arXiv subjects

Elena Seifina

Publications and source records attributed to Elena Seifina.

28 records · Page 2Linked to original sources

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

astro-ph.HE↗

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.

astro-ph.SR↗

arXiv2014

X-ray spectral and timing behavior of Scorpius X-1. Spectral hardening during the flaring branch

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<Gamma_1<2, when source traces the FB. We also detect a decrease kT_{s2} from 1.8 keV to 0.7 keV during the FB. We interpret this apparent quasi-stability of the indices during the HB-NB in the framework of the model in which the spectrum is determined by the Comptonized thermal components. This established for the Comptonized spectral components of the Z-source Sco~X-1 is similar to that was previously found in the atoll sources 4U~1728-34, GX~3+1 and 4U~1820-30 and the Z-source GX~340+0 through all spectral states. However we interpret the index reduction phase detected during the FB in Sco~X-1 within the framework of a model in which the spectrum at the FB is determined by high radiation pressure from the NS surface.

astro-ph.SR↗

arXiv2014

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 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.

astro-ph.SR↗

arXiv2013

Spectral state evolution of 4U~1820-30: the stability of the spectral index of Comptonization tail

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.

astro-ph.HE↗

arXiv2013

Stability of the photon indices in Z-source GX 340+0 for spectral states

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.

astro-ph.HE↗

arXiv2012

GX~3+1: the stability of spectral index as a function of mass accretion rate

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.

astro-ph.GA↗

arXiv2011

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 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.

astro-ph.GA↗

arXiv2010

On the Nature of the Compact Object in SS~433. Observational Evidence of X-ray Photon Index Saturation

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.

astro-ph.GA↗

arXiv2009

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 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.

astro-ph.HE↗