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

In a recent work we demonstrated that a novel X-ray scaling method, originally introduced for Galactic black holes (GBHs), can be reliably extended to estimate the mass of supermassive black holes accreting at a moderate to high level. Here we investigate the limits of applicability of this method to low-accreting active galactic nuclei, using a control sample with goodquality X-ray data and dynamically measured mass. For low-accreting AGNs (LX/LEdd 6 10 4 ), because the basic assumption that the photon index positively correlates with the accretion rate no longer holds the X-ray scaling method cannot be used. Nevertheless, the inverse correlation in the LX/LEdd diagram, found in several low-accreting BHs and confirmed by this sample, can be used to constrain MBH within a factor of � 10 from the dynamically determined values. We provide a simple recipe to determine MBH using solely X-ray spectral data, which can be used as a sanity check for MBH determination based on indirect optical methods.

We will discuss preliminary results of a systematic investigation devoted to study the time resolved broad-band spectra of the prompt emission of a sample of GRBs. These events were simultaneously detected with the BeppoSAX WFCs and the BATSE instrument aboard CGRO. We will discuss the fit of these spectra with the Band function, with a blackbody plus a power–law model, and with a Comptonization model developed by Titarchuk & Farinelli (2011, to be submitted).

The physical mechanisms responsible for production of the non-thermal emission in accreting black holes (BH) should be imprinted in the observational apperances of the power law tails in the X-ray spectra from these objects. Different spectral states exhibited by galactic BH binaries allow examination of the photon upscattering under different accretion regimes. We revisit the data collected by Rossi X-ray Timing Explorer (RXTE) from the BH X-ray binary XTE J1550-564 during two periods of X-ray activity in 1998 and 2000 focusing on the behavior of the high energy cutoff of the power law part of the spectrum. For the 1998 outburst the transition from the low-hard state to the intermediate state was accompanied by a gradual decrease in the cutoff energy which then showed an abrupt reversal to a clear increasing trend as the source evolved to the very high and high-soft states. The 2000 outburst showed only the decreasing part of this pattern. Notably, the photon indexes corresponding to the cutoff increase for the 1998 event are much higher than the index values reached during the 2000 rise transition. We attribute this difference in the cutoff energy behavior to the different partial contributions of the thermal and non-thermal (bulk motion) Comptonization in photon upscattering. Namely, during the 1998 event the higher accretion rate presumably provided more cooling to the Comptonizing media and thus reducing the effectiveness of the thermal upscattering process. Under these conditions the bulk motion takes a leading role in boosting the input soft photons. Monte Carlo simulations of the Comptonization in a bulk motion region near an accreting black hole by Laurent & Titarchuk 2010 strongly support this scenario., 7 pages, 8 figures, ApJ format

International audience; We present a model of a gamma-ray burst (GRB) related to “very massive” stars. In the framework of our model, the GRB phenomenon is a result of helium burning in degenerate conditions in a massive star (≳130 M⊙), in which the thermal nuclear burning occurs in the deflagration regime and has a pulsating temporal pattern. The shock runs away from the burning (reaction) zone, which leads to the development of a coronal outflow (jet-like) structure. In our scenario the GRB observable prompt fast rise and decay part can be a result of photon propagation through the hot corona (Comptonization photosphere) of the star. On the other hand, the GRB afterglow is a cooling phase of the expanding and outflowing envelope. Presumably, the X-ray part of the GRB emergent spectrum is formed due to upscattering of soft photons of outer layers of the star off hot coronal electrons, and thus it should have a specific shape of the Comptonization spectrum.

We study the time variability and spectral evolution of the Black Hole Candidate source XTE J1650-500 using the BeppoSax wide energy range (0.12-200 keV) observations performed during the 2001 X-ray outburst. The source evolves from a low/hard state (LHS) toward a high/soft state (HSS). In all states the emergent photon spectrum is described by the sum of Comptonization and soft (disk) blackbody components. In the LHS, the Comptonization component dominates in the resulting spectrum. On the other hand, during the HSS the soft (disk) component is already dominant. In this state the Comptonization part of the spectrum is much softer than that in the LHS (photon index is ~ 2.4 in the HSS vs. that is ~1.7 in the LHS). In the BeppoSAX data we find a strong signature of the index saturation with the mass accretion rate which can be considered as an observational evidence of the converging flow (black hole) in XTE J1650-500. We derive power spectra (PS) of the source time variability in different spectral states as a function of energy band. When the source undergoes a transition to softer states, the PS as a whole is shifted to higher frequencies which can be interpreted as a contraction of the Compton cloud during hard-soft spectral evolution. It is worthwhile to emphasize a detection of a strong low-frequency red noise component in the HSS PS which can be considered a signature of the presence of the strong extended disk in the HSS. Also as a result of our data analysis, we find a very weak sign of K_alpha line appearance in these BeppoSAX data. This finding does not confirm previous claims by Miniutti et al. on the presence of a broad and strongly relativistic iron emission line in this particular set of the BeppoSAX data., Comment: 31 pages, 10 figures, accepted for publication in the ApJ

Correlations between the quasi-periodic oscillations (QPOs) and the spectral power-law index have been reported for a number of black hole candidate sources and for four neutron star (NS) sources, 4U 0614+09, 4U 1608-52, 4U 1728-34 and Sco X-1. An examination of QPO frequencies and index relationship in Cyg X-2 is reported herein. The RXTE spectrum of Cyg X-2 can be adequately represented by a simple two-component model of Compton up-scattering with a soft photon electron temperature of about 0.7 keV and an iron K-line. Inferred spectral power-law index shows correlation with the low QPO frequencies. We find that the Thomson optical depth of the Compton cloud (CC) tau, in framework of spherical geometry, is in the range of ~4-6, which is consistent with the neutron star's surface being obscured. The NS high frequency pulsations are presumably suppressed as a result of photon scattering off CC electrons because of such high values of tau. We also point out a number of similarities in terms timing (presence of low and high frequency QPOs) and spectral (high CC optical depth and low CC plasma temperature) appearances between Cyg X-2 and Sco X-1., Comment: 7 pages, 4 figures, accepted for publication in ApJ (October 1, 2007, v667n2 issue)

Correlations between 1-10 Hz quasi-periodic oscillations (QPOs) and spectral power-law index have been reported for black hole (BH) candidate sources and one neutron star source, 4U 1728-34. An examination of QPO frequency and index relationships in Sco X-1 is reported here. We discover that Sco X-1, representing Z-source groups, can be adequately modeled by a simple two-component model of Compton up-scattering with a soft photon electron temperature of about 0.4 keV, plus an Iron K line. The results show a strong correlation between spectral power-law index and kHz QPOs. Because Sco X-1 radiates near the Eddington limit, one can infer that the geometrical configuration of the Compton cloud (CC) is quasi-spherical from high radiation pressure in the CC. Thus, we conclude that the high Thomson optical depth of the Compton cloud, in the range of ~5-6 from the best-fit model parameters, is consistent with the neutron star's surface being obscured by material. Moreover, a spin frequency of Sco X-1 is likely suppressed due to photon scattering off CC electrons. In addition, we demonstrate how the power spectrum evolves when Sco X-1 transitions from the horizontal branch to the normal branch.

It is well established that timing and spectral properties of Galactic Black Hole (BH) X-ray binaries (XRB) are strongly correlated. In particular, it has been shown that low frequency Quasi-Periodic Oscillation (QPO) frequency - photon index correlation curves have a specific pattern. In a number of sources the shape of the index-low frequency QPO correlations are self-similar with a position offset in the QPO frequency-Gamma plane. Titarchuk & Fiorito presented strong theoretical and observational arguments that the QPO frequency values in this QPO frequency-Gamma correlation should be inversely proportional to M_{BH}. A simple translation of the correlation for a given source along the frequency axis leads to the observed correlation for another source. As a result of this translation one can obtain a scaling factor which is simply a BH mass ratio for these particular sources. This property of the correlations offers a fundamentally new method for BH mass determination in XRBs. Here we use the observed QPO-index correlations observed in three BH sources: GRO J1655-40, GRS 1915+105 and Cyg X-1. The BH mass of (6.3 +/- 0.5) M_Sun in GRO J1655-40 is obtained using optical observations. RXTE observations during the recent 2005 outburst yielded sufficient data to establish the correlation pattern during both rise and decay of the event. We use GRO J1655-40 as a standard reference source to measure the BH mass in Cyg X-1. We also revisit the GRS 1915+105 data as a further test of our scaling method. We infer the value of BH mass of (15.6 +/- 1.5) M_Sun in this source which is consistent with the previous BH mass estimate in GRS 1915 of (13.3 +/- 4) M_Sun. We obtain the BH mass in Cyg X-1 in the range (8.7 +/- 0.8) M_Sun., 5 Printed Pages + 4 Color Figure, Accepted and scheduled for publication in The Astrophysical Journal July 1, 2007, 663, 1 issue

We report results of a new spectral analysis of two BeppoSAX observations of the Z source GX 17+2. In one of the observations, the source exhibited a power-law-like hard (>30 keV) X-ray tail, which was described in a previous work with a hybrid Comptonization model. Recent high-energy observations with INTEGRAL of a sample of low-mass X-ray binaries including both Z and atoll classes have shown that dynamical (bulk) Comptonization of soft photons is a possible alternative mechanism for producing hard X-ray tails in such systems. We start from the INTEGRAL results and exploit the broadband capability of BeppoSAX to better investigate the physical processes at work. We use GX 17+2 as a representative case. Moreover, we suggest that weakening (or disappearance) of the hard X-ray tail can be explained by increasing radiation pressure that originates at the surface of the neutron star (NS). As a result, the high radiation pressure stops the bulk inflow, and consequently, this radiation feedback from the NS surface leads to quenching of the dynamical (bulk) Comptonization.

We present a Comptonization model for the observed properties of the energy-dependent soft/hard time lags and pulsed fraction (amplitude) associated with the pulsed emission of a neutron star (NS). We account for the soft lags by downscattering of hard X-ray photons in the relatively cold plasma of the disk or NS surface. A fraction of the soft X-ray photons coming from the disk or NS surface upscatter off hot electrons in the accretion column. This effect leads to hard lags as a result of thermal Comptonization of the soft photons. This model reproduces the observed soft and hard lags due to the down- and upscattered radiation as a function of the electron number densities of the reflector, n_{e}^{ref}, and the accretion column, n_{e}^{hot}. In the case of the accretion-powered millisecond pulsars IGR J00291+5934, XTE J1751-305, and SAX J1808.4-3658, the observed time lags agree well with the model. Soft lags are observed only if n_e^{ref} << n_e^{hot}. Scattering of the pulsed emission in the NS environment may account for the observed time lags as a non-monotonic function of energy. The time lag measurements can be used as a probe of the innermost parts of the NS and accretion disk. We determine the upper and lower limits of the density variation in this region using the observed time lags. The observed energy-dependent pulsed amplitude allows us to infer a variation of the Thomson optical depth of the Compton cloud in which the accretion column is embedded., Comment: 4 pages, 3 figures, ApJ accepted

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 report the results of Swift and Chandra observations of an ultra-luminous X-ray source, ULX-1 in M101. We show strong observational evidence that M101 ULX-1 undergoes spectral transitions from the low/hard state to the high/soft state during these observations. The spectra of M101 ULX-1 are well fitted by the so-called bulk motion Comptonization (BMC) model for all spectral states. We have established the photon index (\Gamma) saturation level, \Gamma_{sat}=2.8 +/- 0.1, in the \Gamma vs. mass accretion rate (\dot M) correlation. This \Gamma-\dot M correlation allows us to evaluate black hole (BH) mass in M101 ULX-1 to be M_{BH}~(3.2 - 4.3)x10^4 solar masses assuming the spread in distance to M101 (from 6.4+/- 0.5 Mpc to 7.4+/-0.6 Mpc). For this BH mass estimate we use the scaling method taking Galactic BHs XTE~J1550-564, H~1743-322 and 4U~1630-472 as reference sources. The Gamma vs. \dot M correlation revealed in M101~ULX-1 is similar to that in a number of Galactic BHs and exhibits clearly the correlation along with the strong \Gamma saturation at ~2.8. This is robust observational evidence for the presence of a BH in M101 ULX-1. We also find that the seed (disk) photon temperatures are quite low, of order of 40-100 eV which is consistent with high BH mass in M101~ULX-1. Thus, we suggest that the central object in M101 ULX-1 has intermediate BH mass of order 10^{4} solar masses, Comment: 12 pages, 9 figures, accepted by Astronomy & Astrophysics

We present an analysis of archival RXTE and BeppoSAX data of the X-ray source 4U2206+54 . For the first time, high energy data (> 30 kev) are analyzed for this source. The data are well described by comptonization models (CompTT and BMC) in which seed photons with temperatures between 1.1 kev and 1.5 kev are comptonized by a hot plasma at 50 kev thereby producing a hard tail which extends up to, at least, 100 kev. We offer a new method of identification of neutron star systems using a temperature - luminosity relation. If a given X-ray source is characterized by a low bolometric luminosity and a relatively high color blackbody temperature (>1 kev) it has necessarily to be a neutron star rather than a black hole. From these arguments it is shown that the area of the soft photon source must be small (r ~ 1 km) and that the accretion disk, if present, must be truncated very far from the compact object. Here we report on the possible existence of a cyclotron line around 30 kev. The presence of a neutron star in the system is strongly favored by the available data., Accepted for publication in A&A. 9 pages, 7 figures. Submitted to journal in November 2003

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., 15 pages, 3 tables, 6 figures. Accepted for publication in ApJ, 1st of August

We explain why it is possible to detect directly X-ray emission from near the surface of the neutron star (NS) in SAX J1808.4-3658 but not in most other low-mass X-ray binaries (LMXBs), with the exception that emission from the surface can be seen during bursts events. We show that the X-ray emission from SAX J1808.4-3658 mostly originates in the Comptonization process in a relatively optical thin hot region (with an optical depth $\tau_0$ around 4 and temperature is around 20 keV). Such a transparent region does not prevent us from detecting coherent X-ray pulsation due to hot spots on the NS surface. When the characteristic scattering time in the region is of order of inverse rotational frequency omega or higher then the amplitude of the harmonic oscillation decays exponentially with (tau_0 x omega). We give a precise model for the loss of modulation, such suppression of the QPO amplitude due to scattering can explain the disappearance of kHz QPOs with increasing QPO frequency. We also formulate general conditions under which the millisecond X-ray pulsation can be detected in LMXBs. We demonstrate that the observed soft phase lag of the pulsed emission is a result of the downscattering of the hard X-ray photons in the relatively cold material near the NS surface. In the framework of this downscattering model we propose a method to determine the atmosphere density in that region from soft-lag measurements., Comment: 9 pages, 3 figures, accepted for publication in Ap. J. Letters

We study the spectral and temporal properties of three Type I X-ray bursts observed from Cyg X-2 with the Rossi X-ray Timing Explorer. Despite the short time durations (~ 5 sec) these bursts show radial expansion of order of several neutron star (NS) radii. We apply the analytical models of spectral formation during the expansion and contraction stages to derive physical conditions for the matter in the burning zone close to the surface of the NS as well as the NS mass-radius relation. Our results, combined with statistical errors, show that the central object is a compact star with mass ~1.4 solar masses and radius ~9 km. Our results favor the softer equation of state for neutron star matter., Comment: 9 pages, 2 figures. To be published in the Astrophysical Journal Letters

We present an analytical theory of thermonuclear X-ray burst atmosphere structure. Newtonian gravity and diffusion approximation are assumed. Hydrodynamic and thermodynamic profiles are obtained as a numerical solution of the Cauchy problem for the first-order ordinary differential equation. We further elaborate a combined approach to the radiative transfer problem which yields the spectrum of the expansion stage of X-ray bursts in analytical form where Comptonization and free-free absorption-emission processes are accounted for and tau ~ r^{-2} opacity dependence is assumed. Relaxation method on an energy opacity grid is used to simulate radiative diffusion process in order to match analytical form of spectrum, which contains free parameter, to energy axis. Numerical and analytical results show high similarity. All spectra consist of a power-law soft component and diluted black-body hard tail. We derive simple approximation formulae usable for mass-radius determination by observational spectra fitting., Comment: 31 pages, 4 figures, accepted by 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 XMM-Newton time-resolved spectral analysis of the NLS1 galaxy Mrk 766. We analyse eight available observations of the EPIC-pn camera taken between May 2000 and June 2005 to investigate the X-ray spectral variability as produced by changes in the mass accretion rate. The 0.2-10 keV spectra are extracted in time bins longer than 3 ks to accurately trace the variations of the best fit parameters of our adopted Comptonisation spectral model. We test a bulk-motion Comptonisation (BMC) model which is in general applicable to any physical system powered by accretion onto a compact object, and assumes that soft seed photons are efficiently up-scattered via inverse Compton scattering in a hot and dense electron corona. The Comptonised spectrum has a characteristic power-law shape, whose slope was found to increase for large values of the normalisation of the seed component, that is proportional to the mass accretion rate (in Eddington units). Our baseline spectral model also includes a warm absorber lying on the line of sight and radiation reprocessing from the accretion disk or from outflowing matter in proximity of the central compact object. Our study reveals that the normalisation-slope correlation, observed in Galactic Black Hole sources (GBHs), also holds for Mrk 766: variations of the photon index in the range Gamma~1.9-2.4 are indeed likely to be related to the variations of m-dot, as observed in X-ray binary systems. We finally applied a scaling technique based on the observed correlation to estimate the BH mass in Mrk 766. This technique is commonly and successfully applied to measure masses of GBHs, and this is the first time it is applied in detail to estimate the BH mass in an AGN. We obtain a value of M_{BH}=1.26^{+1.00}_{-0.77}x10^6 M_{sun} that is in very good agreement with that estimated by the reverberation mapping, Comment: 26 pages, 7 figures, 4 tables to be published in Astronomy and Astrophysics

We present an analysis of the spectral and timing properties of X-ray emission from the Z-source Sco~X-1 during its evolution between the Horizontal (HB) and Flaring(FB) branches observed with the RXTE during the 1996 -- 2002 period. We find that the broad-band (3 - 250 keV) energy spectra during all spectral states can be adequately reproduced by a model, consisting of two Comptonized components and an iron-line. We suggest that the seed photons of kT_s1~0.7 keV coming from the disk and of temperature kT_s2~1.8 keV coming from the neutron star (NS) are each upscattered by hot electrons of a "Compton cloud" (herein Comptb1 and Comptb2 components respectively with which are associated similarly subscripted parameters). The photon power-law index Gamma_{2} is almost constant (Gamma_{2}~2) for all spectral states. In turn, Gamma_{1} demonstrates a two-phase behavior with the spectral state: Gamma_{1} is quasi-constant at the level Gamma_{1}~2 for the HB-NB and Gamma_{1} is less than 2, namely in the range of 1.3, Comment: 27 pages, 13 figures, 3 tables, accepted by the ApJ and scheduled for publication for the July 10, 2014, v 789, 2nd issue

We present a method for determining the B-field around neutron stars based on observed kHz and viscous QPO frequencies used in combination with the best-fit optical depth and temperature of a Comptonization model. In the framework of the transition layer QPO model, we analyze magnetoacoustic wave formation in the layer between a neutron star surface and the inner edge of a Keplerian disk. We derive formulas for the magnetoacoustic wave frequencies for different regimes of radial transition layer oscillations. We demonstrate that our model can use the QPO as a new kind of probe to determine the magnetic field strengths for 4U 1728-42, GX 340+0, and Sco X-1 in the zone where the QPOs occur. Observations indicate that the dependence of the viscous frequency on the Keplerian frequency is closely related to the inferred dependence of the magnetoacoustic wave frequency on the Keplerian frequency for a dipole magnetic field. The magnetoacoustic wave dependence is based on a single parameter, the magnetic moment of the star as estimated from the field strength in the transition layer. The best-fit magnetic moment parameter is about (0.5-1)x 10^{25} G cm^3 for all studied sources. From observational data, the magnetic fields within distances less 20 km from neutron star for all three sources are strongly constrained to be dipole fields with the strengths 10^{7-8} G on the neutron star surface.

We suggest that persistent low-frequency quasi-periodic oscillations (QPOs) detected in the black hole (BH) sources XTE J1118+480, GRO J1655-40, and LMC X-1 at ~0.1 Hz, in HZ Her/Her X-1 at ~0.05 Hz, and in neutron star (NS) binaries 4U 1323-62, 4U 1746-31, and EXO 0748-76 at ~1 Hz are caused by the global disk oscillations in the direction normal to the disk (normal mode). We argue that these disk oscillations are a result of the gravitational interaction between the central compact object and the disk. A small displacement of the disk from the equatorial plane results in a linear gravitational restoring force opposite to this displacement. Our analysis shows that the frequency of this mode is a function of the mass of the central object and that it also depends on the inner and outer radii of the disk, which in turn are related to the orbital period of the binary system. We derive an analytical formula for the frequency of the normal disk mode and show that these frequencies can be related to the persistent lower QPO frequencies observed in the NS and BH sources. We offer a new independent approach to the BH mass determination by interpreting this low QPO frequency as the global disk oscillation frequency. The implementation of this method combined with the independent method recently developed by Shrader & Titarchuk that uses the X-ray energy spectra results in stringent constraints for the BH masses.

Since the discovery of kHz quasi-periodic oscillations (QPO) in neutron star binaries, the difference between peak frequencies of two modes in the upper part of the spectrum, i.e. Delta (omega)=omega_h-omega_K has been studied extensively. The idea that the difference Delta(omega) is constant and (as a beat frequency) is related to the rotational frequency of the neutron star has been tested previously. The observed decrease of Delta(omega) when omega_h and omega_k increase has weakened the beat frequency interpretation. We put forward a different paradigm: a Keplerian oscillator under the influence of the Coriolis force. For such an oscillator, omega_h and the assumed Keplerian frequency omega_k hold an upper hybrid frequency relation: omega^2_h-omega^2_K=4*Omega^2, where Omega is the rotational frequency of the star's magnetosphere near the equatorial plane. For three sources (Sco X-1, 4U 1608-52 and 4U 1702-429), we demonstrate that the solid body rotation Omega=Omega_0=const. is a good first order approximation. Within the second order approximation, the slow variation of Omega as a function of omega_K reveals the structure of the magnetospheric differential rotation. For Sco X-1, the QPO have frequencies approximately 45 and 90 Hz which we interpret as the 1st and 2nd harmonics of the lower branch of the Keplerian oscillations for the rotator with vector Omega not aligned with the normal of the disk: omega_L/2 pi=(Omega/pi)(omega_K/omega_h)sin(delta) where delta is the angle between vector Omega and the vector normal to the disk.

An accreting black hole is, by definition, characterized by the drain. Namely, the matter falls into a black hole much the same way as water disappears down a drain - matter goes in and nothing comes out. As this can only happen in a black hole, it provides an unique way to see it. The accretion proceeds almost in free fall close to the black hole horizon. In this paper we calculate (by using Monte -Carlo simulations) the specific features of X-ray spectra formed as a result of upscattering of the soft (disk) photons in the converging inflow (CI) within about 3 Schwarzschild radii of the black hole. The full relativistic treatment has been implemented to reproduce these spectra. We show that spectra in the soft state of black hole systems can be described as the sum of a thermal (disk) component and the convolution of some fraction of this component with the CI upscattering spread function. The latter boosted photon component is seen as an extended power-law at energies much higher than the characteristic soft photons energy. We demonstrate the stability of the power spectral index (alpha= 1.8) over a wide range of the plasma temperature 0-10 keV and mass accretion rates (higher than 2 in Eddington units). We also demonstrate that the sharp high energy cutoff occurs at energies of 200-400 keV which are related to the average rest energy of electrons impinging upon the horizon. The spectrum is practically identical to the standard thermal Comptonization spectrum when the CI plasma temperature is getting of order of 50 keV (hard state of BHS). Also, the change of spectral shapes from the soft to the hard X-ray state is clearly to be related with the temperature of the bulk flow. These Monte-Carlo simulated CI spectra are then a inevitable stamp of the BHS., Comment: 30 pages TeX format, 6 PS figures, accepted for ApJ Main Journal

We study the formation of the H-like iron (Fe XXVI) Lyα line at 6.97 keV in narrow-line Seyfert 1 galaxies in the framework of current models for accretion into a black hole. We find that Fe XXVI Lyα emission is most likely formed by resonant fluorescence resulting from the strong radiation field in the region near the black hole, or possibly in the interphase between a hot corona and the optically thick α disk. Alternatively, the line could be formed by collisional excitation at a temperature of approximately 12 keV in accretion flows dominated by advection or gravitational accretion flows with strong shocks. For either resonant fluorescence or collisional excitation, the equivalent width of the Fe XXVI Lyα line is a powerful temperature diagnostic of the emitting region. We propose some observational tests to distinguish between collisional and resonant fluorescent excitation of the line.

We analyze the exact general relativistic exact integro-differential equation of radiative transfer describing the interaction of low energy photons with a Maxwellian distribution of hot electrons in gravitational field of a Schwarzschild black hole. We prove that due to Comptonization an initial arbitrary spectrum of low energy photons unavoidably results in spectra characterized by an extended power-law feature. We examine the spectral index by using both analytical and numerical methods for a variety of physical parameters as such the plasma temperature and the mass accretion rate. The presence of the event horizon as well as the behaviour of the null geodesics in its vicinity largely determine the dependence of the spectral index on the flow parameters. We come to the conclusion that the bulk motion of a converging flow is more efficient in upscattering photons than thermal Comptonization provided that the electron temperature in the flow is of order of a few keV or less. In this case, the spectrum observed at infinity consists of a soft component produced by those input photons that escape after a few scatterings without any significant energy change and of hard component (described by a power law) produced by the photons that underwent significant upscattering. The luminosity of the power-law component is relatively small compared to that of the soft component. For accretion into black hole the spectral energy index of the power-law is always higher than one for plasma temperature of order of a few keV. This result suggests that the bulk motion Comptonization might be responsible for the power-law spectra seen in the black-hole X-ray sources., Comment: 31 pages, 3 figures; Astrophysical Journal accepted

We model the polarization properties of line emission from an accretion disk under a range of assumptions about the source function and electron-scattering optical depth tau(sub es). For small values of tau(sub es) and modest viewing angles, polarization can be in excess of the Chandrasekhar result for tau(sub es) = infinity. The polarization vector can be either parallel or perpendicular to the projected direction of the disk axis. The polarization properties of the double-peaked H-alpha emission line of the broad-line radio galaxy Arp 102B observed by Antonucci, Hurt, and Agol can be understood in terms of electron scattering and line broadening within the line-emitting region if tau(sub es) is of order unity, and if the position angle of polarization is parallel to the projected disk axis. The required small tau(sub es) is consistent with the hypothesis that the Balmer lines in Arp 102B are produced by photoionization of the disk atmosphere.

We present the results of our 1994 January and 1995 January observations of the quasar 3C 273 obtained with the International Ultraviolet Explorer (IUE) and the Extreme-Ultraviolet Explorer (EUVE). These observations were part of a large multiwavelength campaign to observe 3C 273 from radio through gamma-rays. Our 1995 January photometric observations with the EUVE Lexan/B Deep Survey (DS) instrument indicate strong evidence for variability, at a 99% confidence level, during the 12 day observing period. We have utilized ROSAT PSPC soft X-ray power-law models to correlate with EUVE count rates. Besides variations in the normalization level between both observations, our EUV count rates are consistent with a simple power-law model with spectral index alpha approx. 1.77 (F(sub upsilon) proportional to upsilon(sup -alpha) that can be extrapolated from the soft X-rays to the EUV range. The active galactic nucleus 3C 273 is an important blazar to study because in our picture it reveals the presence of both disk and relativistic beam spectral contributions.

We present calculations of the time lags and the coherence function of X-ray photons for a novel model of radiation emission from accretion-powered high-energy sources. Our model involves only Comptonization of soft photons injected near the compact object in an extended but nonuniform atmosphere around the compact object. Our results show that this model produces time lags between the hard and soft bands of the X-ray spectrum that increase with Fourier period, in agreement with recent observations; it also produces a coherence function equal to one over a wide range of frequencies if the system parameters do not change significantly during the observation, also in agreement with the limited existing observations. We explore various conditions that could affect coherence functions. We indicate that measurements of these statistical quantities could provide diagnostics of the radial variation of the density of this class of sources.

This paper summarizes the development of a successful project, LAUE, supported by the Italian Space Agency (ASI) and devoted to the development of long focal length (up to 100 m) Laue lenses for hard X--/soft gamma--ray astronomy (80-600 keV). The apparatus is ready and the assembling of a prototype lens petal is ongoing. The great achievement of this project is the use of bent crystals. From measurements obtained on single crystals and from simulations, we have estimated the expected Point Spread Function and thus the sensitivity of a lens made of petals. The expected sensitivity is a few $\times10^{-8}$ photons cm$^{-2}$ s$^{-1}$ keV$^{-1}$. We discuss a number of open astrophysical questions that can settled with such an instrument aboard a free-flying satellite., 17 pages, 18 figures, published in Proceedings of the SPIE, Volume 8861, id. 886106 17 pp. (2013)

We report results of a systematic study of the broad--band (2--2000 keV) time--resolved prompt emission spectra of a sample of Gamma-Ray Bursts (GRBs) detected with both the Wide Field Cameras (WFCs) aboard the BeppoSAX satellite and the BATSE experiment aboard CGRO. The main goal of the paper is to test spectral models of the GRB prompt emission that have recently been proposed. In particular, we test the photospheric model proposed by Ryde and Pe'er (2009), i.e., blackbody plus power--law, the addition of a blackbody emission to the Band function in the cases in which this function does not fit the data, and the Comptonization model developed by Titarchuk et al. (2012). By considering the few spectra for which the simple Band function does not provide a fully acceptable fit to the data (Frontera et al. 2012), only in one case we find a statistically significant better fit by adding a blackbody to this function. We confirm the results found fitting the BATSE spectra alone with a blackbody plus a power law. Instead when the BATSE GRB spectra are joined to those obtained with WFCs (2--28 keV), this model becomes unacceptable in most of time intervals in which we subdivide the GRB light curves. We find instead that the Comptonization model is always acceptable, even in the few cases in which the Band function is inconsistent with the data. We discuss the implications of these results., Comment: 52 pages, 18 figures, published in The Astrophysical Journal Volume 779, Issue 2, article id. 175, 21 pp. (2013)

We report the results of Swift/XRT observations (2008-2015) of a hyper-luminous X-ray source, ESO 243-49 HLX-1. We found a strong observational evidence that ESO 243-49 HLX-1 underwent spectral transitions from the low/hard state to the high/soft state during these observations. The spectra of ESO 243-49 HLX-1 are well fitted by the so-{called} bulk motion Comptonization model for all spectral states. We have established the photon index Gamma saturation level, Gamma_{sat}$=3.0+/-0.1, in the correlation of Gamma versus mass accretion rate dot M. This Gamma-dot M correlation allows us to estimate the black hole (BH) mass in ESO 243-49 HLX-1 to be M_{BH}~ 7x 10^4 solar masses, assuming the distance to ESO 243-49 of 95 Mpc. For the BH mass estimate we used the scaling method, taking Galactic BHs XTE~J1550-564, H~1743-322 and 4U~1630-472, and an extragalactic BH source, M101 ULX-1 as reference sources. The Gamma-dot M correlation revealed in ESO 243-49 HLX-1 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 ~ 3. This is a reliable observational evidence of a BH in ESO 243-49 HLX-1. We also found that the seed (disk) photon temperatures are quite low, of order of 50-140 eV which are consistent with a high BH mass in ESO 243-49 HLX-1., Comment: 21 pages, 8 figures, accepted by Astronomy and Astrophysics on August 29, 2016

We present an X-ray spectral analysis of the high-mass binary 4U~1700-37 during its hard-soft state evolution. We use the BeppoSAX, Suzaku and RXTE (Rossi X-ray Timing Explorer), Suzaku and BeppoSAX observations for this investigation. We argue that the X-ray broad-band spectra during all spectral states can be adequately reproduced by a model, consisting of a low-temperature Blackbody component, two Comptonized components both due to the presence of a Compton cloud (CC) that up-scatters seed photons of $T_{s1}$~< 1.4 keV, and $T_{s2}, Comment: 18 pages, 7 figures, 7 Tables, accepted by the ApJ, on February 29, 2016

It is recognized that very likely the correlation between peak energy [Formula: see text] and bolometric intensity is intrinsic to GRBs. However, its physical origin is still debated. In this paper, we will discuss a possible interpretation of the correlation in the light of a GRB prompt emission spectral model, grbcomp, proposed in [L. Titarchuk, R. Farinelli, F. Frontera and L. Amati, Astrophys. J. 752 (2012) 116]. grbcomp is essentially a photospheric model for the prompt emission of GRBs. Its main ingredients are a thermal bath of soft seed photons and a subrelativistically expanding outflow plasma, consequence of the star explosion. The emerging spectrum is the result of two phases: first, up to the photospheric radius, Comptonization of a subrelativistic electron outflow with thermal bath of soft photons, then, convolution of the Comptonized photons in the first phase with a Green function. The result of this convolution is consistent with different physical processes, in particular Inverse Compton. grbcomp has been successfully tested using a significant sample of GRB time resolved spectra in the broad energy band from 2[Formula: see text]keV to 2[Formula: see text]MeV [F. Frontera, L. Amati, R. Farinelli, S. Dichiara, C. Guidorzi, R. Landi and L. Titarchuk, Astrophys. J. 779 (2013) 175].

We present an analysis of the spectral and timing properties observed in X-rays from neutron star binary GX~3+1. We analyze all observations of this source obtained with the RXTE and BeppoSAX satellites. We find that the X-ray broad-band energy spectra during these spectral transitions can be adequately reproduced by a composition of a low-temperature blackbody component, a Comptonized component (COMPTB) and Gaussian component. We argue that the electron temperature kT_e of the Compton cloud monotonically increases from 2.3 keV to 4.5 keV, when GX~3+1 makes a spectral transition. Using a disk seed photon normalization of COMPTB, which is proportional to mass accretion rate, we find that the photon power-law index Gamma is almost constant (Gamma=2.00+/- 0.02) when mass accretion rate changes by factor 4. We interpret this quasi-stability of the index Gamma and a particular form of the spectrum in the framework of a model in which the energy release in the transition layer located between the accretion disk and neutron star surface dominates that in the disk. Moreover, this index stability effect now established for GX~3+1 was previously found in the atoll source 4U~1728-34 and suggested for a number of other LMXB NS binaries (see Farinelli & Titarchuk). This intrinsic behavior of neutron stars, in particular for atoll sources, is fundamentally different from that seen in black hole binary sources where the index monotonically increases during spectral transition from the low state to the high state and then finally saturates at high values of mass accretion rate., 19 pages, 9 figures and 5 tables. The paper is published in the Astrophysical Journal on February 1, 2012. arXiv admin note: text overlap with arXiv:1106.5745

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

We present an analysis of the spectral properties observed in X-rays from Neutron Star X-ray binary 4U~1728-34 during transitions between the low and the high luminosity states when electron temperature kT_e of the Compton cloud monotonically decreases from 15 to 2.5 keV. We analyze the transition episodes from this source observed with BeppoSAX and RXTE. We find that the X-ray broad-band energy spectra of 4U~1728-34 during all spectral states can be modeled by a combination of a thermal (black body-like) component, a Comptonized component (which we herein denote COMPTB) and a Gaussian component. Spectral analysis using this model evidences that the photon power-law index Gamma is almost constant (Gamma=1.99+/-0.02) when kT_e changes from 15 to 2.5 keV during these spectral transitions. We explain this quasi-stability of the index by the model in which the spectrum is dominated by the strong thermal Comptonized component formed in the transition layer (TL) between the accretion disk and neutron star surface. The index quasi-stability takes place when the energy release in the TL is much higher than the flux coming to the TL from the accretion disk. This intrinsic property of neutron star is fundamentally different from that in black hole binary sources for which the index monotonically increases during spectral transition from the low state to high state and saturates at high values of mass accretion rate., Comment: 22 pages, 14 figures, 4 Tables to be published in the Astrophysical Journal, August 10, 2011, v. 737

Estimating the black hole mass at the center of galaxies is a fundamental step not only for understanding the physics of accretion, but also for the cosmological evolution of galaxies. Recently a new method, based solely on X-ray data, was successfully applied to determine the black hole mass in Galactic systems. Since X-rays are thought to be produced via Comptonization process both in stellar and supermassive black holes, in principle, the same method may be applied to estimate the mass in supermassive black holes. In this work we test this hypothesis by performing a systematic analysis of a sample of AGNs, whose black hole mass has been already determined via reverberation mapping and which possess high quality XMM-Newton archival data. The good agreement obtained between the black hole masses derived with this novel scaling technique and the reverberation mapping values suggests that this method is robust and works equally well on stellar and supermassive black holes, making it a truly scale-independent technique for black hole determination., Comment: 16 pages, 6 figures, 7 tables, accepted for publication in ApJ