Elena Seifina

INTEGRAL observations of SS433: Results of a coordinated campaign

Results of simultaneous INTEGRAL and optical observations of the galactic microquasar SS433 in May 2003 and INTEGRAL /RXTE observations in March 2004 are presented. Persistent precessional variability with a maximum to minimum uneclipsed hard X-ray flux ratio of ∼ 4 is discovered. The 18-60 keV X-ray eclipse is found to be in phase with optical and near infrared eclipses. The orbital eclipse observed by INTEGRAL in May 2003 is at least two times deeper and apparently wider than in the soft X-ray band. The broadband 2-100 keV X-ray spectrum simultaneously detected by RXTE/INTEGRAL in March 2004 can be explained by bremsstrahlung emission from optically thin thermal plasma with kT ∼ 30 keV. Optical spectroscopy with the 6-m SAO BTA telescope confirmed the optical companion to be an A5-A7 supergiant. For the first time, spectorscopic indications of a strong heating effect in the optical star atmosphere are found. The measurements of absorption lines which are presumably formed on the non-illuminated side of the supergiant yield its radial velocity semi-amplitude Kv = 132 ±9 km/s. The analysis of the observed hard X-ray light curve and the eclipse duration, combined with the spectroscopically determined optical star radial velocity corrected for the strong heating effect, allows us to model SS433 as a massive X-ray binary. Assuming that the hard X-ray source in SS433 is eclipsed by the donor star that exactly fills its Roche lobe, the masses of the optical and compact components in SS433 are suggested to be Mv ≈ 30M⊙ and Mx ≈ 9M⊙, respectively. This provides further evidence that SS433 is a massive binary system with supercritical accretion onto a black hole.

DISCOVERY OF PHOTON INDEX SATURATION IN THE BLACK HOLE BINARY GRS 1915+105

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 the Rossi X-ray Timing Explorer, coordinated with Ryle Radio Telescope observations. We show that broadband 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 Γ1 = 1.7–3.0) with turnover at high energies and soft thermal component (BMC2, Γ2 = 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 the BH and that of soft component is due to matter accumulation in the transition layer when mass accretion rate increases. Furthermore, we demonstrate a strong correlation between equivalent width of the iron line and radio flux in GRS 1915+105. In addition to our spectral model components we also find a strong feature of “blackbody (BB)-like” bump whose color temperature is about 4.5 keV in eight observations of the intermediate and soft states. We discuss a possible origin of this “BB-like” emission.

ON THE CONSTANCY OF THE PHOTON INDEX OF X-RAY SPECTRA OF 4U 1728-34 THROUGH ALL SPECTRAL STATES

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 high-luminosity states when the electron temperature kTe of the Compton cloud monotonically decreases from 15 to 2.5 keV. We analyze the transition episodes from this source observed with Beppo SAX and RXTE satellites. We find that the X-ray broadband energy spectra of 4U 1728-34 during all spectral states can be modeled by a combination of a thermal (blackbody-like) component, a Comptonized component (which we herein denote as COMPTB), and a Gaussian component. Spectral analysis using this model provides evidence that the photon power-law index Γ is almost constant (Γ = 1.99 ± 0.02) when kTe changes from 15 to 2.5 keV during these spectral transitions. We explain this quasi-stability of the index Γ 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. Moreover, this index stability effect now established for 4U 1728-34 during spectral evolution of the source was previously suggested for a number of other neutron binaries. This intrinsic property of the 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 the high state and saturates at high values of the mass accretion rate.

Diagnostics of SS433 with the RXTE

We present our analysis of the extensive monitoring of SS433 by the RXTE observatory collected over the period 1996–2005. The difference between energy spectra taken at different precessional and orbital phases shows the presence of strong photoabsorption (NH > 10 23 cm −2 ) near the optical star, probably due to its powerful, dense wind. Therefore the size of the secondary deduced from analysis of X-ray orbital eclipses might be significantly larger than its Roche lobe size, which must be taken into account when evaluating the mass ratio from analysis of X-ray eclipses. Assuming that a precessing accretion disk is geometrically thick, we recover the temperature profile in the X-ray emitting jet that best fits the observed precessional variations in the X-ray emission temperature. The hottest visible part of the X-ray jet is located at a distance of l0/a ∼ 0.06−0.09, or ∼2−3 × 10 11 cm from the central compact object, and has a temperature of about Tmax ∼ 30 keV. We discovered appreciable orbital X-ray eclipses at the “crossover” precessional phases (jets are in the plane of the sky, disk is edge-on), which under model assumptions put a lower limit on the size of the optical component R/a > 0.5 and an upper limit on a mass ratio of binary companions q = Mx/Mopt < 0.3−0.35, if the X-ray opaque size of the star is not larger than 1.2RRoche,secondary.

Black Hole Mass Determination In the X-Ray Binary 4U 1630-47: Scaling of Spectral and Variability Characteristics

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 Γ with low-frequency quasi-periodic oscillations (QPOs), νL, and mass accretion rate, ˙ M, can be applied to the black hole mass and the inclination angle estimates. We analyze the transition episodes observed with the Rossi X-Ray Timing Explorer 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 Γ 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 Γ versus νL also show saturation at Γ ∼ 3. Γ– ˙ M and Γ–νL correlations with their index saturation revealed in 4U 1630–47 are similar to those established in a number of other black hole candidates 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 H1743–322 as reference sources, allows us to evaluate a black hole mass in 4U 1630–47 yielding MBH ∼ 10 ± 0.1 solar masses and to constrain the inclination angle of i 70 ◦ .

INTEGRAL observations of SS433, a supercritically accreting microquasar with hard spectrum ?

Observations of SS433 by INTEGRAL carried out in March-May 2003 are presented. SS433 is evidently detected on the INTEGRAL images of the corresponding sky region in the energy bands 25-50 and 50-100 keV. The precessional variability of the hard X-ray flux is clearly seen. The X-ray eclipse caused by the binary orbital motion is also detected. A possible origin of the hard continuum is briefly discussed.

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} 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.

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

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

T_{s1}

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

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