in all Z sources and bright "GX" Atolls, a systematic positive correlation between the X-ray hard tail (40-100keV) and the radio luminosity. This suggests that hard tails and energetic electrons causing the radio emission may have the same origin, most likely the Compton cloud located inside the NS magnetosphere., 12 pages, 7 figures, A&A in press; updated Fig 4 with an additional spectral state, typos corrected and added further comments for completeness

We 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 K_alpha photons within the highly ionized expanding flow leads to a decrease of their energy (redshift) which is 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 K_alpha originated in the wind is of 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., 9 pages, 3 figures, accepted by Astrophysical Journal Letters

A number of recent lines of evidence point towards the presence of hot, out-flowing plasma from the central regions of compact Galactic and extragalactic X-ray sources. Additionally, it has long been noted that many of these sources exhibit an ``excess'' continuum component, above 10 keV, usually attributed to Compton Reflection from a static medium. Motivated by these facts, as well as by recent observational constraints on the Compton reflection models - specifically apparently discrepant variability timescales for line and continuum components in some cases -- we consider possible effects of out-flowing plasma on the high-energy continuum spectra of accretion powered compact objects. We present a general formulation for photon downscattering diffusion which includes recoil and Comptonization effects due to divergence of the flow. We then develop an analytical theory for the spectral formation in such systems that allows us to derive formulae for the emergent spectrum. Finally we perform the analytical model fitting on several Galactic X-ray binaries. Objects which have been modeled with high-covering-fraction Compton reflectors, such as GS1353-64 are included in our analysis. In addition, Cyg X-3, is which is widely believed to be characterized by dense circumstellar winds with temperature of order 10^6 K, provides an interesting test case. Data from INTEGRAL and RXTE covering the 3-300 keV range are used in our analysis. We further consider the possibility that the widely noted distortion of the power-law continuum above 10 keV may in some cases be explained by these spectral softening effects., 16 pages, 5 figures, accepted for publication in the Astrophysical Journal and scheduled for 1 December 2004, vol 616 issue

Powerful Type I X-ray burst with strong radial expansion was observed from the low mass X-ray binary 4U 1820-30 with Rossi X-ray Timing Explorer on May 2, 1997. We investigate closely the flux profile during the burst expansion stage. Applying a semi-analytical model we are able to uncover the behavior of a photospheric radius and to simulate the evolution of neutron star (NS)-accretion disk system. The bottom flux L_{bot} is a few times the Eddington limit L_{Edd} for outer layers, because the electron cross-section is a few times less than the Thomson cross-section at such a 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. Indeed, we observe to the paradoxical situation when the burning zone cools, but the apparent flux increases because of the NS-accretion disk geometry. We demonstrate a strong observational evidence of NS-accretion disk occultation in the behavior of the observed bolometric flux. 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 neutron star atmosphere during burst decay stage to infer the neutron star (NS) mass-radius relation., Comment: 15 pages, 3 figures, accepted to ApJL

Recent studies have shown that strong correlations are observed between the low frequencies (1-10 Hz) of quasiperiodic oscillations (QPOs) and the spectral power law index of several Black Hole (BH) candidate sources, in low hard state, steep power-law (soft) state and in transition between these states. We provide a model, that identifies and explains the origin of the QPOs and how they are imprinted on the properties of power-law flux component. We argue the existence of a bounded compact coronal region which is a natural consequence of the adjustment of Keplerian disk flow to the innermost sub-Keplerian boundary conditions near the central object and that ultimately leads to the formation of a transition layer (TL) between the adjustment radius and the innermost boundary. The model predicts two phases or states dictated by the photon upscattering produced in the TL: (1) hard state, in which the TL is optically thin and very hot (kT ~ 50 keV) producing photon upscattering via thermal Componization; the photon spectrum index Gamma~1.7 for this state is dictated by gravitational energy release and Compton cooling in an optically thin shock near the adjustment radius; (2) a soft state which is optically thick and relatively cold (kT~5 keV); the index for this state, Gamma~2.8 is determined by soft-photon upscattering and photon trapping in converging flow into BH., Comment: 25 pages, 12 figures, accepted for publication in the Astrophysical Journal

We analyze a set of Type I X-ray bursts from the low mass X-ray binary 4U 1728 -34, observed with Rossi X-ray Timing Explorer (RXTE). and we implement an analytical model of X-ray spectral formation in the neutron star (NS) atmosphere during a burst. We infer the dependence of the neutron star mass and radius with respect to the assumed distance to the system using an analytical model of X-ray burst spectral formation. The model behavior clearly indicates that the burster atmosphere is helium-dominated. Our results strongly favor the soft equation of state (EOS) of NS for 4U 1728-34. We find that distance to the source should be within 4.5-5.0 kpc range. We obtain rather narrow constrains for the NS radius in 8.7-9.7 km range and interval 1.2-1.6 solar masses for NS mass for this particular distance range. We uncover a temporal behavior of red-shift corrected burst flux for the radial expansion episodes and we put forth a dynamical evolution scenario for the NS--accretion disk geometry during which an expanded envelope affects the accretion disk and increases the area of the neutron star exposed to the Earth observer. Our scenario enables us to explain the timing characteristics and peak flux variation observed during the burst expansion stage, which is now believed to be common phenomenon. In the framework of this scenario we provide a new method for the estimation of the inclination angle which leads to the value of 50 degrees for 4U 1728-34 .

The effects of bulk motion comptonization on the spectral formation in a converging flow onto a black hole are investigated. The problem is tackled by means of both a fully relativistic, angle-dependent transfer code and a semi-analytical, diffusion-approximation method. We find that a power-law high-energy tail is a ubiquitous feature in converging flows and that the two approaches produce consistent results at large enough accretion rates, when photon diffusion holds. Our semi-analytical approach is based on an expansion in eigenfunctions of the diffusion equation. Contrary to previous investigations based on the same method we find that, although the power-law tail at really large energies is always dominated by the flatter spectral mode, the slope of the hard X-ray portion of the spectrum is dictated by the second mode and it approaches Gamma=3 at large accretion rate, irrespective of the model parameters. The photon index in the tail is found to be largely independent on the spatial distribution of soft seed photons when the accretion rate is either quite low (< 5 in Eddington units) or sufficiently high (> 10). On the other hand, the spatial distribution of source photons controls the photon index at intermediate accretion rates, when Gamma switches from the first to the second mode. Our analysis confirms that a hard tail with photon index Gamma, to be published in the Astrophysical Journal, 16 pages and 9 figures

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., Comment: 9 pages and 2 figures. Accepted for publication in the Astrophysical Journal Letters 2002 August 22

The theoretical predictions of the converging inflow, or Bulk-Motion Comptonization model are discussed and some predictions are compared to X- and gamma-ray observations of the high/soft state of Galactic black hole candidate XTE J1550+564. The 100 Hz QPO phenomenon tends to be detected in the high-state at times when the bolometric luminosity surges and the hard-power-law spectral component is dominant. Furthermore, the power in these features increases with energy. We offer interpretation of this phenomenon, as oscillations of the innermost part of the accretion disk, which in turn supplies the seed photons for the converging inflow where the hard power-law is formed through Bulk Motion Comptonization (BMC). We further argue that the noted lack of coherence between intensity variations of the high-soft-state low and high energy bands is a natural consequence of our model, and that a natural explanation for the observed hard and soft lag phenomenon is offered. In addition, we address some criticisms of the BMC model supporting our claims with observational results., 17 pages, 5 figures, accepted by the Astrophysical Journal

We demonstrate that a X-ray spectrum of a converging inflow (CI) onto a black hole is the sum of a thermal (disk) component and the convolution of some fraction of this component with the Comptonization spread (Green's) function. The latter component is seen as an extended power law at energies much higher than the characteristic energy of the soft photons. We show that the high energy photon production (source function) in the CI atmosphere is distributed with the characteristic maximum at about the photon bending radius, 1.5r_S, independently of the seed (soft) photon distribution. We show that high frequency oscillations of the soft photon source in this region lead to the oscillations of the high energy part of the spectrum but not of the thermal component. The high frequency oscillations of the inner region are not significant in the thermal component of the spectrum. We further demonstrate that Doppler and recoil effects (which are responsible for the formation of the CI spectrum) are related to the hard (positive) and soft (negative) time lags between the soft and hard photon energy channels respectively., 9 pages and 4 figures, to be published in the Astrophysical Journal Letters

We present a linear diffusion model for the evolution of the double-peaked outburst in the transient source XTEJ1118+480. The model treats the two outbursts as episodic mass deposition at the outer radius of the disk followed by evolution of disk structure according to a diffusion process. We demonstrate that light curves with fast-rise, exponential decay profile are a general consequence of the diffusion process. Deconvolution of the light curve proves to be feasible and gives an input function specifying mass deposition at the outer disk edge as well as the total mass of the disk, both as functions of time. The derived evolution of total disk mass can be correlated with the observed evolution of the ~0.1 Hz QPO in the source reported in Wood et al. (2000)., 26 pages, 6 figures, accepted for publication in ApJ

We present a theoretical analysis of Rossi X-ray Timing Explorer data of Z source GX 340+0 obtained by Jonker et al. In the frameworks of the recently formulated the transition layer model the delta-angle is an angle between the neutron star (NS) magnetospheric axis and the disk (presumably NS rotational) axis.We determine the angle delta=6^o.3+/- 0.^o 3 which is a combination of the simultaneously observed kHz QPO and HBO frequencies. While these three frequencies change by a factor of three or more their delta-combination stays almost constant. GX 340+0 is the fourth source (in addition to 4U 0614+09, Sco X-1 and 4U 1702-42) for which delta-angle has been determined. With at most one (constrained) parameter we make a complete classification of six observed power spectral features, including the two kHz frequencies, the first and second harmonics of the HBO frequency, low-frequency noise component and break frequencies. We demonstrate that a new component discovered by Jonker et al. in the GX 340+0 power spectrum is related to the viscous frequency branch which has been, in fact reported earlier in 4U 1728-34 by Ford and van der Klis (1998). Finally, we re-classify several previously misidentified features in the power spectrum., 9 pages, 3 figures. to be published in the Astrophysical Letters

We present a dimensional analysis of two characteristic timescales 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 the frequencies of kilohertz quasi-periodic oscillations (kHz QPOs) compare favorably with recent observations of the source 4U 1728-34. This Letter contains a theoretical classification of kHz QPOs in NS binaries and the related low-frequency features. Thus, results concerning the relationship between the low-Lorentzian frequency of viscous oscillations and the break frequency are presented in the framework of our model of kHz QPOs viewed as Keplerian oscillations in a rotating frame of reference.

We present our analysis of the high-energy radiation from black hole (BH) transients, using archival data obtained primarily with RXTE observatory, and a comprehensive test of the bulk motion Comptonization (BMC) model for the high-soft state continuum. The emergent spectra of over 30 separate measurements of GRO J1655-40, GRS 1915+105, GRS 1739-278, 4U 1630-47 XTE J1755-32, and EXO~1846-031 X-ray sources are successfully fitted by the BMC model, which has been derived from basic physical principles in previous work. This in turn provides direct physical insight into the innermost observable regions where matter impinging upon the event horizon can effectively be directly viewed. The BMC model is characterized by three parameters: the disk color temperature, a geometric factor related to the illumination of the black hole (BH) site by the disk and a spectral index related to the efficiency of the bulk motion upscattering. For the case of GRO J1655-40, where there are distance and mass determinations, a self consistency check of the BMC model has been made. Using model parameters we present new, independent constraints on the black hole mass, mass accretion rate and the distance for the aforementioned sources. Notable is the relationship between the color temperature and flux, which for GRO J1655-40 is entirely distinct from a simple T^4 dependence, and consistent with the disk model we have invoked - standard Shakura-Sunyaev's disk. This allows us to impose an important estimation of the hardness parameter, the ratio of the color temperature to the effective temperature - we find T_h~2.6, higher than previous estimates used in the literature., 50 pages, 8 figures, accepted for publication in the Astrophysical Journal (scheduled for the May 20, 1999 issue)

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., 11 pages, including four figures. To appear in ApJ Letters

We discuss the emergent spectra from accreting black holes, considering in particular the case where the accretion is characterized by relativistic bulk motion. We suggest that such accretion is likely to occur in a wide variety of black hole enviroments, where the strong gravitational field is expected to dominate the pressure forces, and this likely to lead to a characteristic high-energy spectriscopic signature; an extended power tail. It is in the high (soft) state that matter impinging upon the event horizon can be viewed directly, and intrinsic power-law seen. A test of the model is presented using observational data from the Compton Gamma Ray Observatory and the Rossi X-ray Timing Explorer, covering 2- 200 keV for recent galactic black hole X-ray nova outbursts., latex, 18 pages, 2 figures. Accepted by ApJLetters

We study Compton upscattering of low-frequency photons in a converging flow of thermal plasma. The photons escape diffusively and electron scattering is the dominant source of opacity. We solve numerically and approximately analytically the equation of radiative transfer in the case of spherical, steady state accretion into black holes. Unlike previous work on this subject, we consider the inner boundary at a finite radius and this has a significant effect on the emergent spectrum. It is shown that the bulk motion of the converging flow is more efficient in upscattering photons than thermal Comptonization, provided that the electron temperature in the flow is of order a few keV or less. In this case, the spectrum observed at infinity consists of a soft component coming from those input photons which escaped after a few scatterings without any significant energy change and of a power law which extends to high energies and is made of those photons which underwent significant upscattering. The luminosity of the power law is relatively small compared to that of the soft component. The more reflective the inner boundary is, the flatter the power-law spectrum becomes. The spectral energy power-law index for black-hole accretion is always higher than 1 and it is approximately 1.5 for high accretion rates. This result tempts us to say that bulk motion Comptonization might be the mechanism behind the power-law spectra seen in black-hole X-ray sources., Comment: 37 pages, LaTex, AAS Macros, 8 ps figures, to appear in ApJ

We have found relations between the temporal and spectral properties of radiation Comptonized in an extended atmosphere associated with compact accreting sources. We demonstrate that the fluctuation power spectrum density (PSD) imposes constraints on the atmosphere scale and profile. Furthermore, we indicate that the slope and low-frequency break of the PSD are related to the Thomson depth, τ0, of the atmosphere and the radius of its physical size, respectively. Since the energy spectrum of the escaping radiation also depends on τ0 (and the electron temperature kTe), the relation between spectral and temporal properties follows. This relation allows, for the first time, an estimate of the accreting matter Thomson depth, τ0, independent of arguments involving Comptonization. We present figures for the light curves and PSDs of different energy bands, the photon energy spectra, and the phase lags as functions of the variability frequency. The temporal properties of the high (soft) and low (hard) state of black hole sources are discussed in this context.

We report the detection with the Proportional Counter Array (PCA) on board the Rossi X-Ray Timing Explorer (RXTE) of millisecond variability in the X-ray emission from the low-mass X-ray binary 4U 1728-34. Pulsations at 363 Hz with amplitudes (rms) of 2.5%-10% are present in six of the eight bursts analyzed to date. The strongest were seen in two successive bursts recorded on 1996 February 16 when the quiescent count rate was near the highest seen by PCA. The pulsations during these bursts show frequency changes of 1.5 Hz during the first few seconds but become effectively coherent as the burst decays. We interpret the 363 Hz pulsations as rotationally induced modulations of inhomogeneous burst emission. This represents the first compelling evidence for a millisecond spin period in a low-mass X-ray binary. Complex, intensity-dependent, millisecond X-ray variability is also present in all the quiescent flux intervals we examined. Most interesting was the behavior as the count rate approached its highest observed level. Two quasi-periodic oscillations (QPOs) were simultaneously observed in the 650-1100 Hz range. Both QPOs increased in frequency together, maintaining a nearly constant frequency separation of about 363 Hz, the spin period inferred from the burst oscillations. This phenomenology is strongly suggestive of the magnetospheric beat frequency model proposed for the horizontal-branch oscillations (HBOs) seen in Z sources. We discuss this and several other possible physical interpretations for the observed X-ray variability.

We present a model for explaining the recent combined X-ray and low-energy gamma-ray observations of the Seyfert galaxy NGC 4151. According to this model, soft photons become Comptonized in a hot spot producing simultaneously the low-energy power law as observed by Ginga and the high-energy cutoff observed by the Oriented Scintillation Spectrometer Experiment (OSSE). Implementing recently developed theoretical calculations toward a generalized theory of Comptonization, we were able to find fits to the observations using only two parameters which characterize the physical quantities of the emission region: the plasma cloud optical depth and its temperature. We find that there is no need for additional nonthermal, reflection, or higher temperature thermal components to fit the aforementioned OSSE and Ginga observations. We derive in addition the size of the photon region and the temperature of the upscattered soft photons. We should emphasize, also, that any attempt at fitting only the high-energy parts of the spectrum (photon energies greater than 60 keV) by the Sunyaev & Titarchuk (1980) nonrelativistic Comptonization model leads to an underestimate of the Comptonization parameter y (or, equivalently, to an overestimation of the X-ray power-law spectral slope) and leads, as a result, to incorrect proportions between the low-energy and high-energy parts of the spectrum.