Keplerian Angular Momentum Research Articles

Construction of a pseudo-Newtonian potential for a spinning black hole embedded in quintessence background: Brief accretion studies

It is found difficult to construct a complete general relativistic theory for an accretion disc around a relativistic compact star. There exists a trend to reproduce the effects of general relativistic attraction by using a pseudo-Newtonian approach. One such pseudo-Newtonian force for a spinning black hole sitting in quintessence background is constructed in this paper. Comparison with general relativity at different parametric values is picturised thoroughly. Next, an accretion model using this newly constructed pseudo-Newtonian force is built. The radial inward speed gradient is found to be expressed as a ratio of two functions. It can be realized easily that among these two functions, the denominator might vanish for a particular value of radial speed in the interval zero to speed of light. To make the flow physical, the numerator should vanish at the same radial distance. This will ultimately generate two speed profiles starting from the particular radial distance. This is the cause of generation for accretion and wind branches. It has been followed that the disc is truncated at a finite distance due to the fact that the specific angular momentum becomes larger than the Keplerian angular momentum beyond that. Effects of the spin of the central gravitating object are followed along with the intense effect of the quintessence parameters concerned. Accretion and wind density profiles are also studied to find the density variations near the central engine. Ratio of shear viscosity to entropy density is analyzed in the presence of quintessence for viscous accretion flows.

Accretion onto a quintessence contaminated rotating black hole: violating the lower limit for eta over s

Viscous accretion flow around a rotating supermassive black hole sitting in a quintessence tub is studied in this article. To introduce such a dark energy contaminated black hole’s gravitational force, a new pseudo-Newtonian potential is used. This pseudo-Newtonian force can be calculated if we know the distance from the black hole’s center, spin of the black hole and equation of state of the quintessence inside which the black hole is considered to lie. This force helps us to avoid complicated nonlinearity of general relativistic field equations. Transonic, viscous, continuous and Keplerian flow is assumed to take place. Fluid speed, sonic speed profile and specific angular momentum to Keplerian angular momentum ratio are found out for different values of spin parameter and quintessence parameter. Density variation is built and tallied with observations. Shear viscosity to entropy density ratio is constructed for our model and a comparison with theoretical lower limit is done.

General-relativistic instability in rapidly accreting supermassive stars: The impact of rotation

Context. Supermassive stars (SMSs) collapsing via the general-relativistic (GR) instability are invoked as the possible progenitors of supermassive black holes. Their mass and angular momentum at the onset of the instability are key in many respects, in particular regarding the possibility for observational signatures of direct collapse. Accretion dominates the evolution of SMSs and, similar to rotation, it has been shown to impact their final properties significantly. However, the combined effect of accretion and rotation on the stability of these objects is not known. Aims. Here, we study the stability of rotating, rapidly accreting SMSs against GR perturbations and derive the properties of these stars at death. Methods. On the basis of hylotropic structures, which are relevant for rapidly accreting SMSs, we define rotation profiles under the assumption of local angular momentum conservation in radiative regions, which allows for differential rotation. We account for rotation in the stability of the structure by adding a Newtonian rotation term in the relativistic equation of stellar pulsation, which is justified by the slow rotations imposed by the ΩΓ-limit. Results. We find that rotation favours the stability of rapidly accreting SMSs as soon as the accreted angular momentum represents a fraction of f ≳ 0.1% of the Keplerian angular momentum. For f ∼ 0.3−0.5%, the maximum masses consistent with GR stability are increased by an order of magnitude compared to the non-rotating case. For f ∼ 1%, the GR instability cannot be reached if the stellar mass does not exceed 107 − 108 M⊙. Conclusions. These results imply that, as in the non-rotating case, the final masses of the progenitors of direct collapse black holes range in distinct intervals depending on the scenario considered: 105 M⊙ ≲ M ≲ 106 M⊙ for primordial atomically cooled haloes and 106 M⊙ ≲ M ≲ 109 M⊙ for metal-rich galaxy mergers. The models suggest that the centrifugal barrier is inefficient to prevent the direct formation of a supermassive black hole at the collapse of a SMS. Moreover, the conditions of galaxy mergers appear to be more favourable than those of atomically cooled haloes for detectable gravitational wave emission and ultra-long gamma-ray bursts at black hole formation.

Viscous dark energy accretion activities : sonic speed, angular momentum and Mach number studies

In this present article, we study different accretion properties regarding viscous accretion of dark energy. Modified Chaplygin gas is chosen as the dark energy candidate. Viscosity is encountered with the help of Shakura–Sunyaev viscosity parameter. We study sonic speed vs radial distance curves. We compare between adiabatic and dark energy dominated cases and follow that sonic speed falls as we go nearer to the central gravitating object. As viscosity is imposed, a threshold drop in accretion sonic speed is followed. Average rate of fall in accretion sonic speed is increased with black hole’s spin. This is signifying that this kind of accretion is weakening the overall matter/energy infall. Specific angular momentum to Keplerian angular momentum ratio is found to fall as we go far from the black hole. Accretion Mach number turns high as we go towards the inner region and high wind Mach number is not allowed as we are going out. Combining, we conclude that the system weakens the feeding process of accretion.

On the Rotation of Supermassive Stars

Abstract Supermassive stars (SMSs) born from pristine gas in atomically cooled halos are thought to be the progenitors of supermassive black holes at high redshifts. However, the way they accrete their mass is still an unsolved problem. In particular, for accretion to proceed, a large amount of angular momentum has to be extracted from the collapsing gas. Here, we investigate the constraints stellar evolution imposes on this angular momentum problem. We present an evolution model of a supermassive Population III star simultaneously including accretion and rotation. We find that, for SMSs to form by accretion, the accreted angular momentum has to be about 1% of the Keplerian angular momentum. This tight constraint comes from the limit, at which the combination of radiation pressure and centrifugal force cancels gravity. It implies that SMSs are slow rotators, with a surface velocity less than 10%–20% of their first critical velocity, at which the centrifugal force alone cancels gravity. At such low velocities, the deformation of the star due to rotation is negligible.

Hot Accretion onto Black Holes with Outflow

Classic Bondi accretion flow can be generalized to rotating viscous accretion flow. Study of hot accretion flow onto black holes show that its physical charateristics change from Bondi-like for small gas angular momentum to disk-like for Keperian gas angular momentum. Especially, the mass accretion rate divided by the Bondi accretion rate is proportional to the viscosity parameter alpha and inversely proportional to the gas angular momentum divided by the Keplerian angular momentum at the Bondi radius for gas angular momentum comparable to the Keplerian value. The possible presence of outflow will increase the mass inflow rate at the Bondi radius but decrease the mass accretion rate across the black hole horizon by many orders of magnitude. This implies that the growth history of supermassive black holes and their coevolution with host galaxies will be dramatically changed when the accreted gas has angular momentum or develops an outflow.

Maximum mass, moment of inertia and compactness of relativistic stars

A number of recent works have highlighted that it is possible to express the properties of general-relativistic stellar equilibrium configurations in terms of functions that do not depend on the specific equation of state employed to describe matter at nuclear densities. These functions are normally referred to as "universal relations" and have been found to apply, within limits, both to static or stationary isolated stars, as well as to fully dynamical and merging binary systems. Further extending the idea that universal relations can be valid also away from stability, we show that a universal relation is exhibited also by equilibrium solutions that are not stable. In particular, the mass of rotating configurations on the turning-point line shows a universal behaviour when expressed in terms of the normalised Keplerian angular momentum. In turn, this allows us to compute the maximum mass allowed by uniform rotation, M_{max}, simply in terms of the maximum mass of the nonrotating configuration, M_{TOV}, finding that M_{max} ~ (1.203 +- 0.022) M_{TOV} for all the equations of state we have considered. We further show that a universal relation can be found between the dimensionless moment of inertia and the stellar compactness. Although this relation is not surprising as it involves two quantities that have been shown to exhibit universal behaviour with other stellar properties, our parameterisation represents a refinement over a similar relation by Lattimer and Schutz (2005), where a different normalisation was used, and could provide an accurate tool to constrain the equation of state of nuclear matter when measurements of the moment of inertia become available.

The pulse profile and spin evolution of the accreting pulsar in Terzan 5, IGR J17480−2446, during its 2010 outburst

(abridged) We analyse the spectral and pulse properties of the 11 Hz transient accreting pulsar, IGR J17480-2446, in the globular cluster Terzan 5, considering all the available RXTE, Swift and INTEGRAL observations performed between October and November, 2010. By measuring the pulse phase evolution we conclude that the NS spun up at an average rate of <nu_dot>=1.48(2)E-12 Hz/s, compatible with the accretion of the Keplerian angular momentum of matter at the inner disc boundary. Similar to other accreting pulsars, the stability of the pulse phases determined by using the second harmonic component is higher than that of the phases based on the fundamental frequency. Under the assumption that the second harmonic is a good tracer of the neutron star spin frequency, we successfully model its evolution in terms of a luminosity dependent accretion torque. If the NS accretes the specific Keplerian angular momentum of the in-flowing matter, we estimate the inner disc radius to lie between 47 and 93 km when the luminosity attains its peak value. Smaller values are obtained if the interaction between the magnetic field lines and the plasma in the disc is considered. The phase-averaged spectrum is described by thermal Comptonization of photons with energy of ~1 keV. A hard to soft state transition is observed during the outburst rise. The Comptonized spectrum evolves from a Comptonizing cloud at an electron temperature of ~20 keV towards an optically denser cloud at kT_e~3 keV. At the same time, the pulse amplitude decreases from 27% to few per cent and becomes strongly energy dependent. We discuss various possibilities to explain such a behaviour, proposing that at large accretion luminosities a significant fraction of the in-falling matter is not channelled towards the magnetic poles, but rather accretes more evenly onto the NS surface.

HUGE EJECTION OF ANTIELECTRON NEUTRINOS FROM MASSIVE ACCRETION DISKS

The standard massive accretion disk with Keplerian angular momentum (standard accretion disk) producing gamma-ray bursts (GRBs) is investigated on the bases of the microphysics of neutrinos and general relativity. Since the accretion disk gradually heated by viscosity is efficiently cooled by antielectron neutrinos, , the accreting flow maintains a relatively low temperature, T 3 × 1010 K, over a long range of accreting radius that produces very high dense matter around a rotating black hole, ρ ≥ 1013 g cm–3. Thus, the massively accreting matter is in the domain of heavy nuclei all over the accreting flow onto a central black hole where the fraction of evaporated free neutrons is large, Yn 0.8, and that of protons is infinitesimal, Yp 10–4. The electron neutrinos in the disk are almost absorbed by rich neutrons while the antielectron neutrinos are little absorbed by rarefied protons. The mean energy of antielectron neutrinos ejected from the disk is extraordinarily high, MeV, because the antielectron neutrinos are degenerated in the high dense disk. The huge antielectron neutrinos with high mean energy and large luminosity, erg s–1, are ejected from the massive accretion disk. The antielectron neutrinos are possibly the sources of the relativistic jets producing GRBs.

Two temperature viscous accretion flows around rotating black holes: Description of under-fed systems to ultra-luminous X-ray sources

We discuss two temperature accretion disk flows around rotating black holes. As we know that to explain observed hard X-rays the choice of Keplerian angular momentum profile is not unique, we consider the sub-Keplerian regime of the disk. Without any strict knowledge of the magnetic field structure, we assume the cooling mechanism is dominated by bremsstrahlung process. We show that in a range of Shakura–Sunyaev viscosity parameter 0.2 ≳ α ≳ 0.0005 , flow behavior varies widely, particularly by means of the size of disk, efficiency of cooling and corresponding temperatures of ions and electrons. We also show that the disk around a rotating black hole is hotter compared to that around a Schwarzschild black hole, rendering a larger difference between ion and electron temperatures in the former case. With all the theoretical solutions in hand, finally we reproduce the observed luminosities ( L) of two extreme cases—the under-fed AGNs and quasars (e.g. Sgr A ∗ ) with L ≳ 10 33 erg / s to ultra-luminous X-ray sources with L ∼ 10 41 erg/s, at different combinations of mass accretion rate, ratio of specific heats, Shakura–Sunyaev viscosity parameter and Kerr parameter, and conclude that Sgr A ∗ may be an intermediate spinning black hole.

Oscillation modes of relativistic slender tori

Accretion flows with pressure gradients permit the existence of standing waves which may be responsible for observed quasi-periodic oscillations (QPO's) in X-ray binaries. We present a comprehensive treatment of the linear modes of a hydrodynamic, non-self-gravitating, polytropic slender torus, with arbitrary specific angular momentum distribution, orbiting in an arbitrary axisymmetric space-time with reflection symmetry. We discuss the physical nature of the modes, present general analytic expressions and illustrations for those which are low order, and show that they can be excited in numerical simulations of relativistic tori. The mode oscillation spectrum simplifies dramatically for near Keplerian angular momentum distributions, which appear to be generic in global simulations of the magnetorotational instability. We discuss our results in light of observations of high frequency QPO's, and point out the existence of a new pair of modes which can be in an approximate 3:2 ratio for arbitrary black hole spins and angular momentum distributions, provided the torus is radiation pressure dominated. This mode pair consists of the axisymmetric vertical epicyclic mode and the lowest order axisymmetric breathing mode.

Radiative Collimation of Electron–Positron Jets II

Abstract We consider a collimation and acceleration mechanism for relativistic jets emanating from an inner region of a relativistic accretion disk surrounding a Kerr black hole, with Keplerian angular momentum under disk radiation fields, taking into account the effect of radiation drag. When the jets consist of electron–positron pair plasmas and the disk luminosity is of the order of the Eddington luminosity, we found that the terminal speed is about $0.9 \,c$ for a Schwarzschild case, while it becomes $\sim 0.96 \,c$ for an extreme Kerr case. In such a case, the opening angle of jets is of the order of $5^\circ$. We also examined the opening angle and the listing degree when the disk radiation is not axisymmetric, but has a one-armed pattern. For the case of 0.3 amplitude, the opening angle is of the order of a few degrees, and the jet direction is bent by several degrees to $10^\circ$.

Global Magnetohydrodynamic Simulations of Cylindrical Keplerian Disks

This paper presents a series of global three-dimensional accretion disk simulations carried out in the cylindrical limit in which the vertical component of the gravitational field is neglected. The simulations use a cylindrical pseudo-Newtonian potential, ∝1/(R - Rg), to model the main dynamical properties of the Schwarzschild metric. The radial grid domain runs out to 60Rg to minimize the influence of the outer boundary on the inner disk evolution. The disks are initially constant density with a Keplerian angular momentum distribution and contain a weak toroidal or vertical field that serves as the seed for the magnetorotational instability. These simulations reaffirm many of the conclusions of previous local simulations. The magnetorotational instability (MRI) grows rapidly and produces MHD turbulence with a significant Maxwell stress that drives accretion. Tightly wrapped low-m spiral waves are prominent. In some simulations radial variations in Maxwell stress concentrate gas into rings, creating substantial spatial inhomogeneities. As in previous global simulations, there is a nonzero stress at the marginally stable orbit. The stress is smaller than seen in stratified torus simulations but nevertheless produces a small decline in specific angular momentum inside the last stable orbit. Detailed comparisons between simulations are used to examine the effects of various choices in computational setup. Because the driving instability is local, a reduction in the azimuthal computational domain to some fraction of 2π does not create large qualitative differences. Similarly, the choice of either an isothermal or adiabatic equation of state has little impact on the initial evolution. Simulations that begin with vertical fields have greater field amplification and higher ratios of stress to magnetic pressure compared with those beginning with toroidal fields. In contrast to MHD, hydrodynamics alone neither creates nor sustains turbulence.

Nonlinear Evolution of the Magnetorotational Instability in Ion‐Neutral Disks

We carry out three-dimensional magnetohydrodynamical simulations of the magnetorotational (Balbus-Hawley) instability in weakly ionized plasmas. We adopt a formulation in which the ions and neutrals are treated as separate fluids coupled only through a collisional drag term. Ionization and recombination processes are not considered. The linear stability of the ion-neutral system has been previously considered by Blaes & Balbus. Here we extend their results to the nonlinear regime by computing the evolution of the Keplerian angular momentum distribution in the local shearing box approximation. We find significant turbulence and angular momentum transport when the collisional frequency is on the order of 100 times the orbital frequency Ω. At higher collision rates, the two-fluid system studied here behaves much like the fully ionized systems studied previously. At lower collision rates, the evolution of the instability is determined primarily by the properties of the ions, with the neutrals acting as a sink for the turbulence. Since in this regime saturation occurs when the magnetic field is superthermal with respect to the ion pressure, we find that the amplitude of the magnetic energy and the corresponding angular momentum transport rate is proportional to the ion density. Our calculations show that the ions and neutrals are essentially decoupled when the collision frequency is less than 0.01Ω; in this case, the ion fluid behaves as in the single-fluid simulations and the neutrals remain quiescent. We find that purely toroidal initial magnetic field configurations are unstable to the magnetorotational instability across the range of coupling frequencies.

The profile of Keplerian angular momentum in relativistic fluids

This paper gives a correct expression for the Keplerian angular momentum of a relativistic fluid, thereby clearing up some previous misunderstanding on this topic. A possible application of the result obtained to the theory of accretion disks is indicated.

General Relativistic Structure of Star-Toroid Systems

view Abstract Citations (26) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS General Relativistic Structure of Star-Toroid Systems Nishida, Shogo ; Eriguchi, Yoshiharu ; Lanza, Antonio Abstract General relativistic equilibrium configurations of a star-toroid system in a stationary axisymmetric space- time have been computed. The system consists of a rapidly rotating polytropic star and a surrounding polytropic self-gravitating thick toroid. As the gravity of the toroid deforms the central star, less angular velocity is needed for the central star to shed mass from the equator. Due to dragging of the massive toroid the angular velocity and the angular momentum can have opposite signs. Thus this situation is similar to the black hole-toroid system investigated previously by Will, Chakrabarti, & Lanza. The Keplerian angular momentum distribution for the spacetime is also discussed. Publication: The Astrophysical Journal Pub Date: December 1992 DOI: 10.1086/172090 Bibcode: 1992ApJ...401..618N Keywords: ACCRETION; ACCRETION DISKS; RELATIVITY; STARS: ROTATION full text sources ADS |

Effects of a self-gravitating disc on test particle motion around a Kerr black hole

The nature of test particle motion in the presence of a rotating ring of self-gravitating matter around a Kerr black hole is studied. A pseudo-Kerr potential is used to describe the rotating hole. The result shows that, as the mass of the disc is increased, the radius of the marginally bound orbit decreases, whereas the location of the marginally stable orbit, as well as the maximum fraction of the binding energy that can be released, increases. The Keplerian angular momentum distribution, the effects due to dragging of inertial frames by the rotating ring and the force acting on a test particle are also studied

Nuclear runaways in a C/O white dwarf accreting H-rich material possessing angular momentum

The nova outburst phenomenon is presently studied in light of the results of numerical calculations for the accretion of H-rich material onto 1-solar mass C/O white dwarfs, with arrival angular momentums of the order of 10-100 percent of the Keplerian values. The range of models investigated lies between the two limiting cases of radial accretion and accretion with Keplerian angular momentum. It is judged in view of the results obtained that certain important physical effects are being excluded; one of these may be the loss of angular momentum from the white dwarf's surface layers, either inwardly to the core or outwardly through the disk. 25 references.