Outer Thin Disk Research Articles

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

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.

Advection‐dominated Accretion Model of the Black Hole V404 Cygni in Quiescence

We have analyzed archival ASCA data on the soft X-ray transient source V404 Cyg in quiescence. We find that in the energy range 0.7-8.5 keV the spectrum is a hard power law with a photon spectral index between 1.8 and 2.6 (90% confidence limits). We present a model of V404 Cyg in which the accretion flow has two components: (1) an outer thin disk with a small annular extent and (2) a large interior region where the flow is advection dominated. Nearly all the radiation in the infrared, optical, UV, and X-ray bands is from the advection-dominated zone; the thin disk radiates primarily in the infrared where it contributes about 10% of the observed flux. The spectrum we calculate with this model is in excellent agreement with the ASCA X-ray data presented here, as well as with previous optical data. Moreover, the fit is very insensitive to the choice of parameters such as black hole mass, orbital inclination, viscosity coefficient α, and magnetic field strength. We consider the success of the model to be strong support for the advection-dominated accretion paradigm, and further evidence of the black hole nature of V404 Cyg. We discuss strategies whereby systems with advection-dominated accretion could be used to prove the reality of event horizons in black holes.

Heating and Cooling of Hot Accretion Flows by Non Local Radiation

AbstractHot optically thin accretion flow solutions are often used to model X-ray binaries and active galactic nuclei. Recently two new classes of advection-dominated models have been proposed: a two-temperature solution (Narayan & Yi 1995, ApJ, 452, 710; Abramowicz et al. 1995, ApJ, 438, L37), and a one-temperature solution (Esin et al. 1996, ApJ, 465, 312). Though detailed numerical spectra of advection-dominated flows include global radiative transfer effects (e.g. Narayan 1996, ApJ, 462, 136), analytical calculations of the physical properties of these accretion solutions generally ignore the effects of non-local radiative transfer (e.g. Narayan & Yi 1995; Esin et al. 1996). However, the optical depth for electron scattering in such flows is generally much lower than unity, and therefore, radiation emitted at one radius can potentially heat or cool the gas at other radii through Compton scattering. In this paper we investigate the importance of this effect. We discuss three situations: 1.Radiation from the inner regions of an advection-dominated flow Compton cooling gas at intermediate radii and Compton heating gas at large radii.2.Soft radiation from an outer thin accretion disk Compton cooling a hot one- or two-temperature flow on the inside.3.Soft radiation from an inner thin accretion disk Compton cooling hot gas in a surrounding one-temperature flow.We describe how previous results are modified by these non-local interactions. We find that Compton heating or cooling of the gas by the radiation emitted in the inner regions of a hot flow is not important. Likewise, Compton cooling by the soft photons from an outer thin disk is negligible when the transition from a cold to a hot flow occurs at a radius greater than some minimum Rtr,min ~ 102.8(Ṁ / ṀEdd)3.5α−7Rschw. However, if the hot flow terminates at R < Rtr,mim, non-local cooling of the hot gas near the transition radius is so strong that the flow is cooled to a thin disk configuration. As a result, the transition radius decreases and the hot gas at the new boundary experiences even stronger external cooling. The resulting runaway process continues until the entire hot flow assumes the thin disk configuration. In the case of a thin disk surrounded by a hot one-temperature flow, we find that Compton cooling by soft radiation dominates over local cooling in the hot gas for Ṁ ≳ 10−3αṀEdd. As a result, the maximum accretion rate for which an advection-dominated one-temperature solution exists, decreases by a factor of ~ 10, compared to the value computed under an assumption of local energy balance.

Structure of line-emitting accretion disks in active galactic nuclei - ARP 102B

The prime objects of the present self-consistent model of a line-emitting accretion disk able to account for the properties observed in a small class of AGNs are Arp 102B and 3C 390.3, whose double-peaked emission lines have been attributed to a Keplerian disk. Improved calculations of the line profile of a relativistic Keplerian disk, generalized to include a variety of emissivity laws as well as local broadening due to electron scattering or turbulence, are noted to fit Arp 102B; analytic and numerical calculations of the solid angle presented by the outer thin disk to an extended isotropic source of illumination demonstrate that the energy budget requirements for line emission from the disk are also satisfied.

New Evidence for Accretion Disks in AGNs

We have refined our calculation of the line profile of a relativistic, Keplerian disk by incorporating a variety of emissivity laws, as well as broadening due to turbulence or electron scattering. The significant improvement in the fit to the double-peaked Ha line profile of the elliptical radio galaxy Arp 102B provides the most convincing direct evidence for an accretion disk in any AGN. Arp 102B appears to be a low-luminosity analog of 3C 390.3, and several lines of evidence point to the existence of small, hot ion tori illuminating an outer thin disk in both of these galaxies. The rarity of these emission-line profiles might be understood if this particular combination of ion torus/thin disk occurs only for a narrow range of ṁ(= Ṁ/ṀEdd).