We review the properties and behavior of 20 X-ray binaries that contain a dynamically-confirmed black hole, 17 of which are transient systems. During the past decade, many of these transien...

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X-Ray Properties of Black-Hole Binaries

We present an analysis of the host properties of 85224 emission-line galaxies selected from the Sloan Digital Sky Survey. We show that Seyferts and LINERs form clearly separated branches on the standard optical diagnostic diagrams. We derive a new empirical classification scheme which cleanly separates star-forming galaxies, composite AGN-H ii galaxies, Seyferts and LINERs and we study the host galaxy properties of these different classes of objects. LINERs are older, more massive, less dusty and more concentrated, and they and have higher velocity dispersions and lower [OIII] luminosities than Seyfert galaxies. Seyferts and LINERs are most strongly distinguished by their [OIII] luminosities. We then consider the quantity L[OIII]/σ 4 , which is an indicator of the black hole accretion rate relative to the Eddington rate. Remarkably, we find that at fixed L[OIII]/σ 4 , all differences between Seyfert and LINER host properties disappear. LINERs and Seyferts form a continuous sequence, with LINERs dominant at low L/LEDD and Seyferts dominant at high L/LEDD . These results suggest that the majority of LINERs are AGN and that the Seyfert/LINER dichotomy is analogous to the high/low-state transition for X-ray binary systems. We apply theoretical photo-ionization models and show that pure LINERs require a harder ionizing radiation field with lower ionization parameter than Seyfert galaxies, consistent with the low and high X-ray binary states.

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The host galaxies and classification of active galactic nuclei

Gas supplied conservatively to a black hole at rates well below the Eddington rate may not be able to radiate effectively and the net energy flux, including the energy transported by the viscous torque, is likely to be close to zero at all radii. This has the consequence that the gas accretes with positive energy so that it may escape. Accordingly, we propose that only a small fraction of the gas supplied actually falls on to the black hole, and that the binding energy it releases is transported radially outward by the torque so as to drive away the remainder in the form of a wind. This is a generalization of and an alternative to an `ADAF' solution. Some observational implications and possible ways to distinguish these two types of flow are briefly discussed.

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On the fate of gas accreting at a low rate on to a black hole

Black hole accretion flows can be divided into two broad classes: cold and hot. Whereas cold accretion flows consist of cool optically thick gas and are found at relatively high mass accretion rates, hot accretion flows, the topic of this review, are virially hot and optically thin, and occur at lower mass accretion rates. They are described by accretion solutions such as the advection-dominated accretion flow and luminous hot accretion flow. Because of energy advection, the radiative efficiency of these flows is in general lower than that of a standard thin accretion disk. Moreover, the efficiency decreases with decreasing mass accretion rate. Observations show that hot accretion flows are associated with jets. In addition, theoretical arguments suggest that hot flows should produce strong winds. Hot accretion flows are believed to be present in low-luminosity active galactic nuclei and in black hole X-ray binaries in the hard and quiescent states. The prototype is Sgr A*, the ultralow-luminosity supermassive black hole at our Galactic center. The jet, wind, and radiation from a supermassive black hole with a hot accretion flow can interact with the external interstellar medium and modify the evolution of the host galaxy.

Hot Accretion Flows Around Black Holes

Black hole X-ray binaries (BHXBs) are known to display five distinct spectral states. In order of increasing luminosity these are the quiescent state, low state, intermediate state, high state, and very high state. We present a self-consistent model of accretion flows around black holes that unifies all of these states except the very high state. The model is an extension of the following paradigm, which has been applied successfully to the quiescent state. The accretion flow consists of two zones, an inner advection-dominated accretion flow (ADAF) that extends from the black hole horizon to a transition radius rtr, and an outer thin accretion disk that is present beyond rtr. Above the disk is a hot corona, which is a continuation of the inner ADAF. The model consistently treats the dynamics of the accreting gas, the thermal balance of the ions and electrons in the two-temperature ADAF and corona, and the radiation processes that produce the observed spectrum. At low mass accretion rates, 0.01 (in Eddington units), the inner ADAF zone in the model radiates extremely inefficiently, and the outer thin disk is restricted to large radii (rtr ~ 102-104, in Schwarzschild units). The luminosity therefore is low, and this configuration is identified with the quiescent state. For 0.01 and up to a critical value crit${r crit}$ --> ~0.08, the radiative efficiency of the ADAF increases rapidly and the system becomes fairly luminous. The spectrum is very hard and peaks around 100 keV. This is the low state. The exact value of crit${r crit}$ --> depends on the viscosity parameter α (crit${r crit}$ --> ~1.3α -->2; the paper assumes α = 0.25). For values of > crit${r crit}$ --> and up to a second critical value about 10% higher, the ADAF progressively shrinks in size, the transition radius decreases, and the X-ray spectrum changes continuously from hard to soft. We identify this stage with the intermediate state. Finally, when is sufficiently large, the inner ADAF zone disappears altogether and the thin accretion disk extends down to the marginally stable orbit. The spectrum is dominated by an ultrasoft component with a weak hard tail. This is the high state. Model spectra calculated with this unified scenario agree well with observations of the quiescent, low, intermediate, and high states. Moreover, the model provides a natural explanation for the low state to high state transition in BHXBs. We also make a tentative proposal for the very high state, but this aspect of the model is less secure. An important feature of the model is that it is essentially parameter free. We test the model against observations of the soft X-ray transient Nova Muscae during its 1991 outburst. The model reproduces the observed light curves and spectra surprisingly well and makes a number of predictions that can be tested by observations of other BHXBs.

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Advection-Dominated Accretion and the Spectral States of Black Hole X-Ray Binaries: Application to Nova Muscae 1991

We show that most of hot, optically thin accretion disk models which ignore advective cooling are not self-consistent. We have found new types of optically thin disk solutions where cooling is dominated by radial advection of heat. These new solutions are thermally and viscously stable.

Thermal equilibria of accretion disks

Black-hole accretion disks

Thermal Equilibria of Accretion Disks

We present global solutions that describe advection-dominated accretion flows around black holes. The solutions are obtained by solving numerically a set of coupled ordinary differential equations corresponding to a steady axisymmetric height-integrated flow. The solutions satisfy consistent boundary conditions at both ends. On the inside, the flow passes through a sonic point and falls supersonically into the black hole with a zero-torque condition at the horizon. On the outside, the flow attaches to a normal thin accretion disk. We obtain consistent transonic solutions for a wide range of values of the viscosity parameter ?, from 0.001 to 0.3. We do not find any need for shocks in our solutions, and we disagree with previous claims that viscous accretion flows with low values of ? must have shocks. We compare the exact global solutions of this paper with a local self-similar solution that has been studied in the past. Although the self-similar solution makes significant errors close to the boundaries, we find that it provides nevertheless a reasonable description of the overall properties of the flow. We compare also two different forms of viscosity: one is based on a diffusion prescription, while the other takes the shear stress to be simply proportional to the pressure. The results with the two prescriptions are similar. We see a qualitative difference between solutions with low values of the visocity parameter, ? 0.01, and those with large values, ? 0.01. The solutions with low ? have their sonic transitions occurring close to the radius of the marginally bound orbit. These flows are characterized by regions of super-Keplerian rotation and have pressure maxima outside the sonic point. The solutions are similar in many respects to the hydrostatic thick tori developed previously as models of active galactic nuclei. In contrast, the solutions with large ? have sonic transitions farther out, close to, or beyond the marginally stable orbit and have no super-Keplerian rotation or pressure maxima. We believe these flows will be nearly quasi-spherical down to the sonic radius and will not have empty funnels along the rotation axis. The large-? solutions are more likely to be representative of real systems, since most observations of astrophysical systems are best fit, within the context of advection-dominated theories, with values of ? 0.1.

https://iopscience.iop.org/article/10.1086/303591/pdf

Global Structure and Dynamics of Advection-dominated Accretion Flows around Black Holes

We present global models of optically thin, advection-dominated two-temperature accretion flows onto black holes, paying careful attention to transonic properties of the flows. We treat the physical quanti­ ties integrated over the vertical direction, and adopt the standard a-viscosity and the pseudo-Newtonian potential. Bremsstrahlung and synchrotron cooling amplified by Comptonizatio n is considered as to be cooling processes of electrons. It is found that when moderate synchrotron cooling is included, the electron temperature distribution in the inner part of the disk is roughly flat with temperatures slightly lower than the electron rest mass energy. This high electron temperature is appropriate to explain the hard X-rays from X-ray sources and AGNs. It is also found that in the inner part of the disks the energy input to elec­ trons from ions by the ion-electron Coulomb collision is negligible as the electron heating, i.e., the electron temperature is determined by the balance between advective heating and synchrotron cooling.

Shoji Kato

Papers

Thermal equilibria of accretion disks

Black-hole accretion disks

Thermal Equilibria of Accretion Disks

Global Structure and Dynamics of Advection-dominated Accretion Flows around Black Holes

Optically Thin, Advection-Dominated Two-Temperature Disks