Search

Precise measurements of black hole (BHs) masses are necessary to understand the coevolution of these sources and their host galaxies. Sometimes in the center of a galaxy there is not one, but two BHs. The BH duality of the quasar nucleus SDSS~J075217.84+193542.2 (herein SDSS~J0752) was recently proposed based on the observed strict periodicity of optical emission from the source. We tested this assumption using X-ray observations with Swift/XRT (2008--2010). We fitted the SDSS~J075217 spectrum using a Comptonization model and discovered soft X-ray variability in the 0.3--10~keV energy range. We pursued a scenario in which two supermassive BHs at the center of SDSS~J0752 form a pair; and the less massive (secondary) BH periodically crosses/punctures the disk around the more massive (primary) BH. We associate these periodic crossings with tidal disruptions of the disk and, as a consequence, with an increase in X-rays seen as a flare in SDSS~J0752. During such an X-ray flare event (2008--2010), we discovered a change in the source spectral states and the photon index saturation at the $\Gamma\sim3$ level with mass accretion rate $\dot M$. For BH mass scaling we used sources: OJ~287, M101 ULX--1 and HLX--1 ESO~243--49, as a reference ones, and found that M$_{SDSS}=9\times 10^7$ solar masses, assuming $d_{SDSS}= 500$ Mpc. Thus, we obtained a lower limit to a BH mass due the unknown inclination. In addition, we used the virial mass of the secondary BH based on $H_\alpha$-line measurements and we estimated the binary's inclination at SDSS~J0752, $i=80^{\circ}$, using a scaling technique., Comment: 29 pages, 6 figures. arXiv admin note: text overlap with arXiv:2302.06068

Neutron stars (NSs) and white dwarfs (WDs) are characterized by different geometric and physical properties, but their observed properties are often similar, making them difficult to distinguish. Therefore, it is desirable to search for their spectral features that could be easily identified from observations. We present spectral and timing signatures of NSs and WDs hosted in accreting X-ray binaries that can be easily identified from X-ray observations. We perform spectral and timing analysis of 4U~1636--53 and SS~Cygni, as typical representatives of such NS and WD binaries, based on their X-ray observations by RXTE, ASCA, Suzaku and BeppoSAX uising {\it Comptonization} spectral model. As a result, we formulate a criterion that makes it easy to distinguish NS from WD in such binaries: NS X-rays exhibits clear quasi-stable behavior with the index $\Gamma\to2$ and is characterized by quasi periodic oscillations (QPOs) at $\nu_{QPO} >0.5$~Hz, although WD X-rays is stable with $\Gamma \to1.85$ and is accompanied by QPOs at $\nu_{QPO}<0.05$~Hz during source outbursts. In addition, we revealed that in 4U~1636--53 the mHz QPOs anti-correlate with the plasma temperature, $T_e$ of Compton cloud (or the corona around a NS. This allowed us to associate mHz-QPOs with the corona dynamics during outburst cycle. The above index effect, now well established for 4U~1636--53 and SS~Cygni using extensive observations, has previously been found in other low-mass X-ray NS and WD binaries and agrees well with the criterion for distinguishing NSs and WDs presented here., Comment: 14 figures, 9 tables

We presented a study of outburst activity in the BL Lacertae source OJ~287, observed extensively with the X-ray telescope (XRT) onboard Neil Gehrels Swift Observatory. We demonstrated that the results of our analysis of X-ray flaring activity using the Swift/XRT data allow to refine the key characteristics of the OJ~287 secondary (its nature and mass). We discovered that the energy spectra in all spectral states can be fitted using the XSPEC Bulk Motion Comptonization (BMC) model. As a result we found that the X-ray photon index of the BMC model, Gamma correlates with the mass accretion rate, dot. We established that Gamma increases monotonically with dot from the low-hard state, Gamma~ 1.5 to the high-soft state, Gamma~2.8 and finally saturates. The index behavior was similar to that in a number of black hole (BH) candidates in which we showed that its saturation was an observational evidence of the presence of a BH. Based on this correlation, we applied a scaling method and determined that a secondary BH mass in OJ~287 is about ~1.25x10^8 solar masses, using the well-studied X-ray BH binaries XTE~1550--564, H~1743--322, 4U~1630--47, GRS~1915+105 as well as extragalactic BHs ESO~243--49 and M101~ULX--1, as reference sources. Also using the power spectrum analysis we inferred the size of the Compton cloud L_{CC}~ 10^{13} cm where X-ray spectra were formed. Using this value of L_{CC} we confirmed that a BH mass of the secondary in OJ~287 was of order of 10^8 solar masses as we derived using the index, Gamma-correlation (the scaling method) with respect of the mass accretion rate., Comment: 10 pages, 8 figures

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)

We discovered a so called high-temperature blackbody (HBB) component, found in the 15 -- 40 keV range, in the broad-band X-ray energy spectra of black hole (BH) candidate sources. A detailed study of this spectral feature is presented using data from five of the Galactic BH binaries, Cyg X-1, GX 339-4, GRS 1915+105, SS 433 and V4641~Sgr in the low/hard, intermediate, high/soft and very soft spectral states (LHS, IS, HSS and VSS, respectively) and spectral transitions between them using {\it RXTE}, INTEGRAL and BeppoSAX data. In order to fit the broad-band energy spectra of these sources we used an additive XSPEC model, composed of the Comptonization component and the Gaussian line component. In particular, we reveal that the IS spectra have the HBB component which color temperature, kT_HBB} is in the range of 4.5 -- 5.9 keV. This HBB feature has been detected in some spectra of these five sources only in the IS (for the photon index Gamma>1.9) using different X-ray telescopes. We also demonstrate that a timescale of the HBB-feature is of orders of magnitude shorter than the timescale of the iron line and its edge. That leads us to conclude that these spectral features are formed in geometrically different parts of the source and which are not connected to each other. Laurent & Titarchuk (2018) demonstrated a presence of a gravitational redshifted annihilation line emission in a BH using the Monte-Carlo simulations and therefore the observed HBB hump leads us to suggest this feature is a gravitational redshifted annihilation line observed in these black holes, Comment: 32 pages, 16 figures, 18 Tables

We present the discovery of correlations between the X-ray spectral (photon) index and mass accretion rate observed in active galactic nuclei (AGNs) 3C~454.3 and M87. We analyzed spectral transition episodes observed in these AGNs using Chandra, Swift, Suzaku, BeppoSAX, ASCA and RXTE data. We applied a scaling technique for a black hole (BH) mass evaluation which uses a correlation between the photon index (Gamma) and normalization of the seed component which is proportional to a disk mass accretion rate Mdot. We developed an analytical model that shows that Gamma of the BH emergent spectrum undergoes an evolution from lower to higher values depending on Mdot. To estimate a BH mass in 3C~454.3 we consider extra-galactic SMBHs NGC~4051 and NGC~7469 as well as Galactic BHs Cygnus X--1 and GRO~J1550--564 as reference sources for which distances, inclination angles are known and the BH masses are already evaluated. For M87 on the other hand, we provide the BH mass scaling using extra-galactic sources (IMBHs: ESO 243-49 HLX 1 and M 101 ULX--1) and Galactic sources (stellar mass BHs: XTE J1550-564, 4U 1630-472, GRS 1915+105 and H 1743-322) as reference sources. Application of the scaling technique for the photon index-Mdot correlation provides estimates of the BH masses in 3C 454.3 and M87 to be about 3.4x10^9 and 5.6 x10^7 solar masses, respectively. We also compared our scaling BH mass estimates with a recent BH mass estimate of M_{87}=6.5x 10^9 M_{\odot} made using the {Event Horizon Telescope} which gives an image at 1.3 mm and is based on the angular size of the `BH event horizon'. Our BH mass estimate in M87 is at least two orders of magnitude lower than that made by the EHT team., Comment: 27 pages, 20 figures, 13 Tables. Accepted for publication in A&A

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., Comment: 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

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., Comment: 23 pages, 8 Tables, 11 figures

We present an analysis of the spectral properties observed in X-rays from active galactic nucleus BL Lacertae using RXTE, Suzaku, ASCA, BeppoSAX, and Swift observations. The total time covered by these observations is approximately 20 years. We show that this source undergoes X-ray spectral transitions from the low hard state (LHS) through the intermediate state (IS) to the high soft state (HSS) during these observations. During the RXTE observations (1997 -- 2001, {180 ks, for a total 145 datasets}), the source was approximately 75%, 20% and only 5 of the time in the IS, LHS, and HSS, respectively. We also used Swift observations (470 datasets, for a total 800 ks), which occurred during 12 years (2005 -- 2016), the (0.3 -- 200 keV) data of BeppoSAX (1997 -- 2000, 160 ks), and the (0.3 -- 10 keV) data of ASCA (1995 -- 1999, 160 ks). Two observations of Suzaku (2006, 2013; 50 ks) in combinations with long-term RXTE and Swift data-sets allow us to describe all spectral states of BL Lac. The spectra of BL Lac are well fitted by the bulk motion Comptonization (BMC) model for all spectral states. We have established the photon index saturation level, Gamma_{sat}=2.2+/-0.1, in the Gamma vs. mass accretion rate (Mdot) correlation. This Gamma-Mdot correlation allows us to estimate the black-hole (BH) mass in BL Lac to be M_{BH}~3x10^7 M_sol for a distance of 300 Mpc. For the BH mass estimate, we use the scaling method taking stellar-mass Galactic BHs 4U~1543--47 and GX~339--4 as reference sources. The Gamma-Mdot correlation revealed in BL Lac is similar to those in a number of stellar-mass Galactic BHs and two recently studied intermediate-mass extragalactic BHs. It clearly shows the correlation along with the very extended $\Gamma$ saturation at ~ 2.2. This is robust observational evidence for the presence of a BH in BL Lac., Comment: 29 pages, 10 figures and 10 Tables

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}<$1 keV, and an iron-line component. We find using this model that the photon power-law index is almost constant, $\Gamma_{1}\sim 2$ for all spectral states. However, $\Gamma_{2}$ shows a behavior depending on the spectral state. Namely, $\Gamma_{2}$ is quasi-constant at the level of $\Gamma_{2}\sim 2$ while the CC plasma temperature $kT^{(2)}_e$ is less than 40 keV; on the other hand, $\Gamma_{2}$ is in the range of $1.3<\Gamma_{2}<2$, when $kT^{(2)}_e$ is greater than 40 keV. We explain this quasi-stability of $\Gamma$ during most of hard-soft transitions of 4U~1700-37 in a framework of the model in which the resulting spectrum is described by two Comptonized components. We find that these Comptonized spectral components of the HMXB 4U~1700-37 are similar to those previously found in NS sources. This index dependence versus both mass accretion rate and $kT_e$ revealed in 4U~1700-37 is a universal observational evidence for the presence of a NS in 4U 1700-37., Comment: 18 pages, 7 figures, 7 Tables, accepted by the ApJ, on February 29, 2016

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 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}<1.5 keV (herein Comptb) and a second resulting from the seed photons of T_{s2}~1.1-1.3 keV coming from the disk (herein Comptb2). We found that we needed to add a low-temperature blackbody and an iron-line ({Gaussian}) component to the model in order to obtain high-quality fits. The data analysis using this model indicates that the power-law photon index Gamma_{1} of our model is always about 2, independently of the spectral state. Another parameter, Gamma_{2} demonstrates a two-phase behavior depending on the spectral state. Gamma_{2} is quasi-constant at Gamma_{2}~ 2 when the electron temperature kT^{(2)}_e<80 keV and Gamma_{2} is less than 2, in the range of 1.380 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 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

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., Comment: 33 pages, 13 Figures. The paper is accepted and scheduled for publication in The Astrophysical Journal, July 1, 2014, v 789, 1 issue

We analyze the X-ray spectra and their timing properties of the compact X-ray binary 4U~1820-30. We establish a number of spectral transitions in this source seen with BeppoSAX and RXTE. During the RXTE observations (1996 -- 2009), the source were approximately 75% of its time in the soft state making the lower banana and upper banana transitions combined with long-term low-high state transitions. We reveal that all of the X-ray spectra of 4U~1820-30 are fit by a composition of a thermal (blackbody) component, a Comptonization component (COMPTB) and a Gaussian-line component. Thus using this spectral analysis we find that the photon power-law index Gamma of the Comptonization component is almost unchangeable (Gamma~2) while the electron temperature kT_e changes from 2.9 to 21 keV during these spectral events. We also establish that for these spectral events the normalization of COMPTB component (which is proportional to mass accretion rate, \dot M increases by factor 8 when kT_e decreases from 21 keV to 2.9 keV. Before this index stability effect was also found analyzing X-ray data for Z-source GX 340+0 and for atolls, 4U~1728-34, GX~3+1. Thus, we can suggest that this spectral stability property is a spectral signature of an accreting neutron star source. On the other hand in a black hole binary Gamma monotonically increases with \dot M and ultimately its value saturates at large \dot M., Comment: 25 pages, 13 figures and 5 Tables. The paper is accepted and scheduled for publication in The Astrophysical Journal, for the ApJ April 20, 2013, v 767, 2nd issue

We show an analysis of the spectral and timing properties of X-ray radiation from Z-source during its evolutions when the electron temperature of the Transition Layer (TL) kT_e monotonically decreases from 21 to 3 keV. We analyze episodes observed with the BeppoSAX and Rossi X-ray Timing Explorer (RXTE). We reveal that the X-ray broad-band energy spectra during all spectral states can be reproduced by a {physical} model, composed of a soft blackbody component, {two Comptonized components (both due to the presence of that TL that up-scatters both seed photons of T_{s1}<1 keV coming from the disk (first component, Comptb1), and seed photons of temperature T_{s2}<1.5 keV coming from the neutron star (second component, Comptb2) and the iron-line (Gaussian) component. Spectral analysis using this model indicates that the photon power-law indices Gamma_{com1} and Gamma_{com2} of the Comptonized components are almost constant, Gamma_{com1} and Gamma_{com2} ~ 2 when kT_e changes from 3 to 21 keV along Z-track. We interpret the detected quasi-stability of the indices of Comptonized components near a value of 2. Furthermore, this index stability now found for the Comptonized spectral components of Z-source GX 340+0 is similar to that was previously established in the atoll sources 4U~1728-34, GX 3+1 and early proposed for a number of X-ray neutron stars (NSs). This behavior of NSs both for atoll and Z-sources, is essentially different from that observed in black hole binaries where Gamma_{com} increases during a spectral evolution from the low state to the high state and ultimately saturates at high mass accretion rate., Comment: 30 pages, 5 Tables and 12 figures, the paper has been accepted and scheduled for publication in The Astrophysical Journal, for the March 20, 2013, v 766, 1st issue

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., Comment: 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 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

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

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 <1 keV, and the redskewed iron line (LAOR) component. We also present observable correlations between the index and the normalization of the disk "seed" component. The use of "seed" disk normalization, which is presumably proportional to mass accretion rate in the disk, is crucial to establish the index saturation effect during the transition to the soft state. We discovered the photon index saturation of the soft and hard spectral components at values of 4.2 and 3 respectively. We present a physical model which explains the index-seed photon normalization correlations. We argue that the index saturation effect of the hard component (BMC1) is due to the soft photon Comptonization in the converging inflow close to BH and that of soft component is due to matter accumulation in the transition layer when mass accretion rate increases. In addition to our spectral model components we also find a strong feature of "blackbody-like" bump which color temperature is about 4.5 keV in eight observations of the intermediate and soft states. We discuss a possible origin of this "blackbody-like" emission., Comment: 33 pages, 16 figures, accepted for publication in ApJ, on December 10, 2009, v. 707

We analyze the X-ray spectra and their timing properties of the compact Xray binary 4U 1820-30. We establish spectral transitions in this source seen with BeppoSAX and the Rossi X-ray Timing Explorer (RXTE). During the RXTE observations (1996 - 2009), the source were approximately approximately 75% of its time in the soft state making the lower banana and upper banana transitions combined with long-term low-high state transitions. We reveal that all of the X-ray spectra of 4U 1820-30 are fit by a composition of a thermal (blackbody) component, a Comptonization component (COMPTB) and a Gaussian-line component. Thus using this spectral analysis we find that the photon power-law index Gamma of the Comptonization component is almost unchangeable (Gamma approximately 2) while the electron temperature kTe changes from 2.9 to 21 keV during these spectral events. We also establish that for these spectral events the normalization of COMPTB component (which is proportional to mass accretion rate ˙M) increases by factor 8 when kTe decreases from 21 keV to 2.9 keV. Before this index stability effect was also found analyzing X-ray data for Z-source GX 340+0 and for atolls, 4U 1728-34, GX 3+1. Thus, we can suggest that this spectral stability property is a spectral signature of an accreting neutron star source. On the other hand in a black hole binary Γ monotonically increases with ˙Mand ultimately its value saturates at large ˙M.

We present an analysis of the X-ray spectral properties observed from black hole , candidate (BHC) binary SS 433. We have analyzed Rossi X-ray Time Explorer (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. In fact this spectral feature has been recently reproduced in Monte Carlo simulations by Laurent and Titarchuk. 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(sub bh) approximately > 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(sub 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.