arXiv subjects

E. Churazov

Publications and source records attributed to E. Churazov.

At least 199 records · Page 11Linked to original sources

arXiv2001

Simulation of radio plasma in clusters of galaxies

We present three-dimensional hydrodynamical simulations of buoyant gas in a typical cluster environment. The hot matter was injected continuously into a small region off-set from the cluster centre. In agreement with previous analytic estimates we found that the bubbles evolve very differently depending on their luminosity. Using tracer particles we computed radio maps of the bubbles based on different assumptions about the magnetic field. In the radio band the bubbles closely resemble FRI sources. For the bubbles to be detectable for long enough to account for FRI sources, we found that reacceleration has to take place. The bubbles are generally difficult to detect, both, in the radio and in the X-ray band. Thus it is possible to hide a significant amount of energy in the form of bubbles in clusters. Finally, we compute the efficiency of the bubbles to stir the ICM and find that recurrent low-power sources may be fairly effective in mixing the inner cluster region.

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arXiv2001

Merging Binary Clusters

We study three prominent bi-modal X-ray clusters: A3528, A1750 and A3395. Using observations taken with ROSAT and ASCA, we analyze the temperature and surface brightness distributions. We also analyze the velocity distributions of the three clusters using new measurements supplemented with previously published data. We examine both the overall cluster properties, as well as the two sub-cluster elements in each. These results are then applied to the determination of the overall cluster masses, and demonstrate excellent consistency between the various methods used. While the characteristic parameters of the sub-clusters are typical of isolated objects, our temperature results for the regions between the two sub-clusters clearly confirm the presence of merger activity. These three clusters represent a progression of equal-sized sub-cluster mergers, starting from initial contact to immediately before first core passage.

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arXiv2001

New class of low frequency QPOs: signature of nuclear burning or accretion disk instabilities?

We report the discovery of a new class of low frequency quasi--periodic variations of the X-ray flux in the X-ray bursters 4U1608-52 and 4U1636-536. We also report an occasional detection of a similar QPO in Aql X-1. The QPOs, associated with flux variations at the level of percents, are observed at a frequency of 7-9 mHz. While usually the relative amplitude of flux variations increases with energy, the newly discovered QPOs are limited to the softest energies (1-5 keV). The observations of 4U1608-52 suggest that these QPOs are present only when the source X-ray luminosity is within a rather narrow range and they disappear after X-ray bursts. Approximately at the same level of the source luminosity, type I X-ray bursts cease to exist. Judging from this complex of properties, we speculate that a special mode of nuclear burning at the neutron star surface is responsible for the observed flux variations. Alternatively, some instabilities in the accretion disk may be responsible for these QPOs.

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arXiv2001

On the X-ray time lags in the black hole candidates

It is shown that the energy dependence of the time lags in Cygnus X-1 excludes any significant contribution of the standard reflected component to the observed lags. The conclusion is valid in the 0.1--10 Hz frequency range where time lags have been detected with high enough significance. In fact the data hint that reflected component is working in opposite direction, reducing the lags at energies where contribution of the reflected component is significant. We argue that the observed logarithmic dependence of time lags on energy can be understood as due to the small variations of the spectrum power law index in a very simple phenomenological model. We assume that an optically thin flow/corona, emitting a power law like spectrum, is present at a range of distances from the compact object. The slope of the locally emitted spectrum is a function of distance with the hardest spectrum emitted in the innermost region. If perturbations with different time scales are introduced to the accretion flow at different radii the observed X-ray lags naturally appear due to the inward propagation of perturbations on the diffusion time scales.

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arXiv2001

Evolution of Buoyant Bubbles in M87

The morphology of the X-ray and radio emitting features in the central $\sim$ 50 kpc region around the galaxy M87 strongly suggests that buoyant bubbles of cosmic rays (inflated by an earlier nuclear active phase of the galaxy) rise through the cooling gas at roughly half the sound speed. In the absence of strong surface tension, initially spherical bubbles will transform into tori as they rise through an external medium. Such structures can be identified in the radio images of the halo of M87. During their rise, bubbles will uplift relatively cool X-ray emitting gas from the central regions of the cooling flow to larger distances. This gas is colder than the ambient gas and has a higher volume emissivity. As a result, rising ``radio'' bubbles may be trailed by elongated X-ray features as indeed is observed in M87. We performed simple hydrodynamic simulations to qualitatively illustrate the evolution of buoyant bubbles in the M87 environment.

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arXiv2000

Resonant scattering of X-rays by the warm intergalactic medium

For the low density filamentary and sheet-like structures in the warm ($\sim 10^4$ to $\sim 10^6$ K) IGM predicted by numerical simulations the resonant line scattering of X-ray background (XRB) photons by He and H-like ions of heavy elements can exceed the ``local'' thermal emission by a factor of a few or more. Due to the conservative nature of scattering this resonantly scattered radiation can only be identified if a significant fraction of the XRB is resolved and removed. While the combined spectrum of the resolved sources will contain X-ray absorption features, the residual background will contain corresponding emission features with the same intensity. At the relevant densities and temperatures the lines of He and H-like oxygen at 0.57 and 0.65 keV are most promising. These lines (which have a typical width of $\sim$ 1-2 eV) may contain up to 50% of the total 0.5-1 keV emission of the filament. For a nearby ($z \la 0.1$) filament with a Thomson optical depth of $10^{-4}$ XMM should detect about 200 photons in the OVII line during a $10^5$ s exposure if the metallicity of the gas is as large as observed in galaxy clusters. On average up to a few percent of the soft XRB could be resonantly scattered by this phase of the IGM and resonantly scattered photons should account for a significant fraction of the truly diffuse background at low energies. Close to bright X-ray sources like galaxy clusters or AGN the flux of scattered radiation will be further enhanced. Off-line blazars are the most promising illuminating sources. The scattered emission from AGN may also constrain the duration of the active phase of these objects.

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arXiv2000

High frequencies in the power spectrum of Cyg X-1 in the hard and soft spectral states

We analyzed a large number of RXTE/PCA observations of Cyg X-1 in the hard and soft spectral states with total exposure time of ~190 and ~10 ksec respectively and time resolution better than ~250us in order to investigate its variability down to a few milliseconds time scales. The model of modifications of the power density spectra due to the dead time effect was tested using RXTE observations of an extremely bright source - Sco X-1. The results of these tests demonstrated, that although some problems still remain, for sources like Cyg X-1 (which is an order of magnitude less bright than Sco X-1) present knowledge of the instrument is sufficient to reliably analyze the power spectra in the kHz frequency range. In both spectral states of Cyg X-1 we detected statistically significant variability up to ~150-300 Hz with a fractional rms amplitude above 100 Hz at the level of ~2-3%. The power spectrum in the hard state shows a steepening at frequency of ~40-80 Hz with the power law slope changing from ~1.7 to ~2.3-2.4. In the soft state the slope of power spectrum changes from ~1 to ~2 at the frequency of ~15-20 Hz without any evidence of further steepening up to \~100-150 Hz. These break frequencies represent the highest characteristic frequencies detected in the power density spectrum of Cyg X-1 so far.

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arXiv2000

Soft state of Cygnus X-1: stable disk and unstable corona

Two component X-ray spectra (soft multicolor black body plus harder power law) are frequently observed from accreting black holes. These components are presumably associated with the different parts of the accretion flow (optically thick and optically thin respectively) in the vicinity of the compact source. Most of the aperiodic variability of the X-ray flux on the short time scales is associated with the harder component. We suggest that drastically different amplitudes of variability of these two components are simply related to the very different viscous time scales in the geometrically thin and geometrically thick parts of the accretion flow. In the geometrically thin disks variations of viscosity or mass accretion rate occurring at large radius from the black hole on the local dynamical or thermal time scales do not cause any significant variations of the mass accretion rate at smaller radii due to a very long diffusion time. Any variations on the time scales shorter than the diffusion time scale are effectively dampened. On the contrary such variations can easily survive in the geometrically thick flows and as a result the mass accretion rate in the innermost region of the flow will reflect modulations of the mass accretion rate added to the flow at any distance from the black hole. Therefore if primary instabilities operate on the short time scales then the stability of the soft component (originating from the geometrically thin and optically thick flow) and variability of the hard component (coming from the geometrically thick and optically thin flow) are naturally explained.

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arXiv2000

Comptonization, reflection and noise in black hole binaries

Based on the analysis of a large sample of RXTE/PCA observations of several black hole binaries in the low spectral state we show that a correlation exists between the spectral parameters and characteristic noise frequency. In particular, the amplitude of reflection increases and the slope of Comptonized radiation steepens as the noise frequency increases. We consider possible implications of these results on the accretion flow models and discuss a possible observational test aimed to discriminate between different geometries of the accretion flow.

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arXiv2000

Asymmetric, arc minute scale structures around NGC 1275

ROSAT HRI observations show complicated substructure in the X-ray surface brightness within $\sim$5 arcminutes around NGC 1275 -- the dominant galaxy of the Perseus cluster. The typical amplitude of the variations is of the order of 30% of the azimuthally averaged surface brightness at a given distance from NGC 1275. We argue that this substructure could be related to the activity of NGC 1275 in the past. Bubbles of relativistic plasma, inflated by jets, be forced to rise by buoyancy forces, mix with the ambient intracluster medium (ICM), and then spread. Overall evolution of the bubble may resemble the evolution of a hot bubble during a powerful atmospheric explosion. From a comparison of the time scale of the bubble inflation to the rise time of the bubbles and from the observed size of the radio lobes which displace the thermal gas, the energy release in the relativistic plasma by the active nucleus of NGC 1275 can be inferred. Approximate modeling implies a nuclear power output of the order of $10^{45}$ erg s$^{-1}$ averaged over the last $\sim 3~10^7$ years. This is comparable with the energy radiated in X-rays during the same epoch. Detailed measurements of the morphology of the X-ray structure, the temperature and abundance distributions with Chandra and XMM may test this hypothesis.

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arXiv2000

Frequency resolved spectroscopy of Cyg X-1: fast variability of the reflected emission in the soft state

Using the RXTE/PCA data we study the fast variability of the reflected emission in the soft spectral state of Cyg X-1 by means of Fourier frequency resolved spectroscopy. We find that the rms amplitude of variations of the reflected emission has the same frequency dependence as the primary radiation down to time scales of <30-50 msec. This might indicate that the reflected flux reproduces, with nearly flat response, variations of the primary emission. Such behavior differs notably from the hard spectral state, in which variations of the reflected flux are significantly suppressed in comparison with the primary emission, on time scales shorter than ~0.5-1 sec. If related to the finite light crossing time of the reflector, these results suggest that the characteristic size of the reflector -- presumably an optically thick accretion disk, in the hard spectral state is larger by a factor of >5-10 than in the soft spectral state. Modeling the transfer function of the disk, we estimate the inner radius of the accretion disk R_in~100R_g in the hard and R_in<10R_g in the soft state for a 10M_sun black hole.

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arXiv1999

RXTE broad band observations of X-ray Nova XTE J1755-324

The properties of X-ray Nova XTE J1755-324, observed with RXTE and GRANAT observatories in 1997 are reported. The lightcurve of the source was typical for X-ray Novae, except for somewhat shorter decay time scale and the time elapsed between primary and subsequent secondary and tertiary outbursts. At the peak of the lightcurve the source had two-component spectrum, typical for the bright X-ray Novae, with disk blackbody temperature Tin~0.8 keV and photon index of the hard power law tail α~ 2.0. The peak luminosity can be estimated as L~10^{38} erg/s (0.1-100 keV, assuming 8.5 kpc distance). A notable and peculiar feature of the spectral evolution of the source was a short, ~10 days long, episode of increased hardness, occurred shortly before the tertiary maximum of the light curve. During the last pointed observations ~100 days after the primary outburst the source has been found in the hard spectral state with luminosity ~fewX10^{36} erg/s. The overall pattern of the temporal and spectral evolution of XTE J1755-324 resemble in general that of ``canonical'' X-ray Novae (e.g. Nova Muscae 1991) and suggests that the compact object in the binary system is a black hole

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arXiv1999

The 1-12 Hz QPO and dips in GRS 1915+105: tracers of Keplerian and viscous time scales?

We analysed 9 RXTE/PCA observations of GRS 1915+105 in the flaring state, when the hardness of the source spectrum was changing on the time scales from few seconds up to 1000 seconds. The quasi periodic oscillations (QPOs) with the frequency varying between ~ 1 and 12 Hz are associated with the episodes of harder source spectrum. In each observation we found tight correlation between the duration of the hard episode and the characteristic QPO frequency. For a half of the observations this correlation matches the relation between the viscous time scale and the Keplerian frequency, when both quantities are evaluated for various radii in the radiation pressure dominated accretion disk. Assuming that the QPO frequency is proportional to the Keplerian frequency at the boundary between an optically thick accretion disk and a hot comptonization region, the changes of the QPO frequency can then be understood as due to variations of this boundary position on the viscous time scales.

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arXiv1999

Reflection and noise in Cygnus X-1

We analyzed RXTE/PCA observations of Cyg X-1 from 1996-1998. We found a tight correlation between the characteristic noise frequencies (e.g. the break frequency nu_br) and the spectral parameters in the low spectral state. The amplitude of reflection increases and the spectrum of primary radiation steepens as the noise frequency increases (nu_br changes by a factor of ~15 in our sample). This can be understood assuming that increase of the noise frequency is associated with the shift of the inner boundary of the optically thick accretion disk towards the compact object. The related increase of the solid angle, subtended by the disk, and of the influx of the soft photons to the Comptonization region lead to an increase of the amount of reflection and steepening of the Comptonized spectrum. The correlation between the slope of primary radiation and the amplitude of reflection extends to the soft spectral state. A similar correlation between reflection and slope was found for the frequency resolved spectra in the 0.01-15 Hz frequency range. Such a correlation could appear if the longer time scale variations are associated with emission originating closer to the optically thick disk and, therefore, having steeper Comptonized spectra with larger reflection.

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arXiv1999

Frequency resolved spectroscopy of Cyg X-1: fast variability of the Fe K_alpha line

We studied the frequency resolved energy spectra of Cyg X--1 during the standard low (hard) spectral state using the data of the Rossi X-Ray Timing Explorer. We found that the relative amplitude of the reflection features -- the iron fluorescent line at ~6.5 keV and the smeared edge above ~7 keV -- decreases with the increasing frequency. In particular we found that the equivalent width of the iron line decreases above ~1 Hz and drops twice at frequency of ~10 Hz. An assumption that such behavior is solely due to a finite light crossing time of the reflecting media, would imply the characteristic size of the reflector ~5E8 cm, corresponding to ~150 Rg for a 10Msun black hole. Alternatively lack of high frequency oscillations of the reflected component may indicate that the short time scale, ~50-100 msec, variations of the primary continuum appear in geometrically different, likely inner, part of the accretion flow and give a rise to a significantly weaker, if any, reflected emission than the longer time scale events.

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arXiv1999

Deep SIGMA observations of the central square degree of the Galaxy

The Center of the Milky Way emits radiation through all the electromagnetic spectrum due to the presence of several astrophysical phenomena. Various scientifical questions remain unsolved in the picture that emerge from observations, the main one being the presence and activity of a supermassive black hole at the Galactic Center. Also in the X-ray band a strong diffuse emission is detected up to 22 keV, it has been suggested that its origin is linked to past high energy activity of the Galactic Center. The French SIGMA soft gamma ray telescope on board the Russian Granat satellite observed the Galacti Center (GC) for abour 9 X 10^6 seconds from 1990 to the end of 1997. Its unique imaging capabilities coupled with the unprecedented exposure already allowed us to set preliminary constraints on the persistent high energy emission from Sgr A*. Here we present a new analysis of the complete set of observations of this region taking into account the possible presence of diffuse emission in the low energy channels. This allows us both to test current advection models of high energy emission from Sgr A* and to investigate on past high energy activity.

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arXiv1999

Compact and Diffuse Sources in the Galactic Center Region

The 10 by 10 degrees region around dynamic center of our Galaxy is known to host a large number of bright X-ray sources, most of which are low mass binary systems. While high luminosity Z-sources are especially bright in the standard X-ray band (e.g. 2-10 keV), lower luminosity systems are dominating the flux above 30 keV. The two hardest sources in the region, i.e. 1E1740.7-2942 and GRS 1758-258 are thought to be black hole candidates (BHC) based on the similarity of spectral properties and short-term variability to that of dynamically proven BHCs. However, while most of the other galactic BHCs are known to be transients, these two objects could be considered persistent though variable sources. The persistent behavior of these sources implies some constraints on the parameters of the binary systems. Both sources were detected in the first GRANAT observation of the GC region. They are the only two persistent BHCs seen in the 35-75 keV SIGMA image after collecting several million seconds of exposure time over eight years, indicating that they are perhaps the only two persistent BHCs in the region. Another peculiar object in the field is of course a putative supermassive black hole (Sgr A*) at the dynamic center of our Galaxy. The present day's X-ray luminosity from Sgr A* is very small ($\la 10^{36}$ erg/s). However an indication that Sgr A* might have been much brighter in the past was found while studying diffuse X-ray emission in the region. We speculate here on how future X-ray observatories may verify the hypothesis of the violent activity of Sgr A* in the past.

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arXiv1998

Restrictions on the parameters of boundary layer and accretion disk of X-ray bursters in the low state

Using ASCA observations of several X-ray bursters in the low spectral state Lx~1-5x10^{36} erg/s) restrictions on the parameters of the accretion disk and the boundary layer were obtained. The low state spectra of X-ray bursters observed with ASCA often can be well described by a power law with photon index of 1.7--2.2 and are not consistent with significant contribution of the soft spectral component to the total luminosity. For a blackbody spectrum with temperature of 0.2--2 keV the upper limits correspond to <10--20% of the total 0.5--10 keV luminosity. On the other hand, theoretical calculations predict that in the case of a Keplerian accretion disk around a slowly rotating neutron star (with radius of 3Rg) ~2/3 of the total gravitational energy released in the system can be released in the boundary layer between the inner part of the accretion disk and the neutron star surface. More accurate analysis of the ASCA spectra shows that: (1) The inner radius of the standard geometrically thin optically thick part of the accretion disk exceeds $R_{in} > 10-25 R_g$. (2) The characteristic temperature in the inner part of the accretion flow including the boundary layer exceeds $\sim$2 keV independently upon the assumption about the density in the accretion flow. The electron scattering gives the dominant contribution to the opacity in this region.

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