arXiv subjects

Elena Seifina

Publications and source records attributed to Elena Seifina.

At least 19 recordsLinked to original sources

arXiv2025

Linear Polarization in a Black Hole, 4U~1957+115 and its X-ray Spectral Analysis

We present a new spectro-polarimetric method for estimation of the inclination of X-ray binaries using the linear polarization (LP) value in black hole (BH) sources, based on the early paper by Sunyaev \& Titarchuk (1985, ST85). The X-ray LP arises from multiple scattering of initially soft photons in a hot, optically thick Compton cloud (CC) using a flat geometry. ST85 showed that the LP degree $P$ depends on the binary inclination $i$ and remains independent of the photon energy. Moreover, the $P$ follows a characteristic angular distribution based on the CC optical depth $τ_0$, in particular for $τ_0 >$ 10 the LP follows the Chandrasekhar (1950) distribution. In light of the vast number of recent IXPE spectro-polarimetric observations of BHs, testing this approach in combination with spectral analysis of these sources by other missions is desirable. In particular, the BH source, 4U 1957+115 was observed with IXPE in the high/soft state for which $P\sim 2\%$ was found (Marra et al. 2024). To apply the new method, in addition to knowing $P$ for a given source, it is necessary to estimate the CC optical depth $τ_0$ and then to calculate of the best-fit photon index $Γ$ and the plasma temperature $kT_e$. We present the X-ray spectral analysis of 4U~1957+11 using data from IXPE, NuSTAR, NICER, RXTE}, Swift, Suzaku and ASCA. We show that the X-ray source spectra are well described by the Comptonization model with $Γ$ varying from 1.5 to 3. We find a monotonic increase in $Γ$ with the mass accretion rate, $\dot M$, and a final saturation of the index at $Γ\sim3$ for high $\dot M$. We determine a BH mass by the scaling method: $M_{1957}=4.8 \pm 1.8~M_{\odot}$, assuming a source distance of 22 kpc, using H 1743-322, 4U 1630-47, and GRS 1915+105 as reference sources.

astro-ph.HE↗

arXiv2025

X-ray linear polarization prediction in Black-Hole binaries and Active Galactic Nuclei and its measurements by IXPE

We present a theoretical framework for the formation of X-ray linear polarization in black hole (BH) source. The X-ray linear polarization originates from up-scatterings of initially soft photons within a hot, optically thick Compton cloud (CC) characterized by a flat geometry. The IXPE observations of several BHs X-ray binaries and of one Seyfert-1 galaxy confirm our theoretical prediction regarding values of the linear polarization P. The goal of this paper is to demonstrate that the main physical parameters of these Galactic and extragalactic sources can be derived without any free parameter using the polarization and X-ray spectral measurements. We estimate the CC optical depth, τ0 for all BHs observed by IXPE, using the plot of P vs μ = cos i, where i is an observer inclination with respect to the normal, and considering the values P for a given source. Using X ray spectral analysis, we obtain the photon index Γ and, analytically determined the CC plasma temperature kBTe. Different BHs (Cyg X 1, 4U 1630 47, LMC X 1, 4U 1957+115, Swift J1727.8 1613, GX 339 4) and the Seyfert 1 NGC 4151 exhibited polarization between 1 and 8 percent level nearly independently of energy. kBTe is in the range between 5 and 90 keV with a smaller value in High/Soft State with respect to Low/Hard State. We find a similarity between the physical parameter and the IXPE findings and we provide evidence suggesting that the CC exhibits a flat geometry.

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arXiv2024

Changing Look AGN: An X-ray Look

To date, a number of changing-look (CL) active galactic nuclei (AGNs) are known. We studied, in detail what happens to the X-ray spectrum during the CL events using the example of the nearby CL Seyfert NGC1566, which was observed by Swift, NuSTAR, XMM-Newton, and Suzaku. We applied the Comptonization model to describe an evolution of NGC~1566 X-ray spectra during outbursts and compared these results with a typical behavior for other AGNs to identify some differences and common properties that can ultimately help us to better understand the physics of the CL phenomenon. We found that changes in the X-ray properties of NGC1566 are characterized by a different combination of Sy1 (using 1H0707-495 as a representative) and Sy2 properties (using NGC7679 and Mrk3 as their representatives). At high X-ray luminosities NGC1566 exhibits the behavior typical for Sy1, and at low luminosities we see a transition of NGC1566 from the Sy1 behavior to the Sy2 pattern. We revealed the saturation of the spectral indices, ā for these four AGNs during outbursts (ā_1566~1.1, ā _0707~2, ā _7679~0.9 and ā_mrk3~0.9) and determined the masses of the black holes (BHs) in the centers of these AGNs namely, M_0707~6.8x10^7 M_sol, M_7679~8.4x10^6 M_sol, M_mrk3~2.2x10^8 M_sol and M_1566~2x10^5 M_sol, applying the scaling method. Our spectral analysis shows that the changing-look of NGC1566 from Sy1.2 to Sy1.9 in 2019 was accompanied by the transition of NGC1566 to an accretion regime which is typical for the intermediate and highly soft spectral states of other BHs. We also find that when going from Sy2 to Sy1, the spectrum of NGC1566 shows an increase in the soft excess accompanied by a decrease in the Comptonized fraction (0.1<f<0.5), which is consistent with the typical behavior of BH sources during X-ray outburst decay.

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arXiv2024

SDSS~J075217.84+193542.2: X-ray weighing of a secondary BH

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 $Γ\sim3$ 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 $H_α$-line measurements and we estimated the binary's inclination at SDSS~J0752, $i=80^{\circ}$, using a scaling technique.

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arXiv2023

Active galaxy nuclei: current state of the problem

This review presents the main points of current advances in the field of active galactic nuclei (AGN). A brief historical excursion about the search for the nature of AGN is given. The problem of close binary systems consisting of supermassive black holes located in the centers of galaxies is discussed in details. The main characteristics, as well as new methods for studying and ``weighing'' these new objects, are described. This paper is based on a presentation made in the astrophysical seminar, which dedicated to the memory of the outstanding astrophysicist N.G. Bochkarev (took place on May 19, 2023 at the Sternberg Astronomical Institute of Moscow State University).

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arXiv2023

How to distinguish white dwarf and neutron star X-ray binaries during their X-ray outbursts?

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 $Γ\to2$ and is characterized by quasi periodic oscillations (QPOs) at $ν_{QPO} >0.5$~Hz, although WD X-rays is stable with $Γ\to1.85$ and is accompanied by QPOs at $ν_{QPO}<0.05$~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.

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arXiv2023

OJ~287: a new BH mass estimate of the secondary

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.

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arXiv2022

MAXI~J1348--630: Estimating the black hole mass and binary inclination using a scaling technique

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, ν_{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.

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arXiv2020

Detection of a High-Temperature Blackbody Hump in Black Hole Spectra. The strongly redshifted annihilation line

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

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arXiv2019

Spectral index-mass accretion rate correlation and evaluation of black hole masses in AGNs 3C~454.3 and M87

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.

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arXiv2018

Scaling of X-ray spectral properties of a black hole in the Seyfert 1 galaxy NGC~7469

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.

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arXiv2018

NGC 4051: Black hole mass and photon index-mass accretion rate correlation

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.

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arXiv2017

Swift J164449.3+573451 and Swift J2058.4+0516: Black hole mass estimates for tidal disruption event sources

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.

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arXiv2017

BL Lacertae: X-ray spectral evolution and a black-hole mass estimate

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 $Γ$ saturation at ~ 2.2. This is robust observational evidence for the presence of a BH in BL Lac.

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arXiv2016

ESO 243-49 HLX-1: scaling of X-ray spectral properties and black hole mass determination

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.

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arXiv2016

X-Ray Spectra of The High-Mass X-RAY Binary 4U~1700-37 using BeppoSAX, Suzaku and RXTE Observations

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, $Γ_{1}\sim 2$ for all spectral states. However, $Γ_{2}$ shows a behavior depending on the spectral state. Namely, $Γ_{2}$ is quasi-constant at the level of $Γ_{2}\sim 2$ while the CC plasma temperature $kT^{(2)}_e$ is less than 40 keV; on the other hand, $Γ_{2}$ is in the range of $1.3<Γ_{2}<2$, when $kT^{(2)}_e$ is greater than 40 keV. We explain this quasi-stability of $Γ$ 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.

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arXiv2015

Scaling of the photon index vs mass accretion rate correlation and estimate of black hole mass in M101 ULX-1

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 (Γ) saturation level, Γ_{sat}=2.8 +/- 0.1, in the Γvs. mass accretion rate (\dot M) correlation. This Γ-\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 Γsaturation 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

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arXiv2015

BEPPOSAX and RXTE spectral study of the low-mass X-ray binary 4u~1705-44. Spectral hardening during the banana branch

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 1.3 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.

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