arXiv subjects

R. Sunyaev

Publications and source records attributed to R. Sunyaev.

At least 127 records · Page 7Linked to original sources

arXiv2003

IGR J16318-4848: an X-ray source in a dense envelope?

The hard X-ray source IGR J16318-4848 was recently discovered by the INTEGRAL observatory (Courvoisier et al.) and subsequently uncovered in archival data of ASCA observations in 1994 (Murakami et al.). We present results of a detailed analysis of the ASCA data. The spectrum of the source in the 0.5--10 keV band is extraordinarily hard and is virtually unobservable below 4 keV because of strong photoabsorption NHL L>4e23 cm^-2. The 4--10 keV emission is dominated by a Kalpha line of neutral or weakly ionized iron with an equivalent width of \~2.5 keV. There is also an indication for a second line at ~7 keV. Our analysis of archival observations of the IGR J16318-4848 infrared counterpart, discovered by Foschini et al., shows that the point source is detected at different wavelengths in the 1--15 micron range. The available data suggest that IGR J16318-4848 is an X-ray binary system enshrouded by a dense envelope. It is possible that the source is a wind-fed high-mass X-ray binary similar to GX 301-2. We argue that IGR J16318-4848 might be the first representative of a previously unknown population of highly absorbed galactic X-ray sources, which remained undetected with X-ray missions before INTEGRAL.

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arXiv2002

Observing scattered X-ray radiation from gamma-ray bursts: a way to measure their collimation angles

There are observational facts and theoretical arguments for an origin of gamma-ray bursts in molecular clouds in distant galaxies. If this is true, one could detect a significant flux of GRB prompt and early afterglow X-ray radiation scattered into our line of sight by the molecular and atomic matter located within tens of parsecs of the GRB site long after the afterglow has faded away. The scattered flux directly measures the typical density of the GRB ambient medium. Furthemore, if the primary emission is beamed, the scattered X-ray flux will be slowly decreasing for several months to years before falling off rapidly. Therefore, it should be possible to estimate the collimation angle of a burst from the light curve of its X-ray echo and a measured value of the line-of-sight absorption column depth. It is shown that detection of such an echo is for the brightest GRBs just within the reach of the Chandra and XMM-Newton observatories.

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arXiv2002

High Mass X-ray Binaries as a Star Formation Rate Indicator in Distant Galaxies

Based on CHANDRA observations of nearby starburst galaxies and RXTE/ASM, ASCA and MIR-KVANT/TTM studies of high mass X-ray binary (HMXB) populations in the Milky Way and Magellanic Clouds, we propose that the number and/or the collective X-ray luminosity of HMXBs can be used to measure the star formation rate (SFR) of a galaxy. We show that, within the accuracy of the presently available data, a linear relation between HMXB number and the star formation rate exists. The relation between SFR and collective luminosity of HMXBs is non-linear in the low SFR regime, $L_X\propto \SFR^{\approx 1.7}$, and becomes linear only for sufficiently high star formation rate, when the total number of HMXB sources becomes sufficiently large. Such behaviour is caused by the fact, that we measure collective luminosity of a population of the discrete sources. Although more subtle SFR dependent effects are likely to exist, the data are broadly consistent with the existence of a universal luminosity function of HMXBs which can be roughly described as a power law with a differential slope of $\sim 1.6$, a cutoff at $L_X \sim few \times 10^{40}$ erg/sec and a normalisation proportional to the star formation rate. We apply our results to (spatially unresolved) starburst galaxies observed by CHANDRA at redshifts up to $z\sim 1$ in the Hubble Deep Field North and show that the calibration of the collective luminosity of HMXBs as a SFR indicator based on the local sample agrees well with the SFR estimates obtained for these distant galaxies with conventional methods.

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arXiv2002

Constraining the past X-ray luminosity of AGN in clusters of galaxies: the role of resonant scattering

Only a small fraction of galactic nuclei in the nearby universe are luminous; most of them are currently dim. We investigate the feasibility of constraining the X-ray luminosity in the recent past (up to ~10^6 years ago) of the nucleus of a cluster dominant galaxy by measuring the contribution of scattered radiation from the central source to the surface brightness of the intracluster gas dominated by thermal emission. We show that resonance X-ray lines present an advantage over the adjacent continuum, because the relative contribution of the scattered component is typically larger in the line case by a significant factor of 3-10. As an example, we estimate the level of constraint that could be derived from future fine spectroscopic observations on the past X-ray luminosity of the nearby M87 and Cygnus A active galaxies. For instance, scattered line radiation should be detectable from the Virgo cluster if the X-ray luminosity of M87 was a few times 10^44 erg s^-1, or 10^-3 of its Eddington luminosity, until a few times 10^5 years ago. For comparison, upper limits inferred from the available XMM-Newton and Chandra X-ray continuum data are typically 1-2 orders of magnitude weaker. The same method can be applied to distant powerful quasars (at redshifts z~1 if they have cluster-like gaseous coronae, as suggested by Rosat and Chandra observations of active galaxies at z<1. Their surface brightness profiles in the X-ray continuum above ~10 keV>>kT/(1+z) (where T is the gas temperature) should be dominated by redshifted scattered radiation from the quasar. Therefore, measurements with forthcoming mirror-optics hard X-ray telescopes could give information on the lifetime of quasars and parameters of the hot gas around them.

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arXiv2002

The Milky Way in X-rays for an outside observer: Log(N)-Log(S) and Luminosity Function of X-ray binaries from RXTE/ASM data

We study the Log(N)-Log(S) and X-ray luminosity function in the 2-10 keV energy band, and the spatial (3-D) distribution of bright, log(L_X) > 34-35 erg/s, X-ray binaries in the Milky Way. In agreement with theoretical expectations and earlier results we found significant differences between the spatial distributions of low (LMXB) and high (HMXB) mass X-ray binaries. The volume density of LMXB sources peaks strongly at the Galactic Bulge. HMXBs tend to avoid the inner 3-4 kpc of the Galaxy, HMXBs are more concentrated towards the Galactic Plane and show clear signatures of the spiral structure in their spatial distribution. LMXB sources have a flatter Log(N)-Log(S) distribution and luminosity function than HMXBs. The integrated 2-10 keV luminosities of X-ray binaries, averaged over 1996--2000, are 2-3 * 10^39 (LMXB) and 2-3 * 10^38 (HMXB) erg/s. Normalised to the stellar mass and the star formation rate, respectively, these correspond to 5 * 10^28 erg/s/M_sol for LMXBs and 5 * 10^37 erg/s/(M_sol/yr) for HMXBs. Due to the shallow slopes of the luminosity functions the integrated emission of X-ray binaries is dominated by the 5-10 most luminous sources which determine the appearance of the Milky Way in the standard X-ray band for an outside observer. In particular variability of individual sources or an outburst of a bright transient source can increase the integrated luminosity of the Milky Way by as much as a factor of ~2. Although the average LMXB luminosity function shows a break near the Eddington luminosity for a 1.4 M_sol neutron star, at least 11 sources showed episodes of super-Eddington luminosity during ASM observations. We provide the maps of distribution of X-ray binaries in the Milky Way in various projections, which can be compared to images of nearby galaxies taken by CHANDRA and XMM-Newton.

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arXiv2002

Super-Eddington outburst of V4641 Sgr

X-ray transients provide unique opportunity to probe accretion regimes of at a vastly different accretion rates. We analyze a collection of the RXTE observations (Galactic Center scans, ASM monitoring and a pointed observation) of enigmatic transient source high mass X-ray binary V4641 Sgr and argue that they broadly support the hypothesis that giant September 1999 outburst was associated with an episode of super-Eddington accretion onto the black hole. During the outburst an extended optically thick envelope/outflow has been formed around the source making the observational appearance of V4641 Sgr in many aspects very similar to that of SS433. These results suggest that objects like V4641 Sgr and SS433 indeed represent the class of objects accreting matter at a rate comparable or above Eddington value and the formation of an envelope/outflow is a generic characteristic of supercritical accretion. When the accretion rate decreased the envelope vanished and the source short term variability and spectral properties started to resemble those of other galactic black hole candidates accreting at a rate well below the Eddington value. Interestingly that during this phase the source spectrum was very similar to the Cygnus X-1 spectrum in the low state inspite of more than order of magnitude larger X-ray luminosity.

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arXiv2002

Cooling Flows as a Calorimeter of AGN Mechanical Power

The assumption that radiative cooling of gas in the centers of galaxy clusters is approximately balanced by energy input from a central supermassive black hole implies that the observed X-ray luminosity of the cooling flow region sets a lower limit on AGN mechanical power. The conversion efficiency of AGN mechanical power into gas heating is uncertain, but we argue that it can be high even in the absence of strong shocks. These arguments inevitably lead to the conclusion that the time averaged mechanical power of AGNs in cooling flows is much higher than the presently observed bolometric luminosity of these objects. The energy balance between cooling losses and AGN mechanical power requires some feedback mechanism. We consider a toy model in which the accretion rate onto a black hole is set by the classic Bondi formula. Application of this model to the best studied case of M87 suggests that accretion proceeds at approximately the Bondi rate down to a few gravitational radii with most of the power (at the level of a few percent of the rest mass) carried away by an outflow.

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arXiv2001

Polarization of resonance X-ray lines from clusters of galaxies

As is known, resonant scattering can distort the surface-brightness profiles of clusters of galaxies in X-ray lines. We demonstrate that the scattered line emission should be polarized and possibly detectable with near-future X-ray polarimeters. Spectrally-resolved mapping of a galaxy cluster in polarized X-rays could provide valuable independent information on the physical conditions, in particular element abundances and the characteristic velocity of small-scale turbulent motions, in the intracluster gas. The expected degree of polarization is of the order of 10% for richest regular clusters (e.g. Coma) and clusters whose X-ray emission is dominated by a central cooling flow (e.g. Perseus or M87/Virgo).

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arXiv2001

Polarization of X-ray emission from the Sgr B2 cloud

The Sgr B2 giant molecular cloud is claimed to be an "X-ray reflection nebula" - the reprocessing site of a powerful flare of the Sgr A* source, occurred few hundred years ago. The shape of the X-ray spectrum and the strength of the iron fluorescent line support this hypothesis. We argue that the most clean test of the origin of X-rays from Sgr B2 would be a detection of polarized emission from this source.

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arXiv2001

XMM-Newton discovery of a sharp spectral feature at ~7 keV in the Narrow-Line Seyfert 1 galaxy 1H 0707-495

We report the first detection of a sharp spectral feature in a Narrow-Line Seyfert 1 galaxy. Using XMM-Newton we have observed 1H0707-495 and find a drop in flux by a factor of more than 2 at a rest-frame energy of ~7 keV without any detectable narrow Fe K alpha line emission. The energy of this feature suggests a connection with the neutral iron K photoelectric edge, but the lack of any obvious absorption in the spectrum at lower energies makes the interpretation challenging. We explore two alternative explanations for this unusual spectral feature: (i) partial covering absorption by clouds of neutral material and (ii) ionised disc reflection with lines and edges from different ionisation stages of iron blurred together by relativistic effects. We note that both models require an iron overabundance to explain the depth of the feature. The X-ray light curve shows strong and rapid variability, changing by a factor of four during the observation. The source displays modest spectral variability which is uncorrelated with flux.

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arXiv2001

V4641Sgr - Super-Eddington source enshrouded by an extended envelope

An optical spectroscopy of an unusual fast transient V4641 Sgr constrains its mass to 8.7-11.7M_sun (9.6M_sun is the best fit value) and the distance to 7.4--12.3 kpc (Orosz et al. 2001). At this distance the peak flux of 12 Crab in the 2--12 keV energy band, measured by ASM/RXTE, implies the X-ray luminosity exceeding 2-3e39 erg/s, i.e. near or above the Eddington limit for a 9.6M_sun black hole. An optical photometry shows that at the peak of the optical outburst the visual magnitude increased by Delta m_V > 4.7^m relative to the quiescent level and reached m_V < 8.8^m. An assumption that this optical emission is due to irradiated surface of an accretion disk or a companion star with the the black body shape of the spectrum would mean that the bolometric luminosity of the system exceeds L>3e41 erg/s > 300 L_Edd. We argue that the optical data strongly suggest presence of an extended envelope surrounding the source which absorbs primary X-rays flux and reemits it in optical and UV. The data also suggests that this envelope should be optically thin in UV, EUV and soft X-rays. The observed properties of V4641 Sgr at the peak of an optical flare are very similar to those of SS433. This envelope is likely the result of near or super Eddington rate of mass accretion onto the black hole and it vanishes during subsequent evolution of the source when apparent luminosity drops well below the Eddington value. Thus this transient source provides us direct proof of the dramatic change in the character of an accretion flow at the mass accretion rate near or above the critical Eddington value as predicted long time ago by the theoretical models.

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arXiv2001

An upper limit on the X-ray luminosity of the black hole - microlens OGLE-1999-BUL-32

We present an upper limit on the 3--20 keV X-ray flux from the black hole - microlens OGLE-1999-BUL-32, based on RXTE/PCA scans over the Galactic Center region in 1999-2000. It is shown that the X-ray luminosity of the black hole did not exceed L(3-20 keV)<3e33(d/1kpc)^2 ergs/s (where d is the distance to the black hole). Near the maximum of the background star amplification by the microlens (July 6, 1999), the upper limit on the X-ray flux corresponds to an X-ray luminosity L(3-20 keV)<7e33(d/1kpc)^2 ergs/s.

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arXiv2001

Possible 38 day X-ray period of KS1731-260

We report the detection of a 38 day period in the X-ray flux of the transient burster KS1731-260. The narrow peak of periodicity was detected during ~TJD 10150--11050 when the source had a high and relatively stable X-ray flux. After TJD 11100 the source became strongly variable on a time scale of months that contaminates the search for the 38 day periodicity. The detected period can not be a binary period. The binary with Roche lobe overflow has in this case large radii of the secondary and of the accretion disk. Disk and secondary star illumination by X-ray flux from luminous neutron star would lead to high infrared brightness of the binary. That clearly contradicts the infrared data even for the brightest infrared sources within CHANDRA error box of KS1731-260. Remaining possibility is that observed periodicity is connected with the accretion disk precession, similar to that was observed for SS 433, Her X-1, Cyg X-1 etc.

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arXiv2001

Chandra localization of KS1731--260

We present the analysis of the Chandra image of KS 1731-260. The improvement of the source localization (down to ~0.6'') allowed us to rule out 12 of 13 infrared companion candidates proposed by Barret et al. (1998). The remaining possible infrared counterpart of KS 1731-260 (angular distance between sources ~1.45'', or ~2σ) has the brightness m_{J}~16 in the J band. If this star is really the counterpart of KS 1731-260, the lower limit on its luminosity is L_{tot}>L_{J,H}~10L_{\odot}. The strong drop of the X-ray flux from the source allow us to propose the additional check of whether the decribed star is the real counterpart of KS1731-260. If the optical and infrared brightness of the binary system is partially caused by the reprocessing of X-rays, as it is usual for the low mass binary systems, the infrared brightness of real counterpart should strongly reduce in 2001, when the X-ray flux from KS1731-260 turned off (Wijnands et al. 2001)

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arXiv2001

Non-pulsing emission from X-ray pulsars

Results of GRANAT/ART-P observations of three X-ray pulsars in non-pulsing states are presented: 1) a statistically significant non-pulsing flux with a simple power-law spectrum was detected during the ``off''- state of Her X-1; 2) a significant (13 $σ$) non-pulsing flux with a strong iron emission line at energies ~ 6.7-6.9 keV was detected during the eclipse ingress of Cen X-3; 3) a weak non-pulsing flux was detected during the X-ray eclipse of Vela X-1, which probably resulted from scattering of the pulsar emission in the stellar wind of an optical star.

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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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arXiv2000

Energy Release on the Surface of a Rapidly Rotating Neutron Star during Disk Accretion: A Thermodynamic Approach

The total energy E of a star as a function of its angular momentum J and mass M in the Newtonian theory: E = E(J, M) [in general relativity, the gravitational mass M of a star as a function of its angular momentum J and rest mass m, M = M(J, m)], is used to determine the remaining parameters (angular velocity, equatorial radius, chemical potential, etc.) in the case of rigid rotation. Expressions are derived for the energy release during accretion onto a cool (with constant entropy), rapidly rotating neutron star (NS) in the Newtonian theory and in general relativity. A separate analysis is performed for the cases where the NS equatorial radius is larger and smaller than the radius of the marginally stable orbit in the disk plane. An approximate formula is proposed for the NS equatorial radius for an arbitrary equation of state, which matches the exact one at J = 0.

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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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