This review covers the main aspects of black hole accretion disk theory. We begin with the view that one of the main goals of the theory is to better understand the nature of black holes themselves. ...In this light we discuss how accretion disks might reveal some of the unique signatures of strong gravity: the event horizon, the innermost stable circular orbit, and the ergosphere. We then review, from a first-principles perspective, the physical processes at play in accretion disks. This leads us to the four primary accretion disk models that we review: Polish doughnuts (thick disks), Shakura-Sunyaev (thin) disks, slim disks, and advection-dominated accretion flows (ADAFs). After presenting the models we discuss issues of stability, oscillations, and jets. Following our review of the analytic work, we take a parallel approach in reviewing numerical studies of black hole accretion disks. We finish with a few select applications that highlight particular astrophysical applications: measurements of black hole mass and spin, black hole vs. neutron star accretion disks, black hole accretion disk spectral states, and quasi-periodic oscillations (QPOs).
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Context.
The Event Horizon Telescope (EHT) collaboration recently obtained the first images of the surroundings of the supermassive compact object M87* at the center of the galaxy M87. This provides ...a fascinating probe of the properties of matter and radiation in strong gravitational fields. It is important to determine from the analysis of these results what can and cannot be inferred about the nature of spacetime around M87*
Aims.
We want to develop a simple analytic disk model for the accretion flow of M87*. Compared to general-relativistic magnetohydrodynamic models, this new approach has the advantage that it is independent of the turbulent character of the flow and is controlled by only a few easy-to-interpret, physically meaningful parameters. We want to use this model to predict the image of M87*, assuming that it is either a Kerr black hole or an alternative compact object.
Methods.
We computed the synchrotron emission from the disk model and propagate the resulting light rays to the far-away observer by means of relativistic ray tracing. Such computations were performed assuming different spacetimes, such as Kerr, Minkowski, nonrotating ultracompact star, rotating boson star, or Lamy spinning wormhole. We performed numerical fits of these models to the EHT data.
Results.
We discuss the highly lensed features of Kerr images and show that they are intrinsically linked to the accretion-flow properties and not only to gravitation. This fact is illustrated by the notion of the secondary ring, which we introduce. Our model of a spinning Kerr black hole predicts mass and orientation consistent with the EHT interpretation. The non-Kerr images result in a similar quality of numerical fits and may appear very similar to Kerr images, once blurred to the EHT resolution. This implies that a strong test of the Kerr spacetime may be out of reach with the current data. We note that future developments of the EHT could alter this situation.
Conclusions.
Our results show the importance of studying alternatives to the Kerr spacetime to be able to test the Kerr paradigm unambiguously. More sophisticated treatments of non-Kerr spacetimes and more advanced observations are needed to proceed further in this direction.
We consider an accretion flow model originally proposed by Bisnovatyi-Kogan and Ruzmaikin (1974), which has been confirmed in recent 3D MHD simulations. In the model, the accreting gas drags in a ...strong poloidal magnetic field to the center such that the accumulated field disrupts the axisymmetric accretion flow at a relatively large radius. Inside the disruption radius, the gas accretes as discrete blobs or streams with a velocity much less than the free-fall velocity. Almost the entire rest mass energy of the gas is released as heat, radiation and mechanical/magnetic energy. Even for a non-rotating black hole, the efficiency of converting mass to energy is of order 50% or higher. The model is thus a practical analog of an idealized engine proposed by Geroch and Bekenstein.
In all microquasars with double peak high frequency QPOs, the ratio of the frequencies is 3:2, which supports the suggestion that a non-linear resonance between two modes of oscillation in the ...accretion disk plays a role in exciting the observed modulations of the X-ray flux. We discuss evidence in favor of this interpretation and relate the black hole spin to the frequencies expected for various types of resonances that may occur in nearly Keplerian disks in strong gravity. For those microquasars where the mass of the central X-ray source is known, the black hole spin can be deduced from a comparison of the observed and expected frequencies.
We explore the influence of nongeodesic pressure forces present in an accretion disc on the frequencies of its axisymmetric and nonaxisymmetric epicyclic oscillation modes. We discuss its ...implications for models of high-frequency quasi-periodic oscillations (QPOs), which have been observed in the X-ray flux of accreting black holes (BHs) in the three Galactic microquasars, GRS 1915+105, GRO J1655−40, and XTE J1550−564. We focus on previously considered QPO models that deal with low-azimuthal-number epicyclic modes, |
m
| ≤ 2, and outline the consequences for the estimations of BH spin,
a
∈ 0, 1. For four out of six examined models, we find only small, rather insignificant changes compared to the geodesic case. For the other two models, on the other hand, there is a significant increase of the estimated upper limit on the spin. Regarding the falsifiability of the QPO models, we find that one particular model from the examined set is incompatible with the data. If the spectral spin estimates for the microquasars that point to
a
> 0.65 were fully confirmed, two more QPO models would be ruled out. Moreover, if two very different values of the spin, such as
a
≈ 0.65 in GRO J1655−40 and
a
≈ 1 in GRS 1915+105, were confirmed, all the models except one would remain unsupported by our results. Finally, we discuss the implications for a model that was recently proposed in the context of neutron star (NS) QPOs as a disc-oscillation-based modification of the relativistic precession model. This model provides overall better fits of the NS data and predicts more realistic values of the NS mass compared to the relativistic precession model. We conclude that it also implies a significantly higher upper limit on the microquasar’s BH spin (
a
∼ 0.75 vs.
a
∼ 0.55).
Context. At super-Eddington rates accretion flows onto black holes have been described as slim (aspect ratio H/R ≲ 1) or thick (H/R> 1) discs, also known as tori or (Polish) doughnuts. The relation ...between the two descriptions has never been established, but it was commonly believed that at sufficiently high accretion rates slim discs inflate, becoming thick. Aims. We wish to establish under what conditions slim accretion flows become thick. Methods. We use analytical equations, numerical 1 + 1 schemes, and numerical radiative MHD codes to describe and compare various accretion flow models at very high accretion rates. Results. We find that the dominant effect of advection at high accretion rates precludes slim discs becoming thick. Conclusions. At super-Eddington rates accretion flows around black holes can always be considered slim rather than thick.
The “radiation inner edge” of an accretion disk is defined as the inner boundary of the region from which most of the luminosity emerges. Similarly, the “reflection edge” is the smallest radius ...capable of producing a significant X-ray reflection of the fluorescent iron line. For black hole accretion disks with very sub-Eddington luminosities these and all other “inner edges” coexist at the innermost stable circular orbit (ISCO). Thus, in this case, one may rightly consider ISCO as the unique inner edge of the black hole accretion disk. However, even at moderate luminosities, there is no such unique inner edge because differently defined edges are located at different places. Several of them are significantly closer to the black hole than ISCO. These differences grow with the increasing luminosity. For nearly Eddington luminosities, they are so huge that the notion of the inner edge loses all practical significance.
Elliptical accretion disk models for tidal disruption events (TDEs) have been recently proposed and independently developed by two groups. Although these two models are characterized by a similar ...geometry, their physical properties differ considerably. In this paper, we further investigate the properties of the elliptical accretion disk of the nearly uniform distribution of eccentricity within the disk plane. Our results show that the elliptical accretion disks have distinctive hydrodynamic structures and spectral energy distributions, associated with TDEs. The soft X-ray photons generated at pericenter and nearby are trapped in the disk and advected around the ellipse because of large electron scattering opacity. They are absorbed and reprocessed into emission lines and low-frequency continuum via recombination and bremsstrahlung emission. Because of the rapid increase of bound-free and free-free opacities with radius, the low-frequency continuum photons become trapped in the disk at large radius and are advected through apocenter and back to the photon-trapping radius. Elliptical accretion disks predict sub-Eddington luminosities and emit mainly at the photon-trapping radius of thousands of Schwarzschild radii with a blackbody spectrum of nearly single temperature of typically about 3 × 104 K. Because of the self-regulation, the photon-trapping radius expands and contracts following the rise and fall of accretion rate. The radiation temperature is nearly independent of BH mass and accretion rate and varies weakly with the stellar mass and the viscosity parameter. Our results are well consistent with the observations of optical/UV TDEs.
We note that the recently discovered 450 Hz frequency in the X-ray flux of the black hole candidate GRO J1655-40 is in a 3:2 ratio to the previously known 300 Hz frequency of quasi-periodic ...oscillations (QPO) in the same source. If the origin of high frequency QPOs in black hole systems is a resonance between orbital and epicyclic motion of accreting matter, as suggested previously, the angular momentum of the black hole can be accurately determined, given its mass. We find that the dimensionless angular momentum is in the range 0.2<j<0.67 if the mass is in the (corresponding) range of 5.5 to 7.9 solar masses.
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