Publications (23)9.17 Total impact
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Article: Coronae above accretion disks around black holes: The effect of Compton cooling
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ABSTRACT: The geometry of the accretion flow around stellar mass and supermassive black holes depends on the accretion rate. Broad iron emission lines originating from the irradiation of cool matter can indicate that there is an inner disk below a hot coronal flow.These emission lines have been detected in X-ray binaries. Observations with the Chandra X-ray Observatory, XMM Newton and Suzaku have confirmed the presence of these emission lines also in a large fraction of Seyfert-1 active galactic nuclei (AGN). We investigate the accretion flow geometry for which broad iron emission lines can arise in hard and soft spectral state. We study an ADAF-type coronal flow, where the ions are viscously heated and electrons receive their heat only by collisions from the ions and are Compton cooled by photons from an underlying cool disk. For a strong mass flow in the disk and the resulting strong Compton cooling only a very weak coronal flow is possible. This limitation allows the formation of ADAF-type coronae above weak inner disks in the hard state, but almost rules them out in the soft state. The observed hard X-ray luminosity in the soft state, of up to 10% or more of the total flux, indicates that there is a heating process that directly accelerates the electrons. This might point to the action of magnetic flares of disk magnetic fields reaching into the corona. Such flares have also been proposed by observations of the spectra of X-ray black hole binaries without a thermal cut-off around 200 keV.08/2012; -
Article: Broad iron emission lines in Seyfert Galaxies - re-condensation of gas onto an inner disk below the ADAF
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ABSTRACT: Recent observations of Seyfert 1 AGN with Chandra, XMM-Newton and Suzaku revealed broad iron K_alpha emission lines, some relativistically blurred. For galactic black hole X-ray binaries XMM-Newton spectra during hard state also reveal the presence of a relativistic iron emission line and a thermal component, interpreted as an indication for a weak inner cool accretion disk underneath a hot corona. These thermal components were found after the transition from soft to hard spectral state and can be understood as sustained by re-condensation of gas from an advection-dominated flow (ADAF) onto the disk. In view of the similarity of accretion flows around stellar mass and supermassive black holes we discuss whether the broad iron emission lines in Seyfert 1 AGN can be understood as arising from a similar accretion flow geometry. We derive Eddington-scaled accretion rates for Seyfert galaxies with strong lines in samples of Miller (2007) and Nandra et al. (2007). For the evaluation we use the observed X-ray luminosity, bolometric corrections and black hole masses from literature, most values taken from Fabian and Vasudevan (2009). Rates derived are less than 0.1 of the Eddington rate for more than half of the sources. For 10^7 to 10^8 solar mass black holes in Seyfert 1 AGN this limit corresponds to 0.01 to 0.2 solar masses per year. Our investigation shows that for quite a number of Seyfert AGN in hard spectral state iron emission lines can arise from an inner weak disk surrounded by an ADAF as predicted by the re-condensation model. Some of the remaining sources with higher accretion rates may be in a spectral state comparable to the "very high" state of LMXBs. (abridged)01/2011; -
Article: The hard to soft spectral transition in LMXBs - affected by recondensation of gas into an inner disk
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ABSTRACT: Soft and hard spectral states of X-ray transient sources reflect two modes of accretion, accretion via a geometrically thin, optically thick disk or an advection-dominated accretion flow (ADAF). The luminosity at transition between these two states seems to vary from source to source, or even for the same source during different outbursts, as observed for GX 339-4. We investigate how the existence of an inner weak disk in the hard state affects the transition luminosity. We evaluate the structure of the corona above an outer truncated disk and the resulting disk evaporation rate for different irradiation. In some cases, recent observations of X-ray transients indicate the presence of an inner cool disk during the hard state. Such a disk can remain during quiescence after the last outburst as long as the luminosity does not drop to very low values (10^-4 to 10^-3 of the Eddington luminosity). Consequently, as part of the matter accretes via the inner disk, the hard irradiation is reduced. The hard irradiation is further reduced, occulted and partly reflected by the inner disk. This leads to a hard-soft transition at a lower luminosity if an inner disk exists below the ADAF. This seems to be supported by observations for GX 339-4. Comment: 9 pages, 4 figures, accepted for publication in Astronomy and Astrophysics10/2009; -
Article: On the Properties of Inner Cool Disks in the Hard State of Black Hole X-Ray Transient Systems
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ABSTRACT: The formation of a cool disk in the innermost regions of black hole X-ray transient systems in the low hard state is investigated. Taking into account the combined cooling associated with the Compton and conductive energy transport processes in a corona, the radial structure of a disk is described for a range of mass accretion rates. The mass flow in an optically thick inner region can be maintained by the condensation of matter from a corona with the disk temperature and luminosity varying continuously as a function of the accretion rate. Although such a disk component can be present, the contribution of the optically thick disk component to the total luminosity can be small since the mass flow due to condensation in the optically thick disk underlying the corona can be significantly less than the mass flow rate in the corona. The model is applied to the observations of the low quiescent state of the black hole source GX 339-4 at luminosities of around $0.01 L_{Edd}$ and is able to explain the temperature of the thermal component at the observed luminosities. Since conductive cooling dominates Compton cooling at low mass accretion rates, the luminosity corresponding to the critical mass accretion rate above which a weak thermal disk component can be present in the low hard state is estimated to be as low as $0.001 L_{Edd}$. Comment: 30 pages, 8 figures, accepted for publication in ApJ07/2008; -
Article: The Existence of Inner Cool Disks in the Low Hard State of Accreting Black Holes
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ABSTRACT: The condensation of matter from a corona to a cool, optically thick inner disk is investigated for black hole X-ray transient systems in the low hard state. A description of a simple model for the exchange of energy and mass between corona and disk originating from thermal conduction is presented, taking into account the effect of Compton cooling of the corona by photons from the underlying disk. It is found that a weak, condensation-fed inner disk can be present in the low hard state of black hole transient systems for a range of luminosities which depend on the magnitude of the viscosity parameter. For $\alpha \sim 0.1-0.4$ an inner disk can exist for luminosities in the range $\sim 0.001- 0.02$ Eddington value. The model is applied to the X-ray observations of the black hole candidate sources GX 339-4 and Swift J1753.5-0127 in their low hard state. It is found that Compton cooling is important in the condensation process, leading to the maintenance of cool inner disks in both systems. As the results of the evaporation/condensation model are independent of the black hole mass, it is suggested that such inner cool disks may contribute to the optical and ultraviolet emission of low luminosity active galactic nuclei. Comment: 32 pages, 5 figures, accepted for publication in ApJ (Dec. 10, 2007, Vol.671)09/2007; -
Article: Re-condensation from an ADAF into an inner disk - the intermediate state of black hole accretion?
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ABSTRACT: Context: Accretion onto galactic and supermassive black holes occurs in different modes, which are documented in hard and soft spectral states, commonly attributed to an advection-dominated flow (ADAF) inside a truncated disk and standard disk accretion, respectively. At the times of spectral transition an intermediate state is observed, for which the accretion flow pattern is still unclear. Aims: We analyze the geometry of the accretion flow when the mass flow rate in the disk decreases (soft/hard transition) and evaporation of gas into the coronal flow leads to disk truncation. Methods: We evaluate the physics of an advection-dominated flow affected by thermal conduction to a cool accretion disk underneath. We find re-condensation of gas from the ADAF into the underlying inner disk at distances from the black hole and at rates, that depend on the properties of the hot ADAF and vary with the mass accretion rate. This sustains an inner disk for longer than a viscous decay time after the spectral transition occurred, in accordance with the spectra that indicate cool gas in the neighborhood of the accreting black hole. The model allows us to understand why Cyg X-1 does not show hysteresis in the spectral state transition luminosity that is commonly observed for X-ray transient sources. Conclusions: Our results shed new light on the complex mass flow pattern during spectral state transition. Comment: 10 pages, 4 figures, accepted by A&A11/2006; -
Article: An inner disk below the ADAF: the intermediate spectral state of black hole accretion
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ABSTRACT: Aims: The hard and soft spectral states of black hole accretion are understood as connected with ADAF accretion (truncated disk) and standard disk accretion, respectively. However, observations indicate the existence of cool gas in the inner region at times when the disk is already truncated outside. We try to shed light on these not yet understood intermediate states. Methods: The disk-corona model allows to understand the spectral state transitions as caused by changes of the mass flow rate in the disk and provides a picture for the accretion geometry when disk truncation starts at the time of the soft/hard transition, the formation of a gap in the disk filled by an advection-dominated flow (ADAF) at the distance where the evaporation is maximal. We study the interaction of such an ADAF with an inner thin disk below. Results: We show that, when the accretion rate is not far below the transition rate, an inner disk could exist below an ADAF, leading to an intermediate state of black hole accretion. Comment: 4 pages, 1 figure. Accepted for publication in A&A Letters06/2006; -
Article: The effect of heat conduction on the interaction of disk and corona around black holes
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ABSTRACT: Heat conduction plays an important role in the balance between heating and cooling in many astrophysical objects, e.g. cooling flows in clusters of galaxies. Here we investigate the effect of heat conduction on the interaction between a cool disk and a hot corona around black holes. Using the one-radial-zone approximation, we study the vertical structure of the disk corona and derive evaporation and coronal mass flow rates for various reduced thermal conductivities. We find lower evaporation rates and a shift in the evaporation maxima to smaller radii. This implies that the spectral state transition occurs at a lower mass flow rate and a disk truncation closer to the black hole. Reductions of thermal conductivity are thought to be magnetically caused and might vary from object to object by a different configuration of the magnetic fields. Comment: accepted for publication in A & A, 5 pages12/2005; -
Article: Spectral state transitions in low-mass X-ray binaries - the effect of hard and soft irradiation
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ABSTRACT: In neutron star and black hole X-ray binaries the transitions between the two spectral states, hard and soft, signals the change between accretion via a hot advection-dominated flow(ADAF) and disk accretion. In a few cases the hard/soft transition were observed during the rise to the nova outburst, mostly only the soft/hard transition during the luminosity decrease. Surprisingly the luminosities at the second transition is always lower by a factor of 3 to 5. A model for this hysteresis was presented in a preceding paper: It was shown that this switch in the accretion mode at different mass accretion rates and therefore different luminosities is caused by the different amount of Compton cooling or heating of the accretion disk corona as it is irradiated by hard or soft radiation from the central light source, respectively. We now give detailed results on the dependence on hardness of this radiation and on radiation efficiency. We further discuss the influence of the inclination and a possible warping of the disk on the observed hysteresis. Comment: 8 pages, 2 figures, accepted for publication in A&A06/2005; -
Article: Hysteresis in spectral state transitions - a challenge for theoretical modeling
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ABSTRACT: Many low-mass X-ray binaries show both hard and soft spectral states. For several sources the transitions between these states have been observed, mostly from the soft to the hard state during a luminosity decrease. In a few cases also the transition from the hard to the soft state was observed, coincident with an increase of the luminosity. Surprisingly this luminosity was not the same as the one during a following change back to the hard state. The values differed by a factor of about 3 to 5. We present a model for this hysteresis in the light curves of low-mass X-ray binaries (sources with neutron stars or black holes). We show that the different amount of Compton cooling or heating acting on the accretion disk corona at the time of the transition causes this switch in the accretion mode at different mass accretion rates and therefore different luminosities. The inner disk during the soft state provides a certain amount of Compton cooling which is either not present or much less if the inner region is filled with a hot advection-dominated accretion flow (ADAF) that radiates a hard spectrum. Comment: 8 pages, 2 figures, accepted for publication in Astron. Astrophys11/2004; -
Article: A cool disk in the Galactic Center?
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ABSTRACT: We study the possibility of a cool disk existing in the Galactic Center in the framework of the disk-corona evaporation/condensation model. Assuming an inactive disk, a hot corona should form above the disk since there is a continuous supply of hot gas from stellar winds of the close-by massive stars. Whether the cool disk can survive depends on the mass exchange between the disk and corona. If the disk-corona interaction is dominated by evaporation and the rate is larger than the Bondi accretion rate in the Galactic Center, the disk will be depleted within a certain time period and no persistent disk will exist. On the other hand, if the interaction results in hot gas steadily condensing into the disk, an inactive cool disk might survive. For this case we further investigate the Bremsstrahlung radiation from the hot corona and compare it with the observed X-ray luminosity. Our model shows that, for standard viscosity in the corona (alpha=0.3), the mass evaporation rate is much higher than the Bondi accretion rate and the coronal density is much larger than that inferred from Chandra observations. An inactive disk can not survive such strong evaporation. For small viscosity (alpha<0.07) we find condensation solutions. But detailed computations show that in this case there is too much X-ray radiation from the corona to be in agreement with the observations. Therefore, we conclude that there should be no thin/inactive disk presently in the Galactic Center. However, we do not exclude that the alternative non-radiative model of Nayakshin (2004) might instead be realized in nature. Comment: 8 pages, including 3 figures, accepted for publication in A&A03/2004; -
Article: The formation of the coronal flow/ADAF
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ABSTRACT: We develop a new method to describe the accretion flow in the corona above a thin disk around a black hole in vertical and radial extent. The model is based on the same physics as the earlier one-zone model, but now modified including inflow and outflow of mass, energy and angular momentum from and towards neighboring zones. We determine the radially extended coronal flow for different mass flow rates in the cool disk resulting in the truncation of the thin disk at different distance from the black hole. Our computations show how the accretion flow gradually changes to a pure vertically extended coronal or advection-dominated accretion flow (ADAF). Different regimes of solutions are discussed. For some cases wind loss causes an essential reduction of the mass flow.03/2003; -
Article: The effect of disk magnetic fields on the truncation of geometrically thin disks in AGN
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ABSTRACT: We suggest that magnetic fields in the accretion disks of AGN reach into the coronae above and have a profound effect on the mass flow rate in the corona. This strongly affects the location where the accretion flow changes from a geometrically thin disk to a pure vertically extended coronal or advection-dominated accretion flow (ADAF). We show that this can explain the different disk truncation radii in elliptical galaxies and low luminosity AGN with about the same mass flow rate, a discrepancy pointed out by Quataert et al. (1999). Without disk magnetic activity the disk truncation is expected to be uniquely related to the mass flow rate (Meyer et al. 2000b). Whether dynamo action occurs depends on whether the electrical conductivity measured by a magnetic Reynolds number surpasses a critical value (Gammie and Menou 1998).In elliptical galaxies the disk is self-gravitating at the radii where the truncation should occur. It is plausible that instead of a cool disk a ``layer of clouds'' may form (Shlosman et al. 1990, Gammie 2001) for which no dynamo action is expected. For low luminosity AGN the magnetic Reynolds number is well above critical. Simple model calculations show that magnetic fields in the underlying disks reduce the strength of the coronal flow and shift the truncation radius significantly inward. Comment: 4 pages, 1 figure; accepted for publication in A&A Letters07/2002; -
Article: Two-temperature coronal flow above a thin disk
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ABSTRACT: We extended the disk corona model (Meyer & Meyer-Hofmeister 1994; Meyer, Liu, & Meyer-Hofmeister 2000a) to the inner region of galactic nuclei by including different temperatures in ions and electrons as well as Compton cooling. We found that the mass evaporation rate and hence the fraction of accretion energy released in the corona depend strongly on the rate of incoming mass flow from outer edge of the disk, a larger rate leading to more Compton cooling, less efficient evaporation and a weaker corona. We also found a strong dependence on the viscosity, higher viscosity leading to an enhanced mass flow in the corona and therefore more evaporation of gas from the disk below. If we take accretion rates in units of the Eddington rate our results become independent on the mass of the central black hole. The model predicts weaker contributions to the hard X-rays for objects with higher accretion rate like narrow-line Seyfert 1 galaxies (NLS1s), in agreement with observations. For luminous active galactic nuclei (AGN) strong Compton cooling in the innermost corona is so efficient that a large amount of additional heating is required to maintain the corona above the thin disk. Comment: 17 pages, 6 figures. ApJ accepted04/2002; -
Article: The change from accretion via a thin disk to a coronal flow: dependence on the viscosity of the hot gas
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ABSTRACT: We study the transition from the geometrically thin disk to the hot coronal flow for accretion onto black holes. The efficiency of evaporation determines the truncation of the geometrically thin disk as a function of the black hole mass and the mass flow rate in the outer disk. The physics of the evaporation was already described in detail in earlier work (Meyer et al. 2000b). We show now that the value of the viscosity parameter for the coronal gas has a strong influence on the evaporation efficiency. For smaller values of the viscosity evaporation is less efficient. For a given mass flow rate from outside the geometrically thin disk then extends farther inward. Spectral transitions between soft and hard states are then expected for different mass flow rates in the outer disk. The physics is the same for the cases of stellar and supermassive black holes systems. Comment: 6 pages, 1 figure, accepted for publication in A&A12/2001; -
Article: Black hole X-ray binaries: A new view on soft-hard spectral transitions
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ABSTRACT: The theory of coronal evaporation predicts the formation of an inner hole in the cool thin accretion disk for mass accretion rates below a certain value (~ 1/50 of the Eddington mass accretion rate) and the sudden disappearance of this hole when the mass accretion rate rises above that value. The inner edge of the standard thin disk then suddenly shifts inward from about a few hundred Schwarzschild radii to the last stable orbit. This appears to quantitatively account for the observed transitions between hard and soft spectral states at critical luminosities. Due to the evaporation process the matter accreting in the geometrically thin disk changes to a hot coronal flow which proceeds towards the black hole as an advection-dominated accretion flow (ADAF; for a review see Narayan et al. 1998).03/2000; -
Article: Evaporation of accretion disks around black holes: the disk-corona transition and the connection to the advection-dominated accretion flow
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ABSTRACT: We apply the disk-corona evaporation model (Meyer & Meyer-Hofmeister) originally derived for dwarf novae to black hole systems. This model describes the transition of a thin cool outer disk to a hot coronal flow. The mass accretion rate determines the location of this transition. For a number of well studied black hole binaries we take the mass flow rates derived from a fit of the ADAF model to the observed spectra (for a review see Narayan, Mahadevan, & Quataert) and determine where the transition of accretion via a cool disk to a coronal flow/ADAF would be located for these rates. We compare with the observed location of the inner disk edge, as estimated from the maximum velocity of the $\rm H_\alpha$ emission line. We find that the transition caused by evaporation agrees with this in stellar disks. We also show that the ADAF and the ``thin outer disk + corona'' are compatible in terms of the physics in the transition region. Comment: 7 pages, 1 figure, aas2pp4.sty, ApJ Letters in press (Vol.527)11/1999; -
Article: On the source of viscosity in cool binary accretion disks
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ABSTRACT: We suggest that the low viscosity in close binary accretion disks during quiescence is due to magnetic fields from the companion star. In very late evolutionary phase the companion stars become brown dwarfs and have cooled down to such low a temperature that this process cannot work anymore. The extremely low viscosity in WZ Sge stars supports this connection between companion stars and viscosity. We further suggest that magnetic activity in such very cool stars is cut off by their poor electric conductivity.01/1999; -
Article: WZ Sagittae - an old dwarf nova
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ABSTRACT: . We model the evolution of the accretion disk of WZ Sagittae during the long quiescence. We find that the large amount of mass in the disk derived from the outburst luminosity is a severe constraint and demands values of ff c ß 0.001 in contradiction to some recent suggestions. We include in our computations the formation of an inner disk hole and the growth of the disk due to redistribution of angular momentum. We find a new mode of disk evolution. The disk is quasi-stationary. Only about half of the mass transfered from the companion star flows through the disk, the other half is needed to build up the steadily growing outer disk. When the 3:1 resonance radius is reached the disk growth ends. From then on all transferred matter flows inward, the surface density increases, leading to an outburst within a few years. We predict superhumps at low luminosity during this last phase. We discuss X-rays expected, the white dwarf mass and distance to WZ Sagittae. Key words: accretion disks -...10/1998; -
Article: The influence of new opacity data on the vertical structure of accretion disks
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ABSTRACT: Using the most recent opacity tables and the corresponding equation of state (EOS) data, we compute the vertical structure of accretion disks in cataclysmic variables, and give the new numerical results. We also present the resulting viscosity-surface density relation. We conclude that the improvement of opacity and EOS hardly influences the disk structure compared to the uncertainties connected with parameterization of viscosity and mixing length.Astronomy and Astrophysics 11/1997; 328:243-246. · 4.59 Impact Factor
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Institutions
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2007
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Max-Planck-Institut für Astrophysik
Garching bei München, Bavaria, Germany
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