Retrograde versus Prograde Models of Accreting Black Holes

Advances in Astronomy ◽

10.1155/2013/213105 ◽

2013 ◽

Vol 2013 ◽

pp. 1-11 ◽

Cited By ~ 13

Author(s):

David Garofalo

Keyword(s):

Black Holes ◽

Black Hole ◽

Magnetic Fields ◽

Observational Data ◽

Accretion Disks ◽

General Consensus ◽

Rotating Black Holes

There is a general consensus that magnetic fields, accretion disks, and rotating black holes are instrumental in the generation of the most powerful sources of energy in the known universe. Nonetheless, because magnetized accretion onto rotating black holes involves both the complications of nonlinear magnetohydrodynamics that currently cannot fully be treated numerically, and uncertainties about the origin of magnetic fields that at present are part of the input, the space of possible solutions remains less constrained. Consequently, the literature still bears witness to the proliferation of rather different black hole engine models. But the accumulated wealth of observational data is now sufficient to meaningfully distinguish between them. It is in this light that this critical paper compares the recent retrograde framework with standard “spin paradigm” prograde models.

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Black holes and magnetic fields

Proceedings of the International Astronomical Union ◽

10.1017/s1743921307004851 ◽

2006 ◽

Vol 2 (S238) ◽

pp. 139-144

Author(s):

Jiří Bičák ◽

Vladimír Karas ◽

Tomáš Ledvinka

Keyword(s):

Black Holes ◽

Black Hole ◽

Magnetic Fields ◽

Magnetic Flux ◽

Higher Dimensions ◽

Meissner Effect ◽

Maxwell Theory ◽

Potential Threat ◽

Rotating Black Holes ◽

Extreme Black Holes

AbstractStationary axisymmetric magnetic fields are expelled from outer horizons of black holes as they become extremal. Extreme black holes exhibit Meissner effect also within exact Einstein–Maxwell theory and in string theories in higher dimensions. Since maximally rotating black holes are expected to be astrophysically most important, the expulsion of the magnetic flux from their horizons represents a potential threat to an electromagnetic mechanism launching the jets at the account of black-hole rotation.

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MODIFIED NEWTONIAN POTENTIALS FOR OPTICALLY THIN ACCRETION DISKS AROUND BLACK HOLES

International Journal of Modern Physics D ◽

10.1142/s0218271898000310 ◽

1998 ◽

Vol 07 (03) ◽

pp. 471-488 ◽

Cited By ~ 7

Author(s):

T. LØVÅS

Keyword(s):

Black Holes ◽

General Relativity ◽

Black Hole ◽

Gravitational Field ◽

Accretion Disks ◽

Rotating Black Holes ◽

Relativistic Result ◽

Do So

The use of modified Newtonian potentials to describe the gravitational field around black holes has proven successful. I will present here an investigation of the accuracy of several modified Newtonian potentials proposed in the literature, by comparing the result with the exact relativistic solution. I will do so for optically thin accretion disks that are more sensitive to the form of the potential than optically thick standard disks. I find that simple modified Newtonian potentials capture the essential features of general relativity, and the results from using the modified Newtonian potentials deviate from the relativistic result only by 20% at most for nonrotating black holes. For rotating black holes the accuracy depends on the rotation of the black hole.

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Vortex generation in the early Universe

Astronomy and Astrophysics ◽

10.1051/0004-6361/202039127 ◽

2020 ◽

Vol 642 ◽

pp. L6

Author(s):

Chinmoy Bhattacharjee ◽

David J. Stark

Keyword(s):

Black Holes ◽

Black Hole ◽

Magnetic Fields ◽

Accretion Disk ◽

Early Universe ◽

Accretion Disks ◽

Disk Model ◽

Spacetime Curvature ◽

Vorticity Generation ◽

Biermann Battery

Context. Accretion disks formed near primordial black holes can be sources of seed magnetic fields in the early Universe. In particular, the Biermann battery mechanism has been shown to generate primordial magnetic fields in an unmagnetized and turbulence-free accretion disk, but this depends on a delicate misalignment of density and pressure gradients in plasmas. Aims. We aim to reformulate the question of magnetogenesis in the context of plasma generalized vorticity and to search for a more robust mechanism of vorticity generation in the early Universe. Methods. We utilize the electro-vortical formalism in curved spacetime, which treats the plasma flow and electromagnetic field on an equal footing, and apply it to a thin accretion disk model near a rotating black hole. Results. We present a spacetime curvature-driven mechanism that persists even in the absence of the Biermann battery. We explore the vorticity and enstrophy generation rate dependencies on black hole masses and spin rates. Conclusions. Analysis indicates that the accretion disks around lower-mass, faster rotating black holes contribute the greatest amount to the enstrophy and vorticity generation rates from the spacetime curvature drive. The shorter turning radii at which the sign of the vorticity changes – corresponding with this region of phase space – may favor these length scales in vortical structure formation and subsequent evolution.

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Thin accretion disks around rotating black holes in 4D Einstein–Gauss–Bonnet gravity

The European Physical Journal C ◽

10.1140/epjc/s10052-021-09266-7 ◽

2021 ◽

Vol 81 (5) ◽

Author(s):

Mohaddese Heydari-Fard ◽

Malihe Heydari-Fard ◽

Hamid Reza Sepangi

Keyword(s):

Black Holes ◽

Black Hole ◽

Accretion Disk ◽

Accretion Disks ◽

Coupling Parameter ◽

Energy Conversion Efficiency ◽

Bonnet Gravity ◽

Stable Circular Orbit ◽

Rotating Black Holes ◽

Kerr Black Holes

AbstractRecently, Kumar and Ghosh have derived Kerr-like rotating black hole solutions in the framework of four-dimensional Einstein–Gauss–Bonnet theory of gravity and investigated the black hole shadow. Using the steady-state Novikov–Thorne model, we study thin accretion disk processes for such rotating black holes including the energy flux, temperature distribution, emission spectrum, energy conversion efficiency as well as the radius of the innermost stable circular orbit. We also study the effects of the Gauss–Bonnet coupling parameter $$\alpha $$ α on these quantities. The results are compared to slowly rotating relativistic Kerr black holes which show that for a positive Gauss–Bonnet coupling, thin accretion disks around rotating black holes in four-dimensional Einstein–Gauss–Bonnet gravity are hotter and more efficient than that for Kerr black holes with the same rotation parameter a, while for a negative coupling they are cooler and less efficient. Thus the accretion disk processes may be considered as tools for testing Einstein–Gauss–Bonnet gravity using astrophysical observations.

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Epicyclic Oscillations around Simpson–Visser Regular Black Holes and Wormholes

Universe ◽

10.3390/universe7080279 ◽

2021 ◽

Vol 7 (8) ◽

pp. 279

Author(s):

Zdeněk Stuchlík ◽

Jaroslav Vrba

Keyword(s):

Black Holes ◽

Black Hole ◽

Active Galactic Nuclei ◽

Observational Data ◽

High Frequency ◽

Galactic Nuclei ◽

Supermassive Black Holes ◽

Periodic Oscillations ◽

High Length ◽

Circular Geodesic

We study epicyclic oscillatory motion along circular geodesics of the Simpson–Visser meta-geometry describing in a unique way regular black-bounce black holes and reflection-symmetric wormholes by using a length parameter l. We give the frequencies of the orbital and epicyclic motion in a Keplerian disc with inner edge at the innermost circular geodesic located above the black hole outer horizon or on the our side of the wormhole. We use these frequencies in the epicyclic resonance version of the so-called geodesic models of high-frequency quasi-periodic oscillations (HF QPOs) observed in microquasars and around supermassive black holes in active galactic nuclei to test the ability of this meta-geometry to improve the fitting of HF QPOs observational data from the surrounding of supermassive black holes. We demonstrate that this is really possible for wormholes with sufficiently high length parameter l.

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Gaseous Disks in the Nuclei of Elliptical Galaxies

International Astronomical Union Colloquium ◽

10.1017/s0252921100043293 ◽

1997 ◽

Vol 163 ◽

pp. 620-625 ◽

Cited By ~ 4

Author(s):

H. Ford ◽

Z. Tsvetanov ◽

L. Ferrarese ◽

G. Kriss ◽

W. Jaffe ◽

...

Keyword(s):

Black Holes ◽

Black Hole ◽

Angular Momentum ◽

Accretion Disks ◽

Large Scale ◽

Elliptical Galaxies ◽

Central Mass ◽

Massive Black Holes ◽

Radio Jets

AbstractHST images have led to the discovery that small (r ~ 1″ r ~ 100 – 200 pc), well-defined, gaseous disks are common in the nuclei of elliptical galaxies. Measurements of rotational velocities in the disks provide a means to measure the central mass and search for massive black holes in the parent galaxies. The minor axes of these disks are closely aligned with the directions of the large–scale radio jets, suggesting that it is angular momentum of the disk rather than that of the black hole that determines the direction of the radio jets. Because the disks are directly observable, we can study the disks themselves, and investigate important questions which cannot be directly addressed with observations of the smaller and unresolved central accretion disks. In this paper we summarize what has been learned to date in this rapidly unfolding new field.

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Black hole hair removal for N = 4 CHL models

Journal of High Energy Physics ◽

10.1007/jhep02(2021)125 ◽

2021 ◽

Vol 2021 (2) ◽

Author(s):

Subhroneel Chakrabarti ◽

Suresh Govindarajan ◽

P. Shanmugapriya ◽

Yogesh K. Srivastava ◽

Amitabh Virmani

Keyword(s):

Black Holes ◽

Black Hole ◽

Degrees Of Freedom ◽

Microscopic Analysis ◽

Flat Space ◽

Special Care ◽

Hair Removal ◽

Partition Functions ◽

Rotating Black Holes

Abstract Although BMPV black holes in flat space and in Taub-NUT space have identical near-horizon geometries, they have different indices from the microscopic analysis. For K3 compactification of type IIB theory, Sen et al. in a series of papers identified that the key to resolving this puzzle is the black hole hair modes: smooth, normalisable, bosonic and fermionic degrees of freedom living outside the horizon. In this paper, we extend their study to N = 4 CHL orbifold models. For these models, the puzzle is more challenging due to the presence of the twisted sectors. We identify hair modes in the untwisted as well as twisted sectors. We show that after removing the contributions of the hair modes from the microscopic partition functions, the 4d and 5d horizon partition functions agree. Special care is taken to present details on the smoothness analysis of hair modes for rotating black holes, thereby filling an essential gap in the literature.

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Symmetry and the Arrow of Time in Theoretical Black Hole Astrophysics

Journal of Gravity ◽

10.1155/2015/530850 ◽

2015 ◽

Vol 2015 ◽

pp. 1-5

Author(s):

David Garofalo

Keyword(s):

Black Holes ◽

Black Hole ◽

Active Galactic Nuclei ◽

Accretion Disks ◽

Time Reversal ◽

Large Scale ◽

Galactic Nuclei ◽

Arrow Of Time ◽

Time Symmetry ◽

The Universe

While the basic laws of physics seem time-reversal invariant, our understanding of the apparent irreversibility of the macroscopic world is well grounded in the notion of entropy. Because astrophysics deals with the largest structures in the Universe, one expects evidence there for the most pronounced entropic arrow of time. However, in recent theoretical astrophysics work it appears possible to identify constructs with time-reversal symmetry, which is puzzling in the large-scale realm especially because it involves the engines of powerful outflows in active galactic nuclei which deal with macroscopic constituents such as accretion disks, magnetic fields, and black holes. Nonetheless, the underlying theoretical structure from which this accreting black hole framework emerges displays a time-symmetric harmonic behavior, a feature reminiscent of basic and simple laws of physics. While we may expect such behavior for classical black holes due to their simplicity, manifestations of such symmetry on the scale of galaxies, instead, surprise. In fact, we identify a parallel between the astrophysical tug-of-war between accretion disks and jets in this model and the time symmetry-breaking of a simple overdamped harmonic oscillator. The validity of these theoretical ideas in combination with this unexpected parallel suggests that black holes are more influential in astrophysics than currently recognized and that black hole astrophysics is a more fundamental discipline.

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