arXiv subjects

E. Churazov

Publications and source records attributed to E. Churazov.

At least 163 records · Page 9Linked to original sources

arXiv2006

Effect of turbulent diffusion on iron abundance profiles

We compare the observed peaked iron abundance profiles for a small sample of groups and clusters with the predictions of a simple model involving the metal ejection from the brightest galaxy and the subsequent diffusion of metals by stochastic gas motions. Extending the analysis of Rebusco et al. (2005) we found that for 5 out of 8 objects in the sample an effective diffusion coefficient of the order of $10^{29}$ cm$^{2}$ s$^{-1}$ is needed. For AWM4, Centaurus and AWM7 the results are different suggesting substantial intermittence in the process of metal spreading across the cluster. There is no obvious dependence of the diffusion coefficient on the mass of the system. We also estimated the characteristic velocities and the spatial scales of the gas motions needed to balance the cooling losses by the dissipation of the same gas motions. A comparison of the derived spatial scales and the sizes of observed radio bubbles inflated in the ICM by a central active galactic nucleus (AGN) suggests that the AGN/ICM interaction makes an important (if not a dominant) contribution to the gas motions in the cluster cores.

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arXiv2006

Constraints on the late X-ray emission from the low-energy GRB 031203: INTEGRAL data

Comparison of the INTEGRAL upper limits on the hard X-ray flux before and after the low-energy GRB 031203 with the XMM measurements of the dust-scattered radiation at lower energies suggests that a significant fraction of the total burst energy could be released in the form of soft X-rays at an early afterglow stage with a characteristic duration of ~100-1000s. The overall time evolution of the GRB 031203 afterglow may have not differed qualitatively from the behavior of standard (i.e., more intense) bursts studied by the SWIFT observatory. The available data also admit the possibility that the dust-scattered radiation was associated with an additional soft component in the spectrum of the gamma-ray burst itself.

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arXiv2006

Hard X-ray emission from the Galactic ridge

We present results of a study of the Galactic ridge X-ray emission (GRXE) in hard X-rays performed with the IBIS telescope aboard INTEGRAL. The imaging capabilities of this coding aperture telescope make it possible to account for the flux from bright Galactic point sources whereas the wide field of view permits to collect large flux from the underlying GRXE. Extensive study of the IBIS/ISGRI detector background allowed us to construct a model that predicts the detector count rate with $\sim1-2$% accuracy in the energy band 17-60 keV. The derived longitude and latitude profiles of the ridge emission are in good agreement with the Galactic distribution of stars obtained from infrared observations. This, along with the measured hard X-ray spectrum of the Galactic ridge emission strongly indicates its stellar origin. The derived unit stellar mass emissivity of the ridge in the energy band 17-60 keV, $(0.9 - 1.2)\times 10^{27}$\lummass (assuming a bulge mass of $1-1.3 \times 10^{10} M_\odot$) agrees with that of local (in the Solar neigborhood) accreting magnetic white dwarf binaries - dominant contributors to the GRXE at these energies. In addition, the shape of the obtained GRXE spectrum can be used to determine the average mass of white dwarfs in such systems in the Galaxy as $\sim0.5 M_{\sun}$. The total hard X-ray luminosity of the GRXE is $L_{\rm 17-60 keV} =(3.7\pm0.2)\times10^{37}$\lum in the 17--60 keV band. At energies 70--200 keV no additional contribution to the total emission of the Galaxy apart from the detected point sources is seen.

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arXiv2006

Identification of 8 INTEGRAL hard X-ray sources with Chandra

We report the results of identification of 8 hard X-ray sources discovered by the INTEGRAL observatory during the ongoing all-sky survey. These sources have been observed by Chandra. In 6 cases a bright X-ray source was found within the INTEGRAL localization region, which permitted to unambigously identify 5 of the objects with nearby galaxies, implying that they have an active galactic nucleus (AGN), whereas one source is likely an X-ray binary in LMC. 4 of the 5 newly discovered AGNs have measured redshifts in the range 0.025-0.055. The X-ray spectra reveal the presence of significant amounts of absorbing gas (NH in the range 10^22-10^24 cm^-2) in all 5 AGNs, demonstrating that INTEGRAL is starting to fill in the sample of nearby obscured AGNs.

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arXiv2005

X-ray luminosity function of faint point sources in the Milky Way

We assess the contribution to the X-ray (above 2 keV) luminosity of the Milky Way from different classes of low-mass binary systems and single stars. We begin by using the RXTE Slew Survey of the sky at |b|>10 to construct an X-ray luminosity function (XLF) of nearby X-ray sources in the range 10^30<Lx(erg/s)<10^34 (where Lx is the luminosity over 2-10 keV), occupied by coronally active binaries (ABs) and cataclysmic variables (CVs). We then extend this XLF down to Lx~10^27.5 erg/s using the Rosat All-Sky Survey in soft X-rays and available information on the 0.1-10 keV spectra of typical sources. We find that the local cumulative X-ray (2-10 keV) emissivities (per unit stellar mass) of ABs and CVs are (2.0+/-0.8)x10^27 and (1.1+/-0.3)x10^27 erg/s/Msol, respectively. In addition to ABs and CVs, representing old stellar populations, young stars emit locally (1.5+/-0.4)x10^27 erg/s/Msol. We finally attach to the XLF of ABs and CVs a high luminosity branch (up to ~10^39 erg/s) composed of neutron-star and black-hole low-mass X-ray binaries (LMXBs), derived in previous work. The combined XLF covers ~12 orders of magnitude in luminosity. The estimated combined contribution of ABs and CVs to the 2-10 keV luminosity of the Milky Way is ~2x10^38 erg/s, or ~3% of the integral luminosity of LMXBs (averaged over nearby galaxies). The XLF obtained in this work is used elsewhere (Revnivtsev et al.) to assess the contribution of point sources to the Galactic ridge X-ray emission.

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arXiv2005

Identification of four RXTE Slew Survey sources with nearby luminous active galactic nuclei

Based on RXTE scans and observations with the SWIFT/XRT telescope and INTEGRAL observatory, we report the identification of four X-ray sources discovered during the RXTE Slew Survey of the |b|>10deg sky with nearby (z ~ 0.017-0.098) luminous (log L_2-10keV ~ 42.7-44 erg/s) active galactic nuclei. Two of the objects exhibit heavily intrinsically absorbed X-ray spectra (NHL~10^23 cm^-2).

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arXiv2005

Origin of the Galactic ridge X-ray emission

We analyze a map of the Galactic ridge X-ray emission (GRXE) constructed in the 3-20 keV energy band from RXTE/PCA scan and slew observations. We show that the GRXE intensity closely follows the Galactic near-infrared surface brightness and thus traces the Galactic stellar mass distribution. The GRXE consists of two spatial components which can be identified with the bulge/bar and the disk of the Galaxy. The parameters of these components determined from X-ray data are compatible with those derived from near-infrared data. The inferred ratio of X-ray to near-infrared surface brightness I(3-20 keV) (1e-11 erg/s/cm2/deg2)/I_(3.5micron)(MJy/sr)=0.26+/-0.05, and the ratio of X-ray to near-infrared luminosity L_(3-20 keV)/L_(3-4 micron)=(4.1+/-0.3)e-5. The corresponding ratio of the 3-20 keV luminosity to the stellar mass is L_x/M_Sun= (3.5\pm0.5) 10^{27} erg/s, which agrees within the uncertainties with the cumulative emissivity per unit stellar mass of point X-ray sources in the Solar neighborhood, determined in an accompanying paper (Sazonov et al.). This suggests that the bulk of the GRXE is composed of weak X-ray sources, mostly cataclysmic variables and coronally active binaries. The fractional contributions of these classes of sources to the total X-ray emissivity determined from the Solar neighborhood data can also explain the GRXE energy spectrum. Based on the luminosity function of local X-ray sources we predict that in order to resolve 90% of the GRXE into discrete sources a sensitivity limit of ~10^{-16} erg/s/cm2 (2--10 keV) will need to be reached in future observations.

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arXiv2005

Heating the bubbly gas of galaxy clusters with weak shocks and sound waves

Using hydrodynamic simulations and a technique to extract the rotational component of the velocity field, we show how bubbles of relativistic gas inflated by AGN jets in galaxy clusters act as a catalyst, transforming the energy carried by sound and shock waves to heat. The energy is stored in a vortex field around the bubbles which can subsequently be dissipated. The efficiency of this process is set mainly by the fraction of the cluster volume filled by (sub-)kpc scale filaments and bubbles of relativistic plasma.

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arXiv2005

Supermassive Black Holes in Elliptical Galaxies: Switching from Very Bright to Very Dim

Relativistic outflows (mainly observed in the radio) are a characteristic feature of both Galactic stellar mass black holes and supermassive black holes (SMBHs). Simultaneous radio and X-ray observations of Galactic sources have shown that the outflow is strong at low accretion rates, but it weakens dramatically or disappears completely at high accretion rates, manifesting structural changes in the accretion flow. It is reasonable to assume that SMBHs follow the same trend. For low luminosity SMBHs in nearby elliptical galaxies and clusters, recent observations strongly suggest that the outflows play the central role in keeping the gas hot (mechanical feedback). If the outflow is quenched in SMBHs at high accretion rates similarly to the behavior of galactic sources, then the straightforward consequence is a relatively weak feedback of rapidly accreting SMBHs. We argue that elliptical galaxies and their central engines should then evolve through two stages. Early on, the central SMBH rapidly grows by accreting cooling gas at a near-Eddington rate with high radiative efficiency but with weak feedback on the infalling gas. This stage terminates when the black hole has grown to a sufficiently large mass that its feedback (radiative and/or mechanical), despite the low gas heating efficiency, is able to suppress gas cooling. After that the system switches to a stable state corresponding to passively evolving ellipticals, when the accretion rate and radiative efficiency are very low, but the gas heating efficiency is high and energy input from the relativistic outflow keeps the gas hot.

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arXiv2005

Metal Abundances in the ICM as a Diagnostics of the Cluster History

Galaxy clusters with a dense cooling core exhibit a central increase in the metallicity of the intracluster medium. Recent XMM-Newton studies with detailed results on the relative abundances of several heavy elements show that the high central abundances are mostly due to the contribution from supernovae type Ia. The dominant source is the stellar population of the central cluster galaxy. With this identification of the origin of heavy elements and the observed rates of SN Ia in elliptical galaxies, the central abundance peak can be used as a diagnostic for the history of the cluster core region. We find for four nearby cooling core clusters that the enrichment times for the central peaks are larger than 6 - 10 Gyrs even for a higher SN Ia rate in the past. This points to an old age and a relatively quiet history of these cluster core regions. A detailed analysis of the element abundance ratios provides evidence that the SN Ia yields in the central cluster galaxies are more rich in intermediate mass elements, like Si and S, compared to the SN Ia models used to explain the heavy element enrichment in our Galaxy.

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arXiv2005

Impact of stochastic gas motions on galaxy cluster abundance profiles

The impact of stochastic gas motions on the metal distribution in cluster core is evaluated. Peaked abundance profiles are a characteristic feature of clusters with cool cores and abundance peaks are likely associated with the brightest cluster galaxies (BCGs) which dwell in cluster cores. The width of the abundance peaks is however significantly broader than the BCG light distribution, suggesting that some gas motions are transporting metals originating from within the BCG. Assuming that this process can be treated as diffusive and using the brightest X-ray cluster A426 (Perseus) as an example, we estimate that a diffusion coefficient of the order of $2 10^{29} {\rm cm^2 s^{-1}}$ is needed to explain the width of the observed abundance profiles. Much lower (higher) diffusion coefficients would result in too peaked (too shallow) profiles. Such diffusion could be produced by stochastic gas motions and our analysis provides constraints on the product of their characteristic velocity and their spatial coherence scale. We speculate that the activity of the supermassive black hole of the BCG is driving the stochastic gas motions in cluster cores. When combined with the assumption that the dissipation of the same motions is a key gas heating mechanism, one can estimate both the velocity and the spatial scale of such a diffusive processes.

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arXiv2005

Extragalactic source counts in the 20-50 keV energy band from the deep observation of the Coma region by INTEGRAL/IBIS

We present the analysis of serendipitous sources in a deep, 500 ksec, hard X-ray observation of the Coma cluster region with the IBIS instrument onboard INTEGRAL. In addition to the Coma cluster, the final 20-50 keV image contains 12 serendipitous sources with statistical significance >4sigma. We use these data (after correcting for expected number of false detections) to extend the extragalactic source counts in the 20-50 keV energy band down to a limiting flux of 1.0e-11 erg/s/cm^2 (~ 1mCrab). This is a more than a factor of 10 improvement in sensitivity compared to the previous results in this energy band obtained with the HEAO-1 A4 instrument. The derived source counts are consistent with the Euclidean relation, N(>f) ~ f**(-3/2). A large fraction of identified serendipitous sources are low-redshift, z<0.02, AGNs, mostly of Seyfert 1 type. The surface density of hard X-ray sources is 0.014 +- 0.005 per square degree above a flux threshold of 1e-11 erg/s/cm^2. These sources directly account for ~3% of the cosmic X-ray background in the 20-50 keV energy band. Given the low redshift depth of our sample, we expect that similar sources at higher redshifts account for a significant fraction of the hard X-ray background. Our field covers only 3% of the sky; a systematic analysis of other extragalactic INTEGRAL observations can produce much larger source samples and is, therefore, critically important.

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arXiv2004

Positron annihilation spectrum from the Galactic Center region observed by SPI/INTEGRAL

The electron-positron annihilation spectrum observed by SPI/INTEGRAL during deep Galactic Center region exposure is reported. The line energy (510.954$\pm$0.075 keV) is consistent with the unshifted annihilation line. The width of the annihilation line is 2.37$\pm$0.25 keV (FWHM), while the strength of the ortho-positronium continuum suggests that the dominant fraction of positrons (94$\pm$6%) form positronium before annihilation. Compared to the previous missions these deep INTEGRAL observations provide the most stringent constraints on the line energy and width. Under the assumption of an annihilation in a single-phase medium these spectral parameters can be explained by a warm $T_e\sim 7000-4~10^4$ K gas with the degree of ionization larger than a few $10^{-2}$. One of the wide-spread ISM phases - warm ($T_e \sim 8000$ K) and weakly ionized (degree of ionization $\sim$ 0.1) medium satisfies these criteria. Other single-phase solutions are also formally allowed by the data (e.g. cold, but substantially ionized ISM), but such solutions are believed to be astrophysically unimportant. The observed spectrum can also be explained by the annihilation in a multi-phase ISM. The fraction of positrons annihilating in a very hot ($T_e \ge 10^6$ K) phase is constrained to be less than $\sim$8%. Neither a moderately hot ($T_e \ge 10^5$ K) ionized medium nor a very cold ($T_e \le 10^3$ K) neutral medium can make a dominant contribution to the observed annihilation spectrum. However, a combination of cold/neutral, warm/neutral and warm/ionized phases in comparable proportions could also be consistent with the data.

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arXiv2004

Stability of cold fronts in clusters: is magnetic field necessary?

Cold fronts - sharp discontinuities recently discovered by Chandra in many clusters of galaxies - are believed to be due to a hot gas flow over a colder gravitationally bound gas cloud. We analyze the stability of the fronts with respect to Kelvin-Helmholtz instability and show that an intrinsic width of the interface of the order of a few per cent of the curvature radius strongly limits the growth of perturbation. For the best studied case of a front in the Cluster Abell 3667 we conclude that current observational data on the width and extent of the front can be explained even in the absence of dynamically important magnetic fields.

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arXiv2004

Implications of the central metal abundance peak in cooling core clusters of galaxies

Recent XMM-Newton observations of clusters of galaxies have provided detailed information on the distribution of heavy elements in the central regions of clusters with cooling cores providing strong evidence that most of these metals come from recent SN type Ia. In this paper we compile information on the cumulative mass profiles of iron, the most important metallicity tracer. We find that long enrichment times (larger than 5 Gyr) are necessary to produce the central abundance peaks. Classical cooling flows, a strongly convective intracluster medium, and a complete metal mixing by cluster mergers would destroy the observed abundance peaks too rapidly. Thus the observations set strong constraints on cluster evolution models requiring that the cooling cores in clusters are preserved over very long times. We further conclude from the observations that the innermost part of the intracluster medium is most probably dominated by gas originating predominantly from stellar mass loss of the cD galaxy.

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arXiv2004

A Hard X-ray survey of the Galactic Center with INTEGRAL/IBIS. Catalog of sources

During the period Aug.23-Sept.24 2003, the INTEGRAL observatory performed an ultra deep survey of the Galactic Center region with a record sensitivity at energies higher than 20 keV. We have analized images of the Galactic Center region obtained with the ISGRI detector of the IBIS telescope (15-200 keV) and present here a catalog of detected sources. In total, 60 sources with a flux higher than 1.5 mCrab have been detected. 44 of them were earlier identified as Galactic binary systems, 3 are extragalactic objects. 2 new sources are discovered.

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arXiv2004

Reflections of AGN Outbursts in the Gaseous Atmosphere of M87

We combined deep Chandra, ROSAT HRI, and XMM-Newton observations of M87 to study the impact of AGN outbursts on its gaseous atmosphere. Many X-ray features appear to be a direct result of repetitive AGN outbursts. In particular, the X-ray cavities around the jet and counter jet are likely due to the expansion of radio plasma, while rings of enhanced emission at 14 and 17 kpc are probably shock fronts associated with outbursts that began 1-2\times10^7 years ago. The effects of these shocks are also seen in brightenings within the prominent X-ray arms. On larger scales, ~50 kpc from the nucleus, depressions in the surface brightness may be remnants of earlier outbursts. As suggested for the Perseus cluster (Fabian et al.), our analysis of the energetics of the M87 outbursts argues that shocks may be the most significant channel for AGN energy input into the cooling flow atmospheres of galaxies, groups, and clusters. For M87, the mean power driving the shock outburst, 2.4\times 10^{43} ergs/sec, is three times greater than the radiative losses from the entire ``cooling flow''. Thus, even in the absence of other energy inputs, outbursts every 3\times10^7 years are sufficient to quench the flow.

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arXiv2003

Doppler broadening of the iron line and R-Gamma correlation in black hole binaries

RXTE/PCA observations of several black hole X-ray binaries in the low spectral state revealed a tight correlation between spectral parameters and characteristic frequencies of variability of X-ray flux. In particular, the amplitude of reflection increases and the slope of Comptonized radiation steepens as the noise frequencies increase. The data also suggest that there is a correlated increase of the width of the Fe fluorescent line, probably related to the Doppler broadening of the reflection features. Moreover, the width of the line seems to follow DeltaE/E ~ nu_qpo^{1/3} law. If confirmed with higher energy resolution observations, this result will have significant impact on the models of the accretion flow and on our understanding of the nature of characteristic frequencies of variability in X-ray binaries. In particular, it will lend support to the truncated disk picture, confirming that the spectral variations are indeed related to changes of the position of the inner disk boundary and that characteristic variability frequencies are proportional to the Keplerian frequency at the inner boundary of the accretion disk.

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