Rotating dirty black hole and its shadow

In this paper, we examine the effect of dark matter to a Kerr black hole of mass m. The metric is derived using the Newman-Janis algorithm, where the seed metric originates from the Schwarzschild black hole surrounded by a spherical shell of dark matter with mass M and thickness Δr_s. The seed metric is also described in terms of a piecewise mass function with three different conditions. Specializing in the non-trivial case where the observer resides inside the dark matter shell, we analyzed how the effective mass of the black hole environment affects the basic black hole properties. A high concentration of dark matter near the rotating black hole is needed to have considerable deviations on the horizons, ergosphere, and photonsphere radius. The time-like geodesic, however, shows more sensitivity to deviation even at very low dark matter density. Further, the location of energy extraction via the Penrose process is also shown to remain unchanged. With how the dark matter distribution is described in the mass function, and the complexity of how the shadow radius is defined for a Kerr black hole, deriving an analytic expression for Δr_s as a condition for notable dark matter effects to occur remains inconvenient.     

Absorption of massless scalar wave from Schwarzschild black hole surrounded by quintessence

By using the partial wave method, we investigate the absorption of massless scalar wave from Schwarzschild black hole surrounded by the quintessence. We obtained the expression of absorption cross section
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Then we numerically carry out the absorption cross section and we find that the larger angular momentum quantum number l is, the smaller the corresponding maximum value of partial absorption cross section is, and that the total absorption cross section tends to geometric-optical limit σ_abs^hf≈ π b_c². We also find that higher value of ω_q (state parameter of the quintessence) corresponds the higher value of absorption cross section σ_abs.     

Spectral features of radiation from Nordström and Kerr-Newman white holes

Unlike the Schwarzschild white hole, Nordström and Kerr-Newman white holes cannot explode right down from the space time singularityR=0. For example a charged white hole has to commence explosion (i.e., comes into existence) with a radiusR ₀=R _c (2?R _c /R _b )^?1 whereR _c is the ‘classical radius’ andR _b is the final radius attained when the stationary state is reached. That means charged and rotating black holes also cannot hit the singularityR=0 and perish. Here the explosion is decelerated by the presence of charge and rotation and hence the radiation emitted would be not as energetic as in the Schwarzschild case where its energy is infinitely large for emission fromR=0.     

Absorption Cross Section of Static Einstein-Maxwell Dilation Axion Black Hole for Scalar Particles

The absorption cross section of the static Einstein-Maxwell dilaton axion （EMDA） black hole for scalar particles is investigated. It is shown that the ratio of the absorption cross section of the EMDA black hole to that of the Schwarzschild black hole decreases as the absolute value of the dilaton increases, and it becomes zero as the dilaton tends to its extremal value. It is also shown that the absorption cross section decreases as both the v and the absolute value of the dilaton increase, and it decreases as the mass of the particle decreases.     

Quantum dynamics of strings in black hole space times

We quantize a bosonic string in the D-dimensional Schwarzschild geometry following the method recently proposed by the present authors. This allows us to take into account strong-curvature effects of the black hole. We start from the exact motion of the center of mass of the string, and compute the quantum fluctuations around it to first and second order. This provides the dominant term for physical magnitudes in an expansion in powers of √α' / R_S (√α' = l_pl = Planck length, R_S = Schwarzschild's radius). The mass spectrum and critical dimension are the same as in flat space-time but there is non-trivial elastic and inelastic scattering by the black hole. Ingoing and outgoing modes are introduced in a light-cone-gauge formalism. A linear transformation relating these modes desribes two main effects: (i) polarization changes and (ii) mixing of the particle and antiparticle modes reversing, at the same time, their right or left character.     

Hidden conformal symmetry of rotating black holes in the five-dimensional Gödel universe

We have investigated the hidden conformal symmetry of generic non-extremal rotating black holes in the five-dimensional Gödel universe. In a range of parameters, the low-frequency massless scalar wave equation in the “near region” can be described by an SL(2, R) _L × SL(2, R) _R conformal symmetry. We further found that the microscopic entropy via Cardy formula matches the macroscopic Bekenstein-Hawking entropy and the absorption cross section for the massless scalar also agrees with the one for the two dimensional finite temperature conformal field theory (CFT). All these evidences support the conjecture that the generic non-extremal rotating black hole immersed in the Gödel universe can be dual to a two dimensional finite temperature CFT. In addition, we have reformulated the first laws of thermodynamics associated with the inner and outer horizons of the rotating Gödel-type black holes into the forms of conformal thermodynamics.     

Quasinormal Modes of Hayward Regular Black Hole

In this paper, using asymptotic iteration method and eikonal limit, the massive scalar quasinormal modes (QNM) is studied in regular Hayward spacetime, which is much similar to Schwarzschild black hole when r→∞ but there is no singularity at the center. We analyze the QNM frequencies ω by varying the parameter β (it is related to mass of black hole and cosmological constant), spherical harmonic index L and the mass of scalar field m. The results show that the effect of β could lead to the real part of ω increase but the imaginary part decrease, which imply that the existence of cosmological constant would impact on the process of a black hole relaxing after it has been perturbed.     

Scattering and absorption of particles by a black hole involving a global monopole

Under the conditions that the wavelength of a particle is much larger than its radius of central mass, and the Schwarzschild field is weak, the scattering of a particle has been studied by many researchers. They obtained that scalar and vector particles abide by Rutherford’s angle distribution by using the low level perturbation method and the scattered field’s approximation in a weak field. The scattering cross section of a photon coincides with the section in Newton’s field of point mass. We can obtain the photon’s polarization effect by calculating the second-order perturbation in the linear Schwarzschild field. This article discusses the scattering and absorption of a particle by a black hole involving a global monopole by using the aforesaid method.     

Absorption/Scattering of Massless Dirac Wave from Black Hole Spacetimes with Cosmic String

In this paper we investigate the scattering of massless Dirac wave from several different black hole spacetimes (i.e. the Schwarzschild black hole, the RN extremal black hole, the Schwarzschild de Sitter black hole, and the extremal Schwarzschild de Sitter black hole) which are influenced by the cosmic string, respectively. All these cases show us that the total absorption cross sections oscillate around the geometric-optical limit and decrease with linear mass density μ of the cosmic string. All of the total scattering cross sections exhibit that the main scattering angle becomes narrower for the high partial frequency wave. Due to the influence of cosmic string, the glory peak becomes wider for larger values of linear mass density μ of the cosmic string.     

Absorption cross section in the topologically massive gravity at the critical point

The absorption cross section for the warped AdS₃ black hole background shows that it is larger than the area even if the s-wave limit is considered. It raises some question whether the deviation from the areal cross section is due to the warped configuration of the geometry or the rotating coordinate system, where these two effects are mixed up in the warped AdS₃ black hole. So, we study the low-frequency scattering dynamics of propagating scalar fields under the warped AdS₃ background at the critical point which reduces to the BTZ black hole in the rotating frame without the warped factor, which shows that the deformation effect at the critical point does not appear.     

Black Hole Scattering via Spectral Methods

We present an alternative method to solve the problem of scattering by a black hole by adapting the spectral code originally developed by Boyd (Comp Phys 4:83, 1990). In order to show the effectiveness and versatility of the algorithm, we solve the scattering by Schwarzschild, standard acoustic, and charged black holes. We recover the partial and total absorption cross sections and, in the case of charged black holes, the conversion factor of eletromagnetic and gravitational waves. We also study the exponential decay of the reflection coefficient, which is a general feature of any scattering problem.     

Photoionization of the outer electrons in noble gas endohedral atoms

We suggest a prominent modification of the outer shell photoionization cross section in noble gas (NG) endohedral atoms NG@C _n under the action of the electron shell of fullerene C _n . This shell leads to two important effects: a strong enhancement of the cross section due to fullerene shell polarization under the action of the incoming electromagnetic wave and to prominent oscillation of this cross section due to the reflection of a photoelectron from the NG by the fullerene shell. Both factors lead to powerful maxima in the outer shell ionization cross sections of NG@C _n , which we call giant endohedral resonances. The oscillator strength reaches a very large value in the atomic scale, 25. We consider atoms of all noble gases except He. The polarization of the fullerene shell is expressed in terms of the total photoabsorption cross section of the fullerene. The photoelectron reflection is taken into account in the framework of the so-called bubble potential, which is a spherical δ-type potential. It is assumed in the derivations that the NG is centrally located in the fullerene. It is also assumed, in accordance with the existing experimental data, that the fullerene radius R _C is much larger than the atomic radius r _A and the thickness Δ_C of the fullerene shell. As was demonstrated recently, these assumptions allow us to represent the NG@C _n photoionization cross section as a product of the NG cross section and two well-defined calculated factors. The text was submitted by the authors in English.     

Segregation during crystal growth from melt and absorption cross section determination by optical absorption method

Segregation during crystal growth from melt under two conditions is studied by using crystal mass, which can be measured easily, as an independent variable, and a method to determine the effective segregation coefficient and absorption cross section of optical dopant is given. When the segregated solute disperses into the whole or just a part of melt homogenously, the concentration C _s in solid interface will change by different formulas. If the crystal growth interface is conical and segregated solute disperses into melt in total or part, the solute concentration at r = 2/3R, where r is the distance from the growth cross section center and R the crystal radius, is independent on the shape of the crystal growth interface, and its variation at r = 2/3R can be regarded as the result from crystal growth in flat interface. With C _s variation formula in solid and absorption cross section σ for optical dopant, the absorption coefficients along the crystal growth direction can be calculated, and the corresponding experimental value can be obtained through the crystal optical absorption spectra. By minimizing the half sum, whose independent variables are k, ΔW or σ, of the difference square between the calculated and experimental absorption coefficients from one or more absorption peaks along the crystal growth direction, k and σ, or k and ΔW, can be determined at the same time through the Levenberg-Marquardt iteration method. Finally, the effective segregation coefficient k, ΔW and absorption cross sections of Nd:GGG were determined, the results fitted by two formula gave more closed effective segregation coefficient, and the value ΔW also indicates that the segregated dopant had nearly dispersed into the whole melt. Experimental results show that the method to determine effective segregation coefficient k, ΔW and absorption cross sections σ is convenient and reliable, and the two segregation formulas can describe the segregation during the crystal growth from melt relatively commendably. Supported by the National Natural Science Foundation of China (Grant No. 50772112)     

Formation of black holes in quantum cosmology

The formation of black holes in the quantum cosmology scheme has been discussed by means of calculating the wave function of the universe with a black hole, which is described by a Schwarzschild-de Sitter metric. The average radius of the Schwarzschild black holes formed in the process of the birth of the universe is shown to be about l_p⁶H²/a³, where l_p is the Planck length; ∧=3H² is the cosmological constant; a is the radius of the universe when it enters into the classical era.     

Hidden conformal symmetry of self-dual warped AdS<Subscript>3</Subscript> black holes in topological massive gravity

We extend the recently proposal of hidden conformal symmetry to the self-dual warped AdS₃ black holes in topological massive gravity. It is shown that the wave equation of massive scalar field with sufficient small angular momentum can be reproduced by the SL(2, R) Casimir quadratic operator. Due to the periodic identification in the φ direction, it is found that only the left section of hidden conformal symmetry is broken to U(1), while the right section is unbroken, which only gives the left temperature of dual CFT. As a check of the dual CFT conjecture of self-warped AdS₃ black hole, we further compute the Bekenstein–Hawking entropy and absorption cross section and quasinormal modes of scalar field perturbation and show these are just of the forms predicted by the dual CFT.     

Nonsingular black hole

We consider the Schwarzschild black hole and show how, in a theory with limiting curvature, the physical singularity “inside it” is removed. The resulting spacetime is geodesically complete. The internal structure of this nonsingular black hole is analogous to Russian nesting dolls. Namely, after falling into the black hole of radius \(r_{g}\), an observer, instead of being destroyed at the singularity, gets for a short time into the region with limiting curvature. After that he re-emerges in the near horizon region of a spacetime described by the Schwarzschild metric of a gravitational radius proportional to \(r_{g}^{1/3}\). In the next cycle, after passing the limiting curvature, the observer finds himself within a black hole of even smaller radius proportional to \(r_{g}^{1/9}\), and so on. Finally after a few cycles he will end up in the spacetime where he remains forever at limiting curvature.     

Orbital resonances in discs around braneworld Kerr black holes

Rotating black holes in the brany universe of the Randall–Sundrum type with infinite additional dimension are described by the Kerr geometry with a tidal charge b representing the interaction of the brany black hole and the bulk spacetime. For b < 0 rotating black holes with dimensionless spin a > 1 are allowed. We investigate the role of the tidal charge in the orbital resonance model of quasiperiodic oscillations (QPOs) in black hole systems. The orbital Keplerian frequency v _K and the radial and vertical epicyclic frequencies v _r, v _θ of the equatorial, quasicircular geodetical motion are given. Their radial profiles related to Keplerian accretion discs are discussed, assuming the inner edge of the disc located at the innermost stable circular geodesic. For completeness, naked singularity spacetimes are considered too. The resonant conditions are given in three astrophysically relevant situations: for direct (parametric) resonances of the oscillations with the radial and vertical epicyclic frequencies, for the relativistic precession model, and for some trapped oscillations of the warped discs, with resonant combinational frequencies involving the Keplerian and radial epicyclic frequencies. It is shown, how the tidal charge could influence matching of the observational data indicating the 3 : 2 frequency ratio observed in GRS 1915 + 105 microquasar with prediction of the orbital resonance model; limits on allowed range of the black hole parameters a and b are established. The “magic” dimensionless black hole spin enabling presence of strong resonant phenomena at the radius, where v _K: v _θ : v _r = 3 : 2 : 1, is determined in dependence on the tidal charge. Such strong resonances could be relevant even in sources with highly scattered resonant frequencies, as those expected in Sgr A*. The specific values of the spin and tidal charge are given also for existence of specific radius where v _K : v _θ : v _r = s : t : u with 5≥s >t >u being small natural numbers. It is shown that for some ratios such situation is impossible in the field of black holes. We can conclude that analysing the microquasars high-frequency QPOs in the framework of orbital resonance models, we can put relevant limits on the tidal charge of brany Kerr black holes.     

On the strong influence of inner shell resonances upon the outer shell photoionization of endohedral atoms

It is demonstrated by the example of the Xe atom stuffed inside the C₆₀ fullerene, i.e., the endohedral Xe@C₆₀, that the so-called confinement resonances in 4d subshell strongly affect the photoionization cross section of outer 5p and subvalent 5s electrons near the 4d ionization threshold. It is a surprise that these narrow inner 4d shell resonances are not smeared out in the outer shell photoionization cross section. On the contrary; the inner shell resonances affect the outer cross section by enhancing them enormously. Close to its own photoionization thresholds, 5p and 5s photoionization cross sections of Xe@C₆₀ are dominated by their own confinement resonances greatly affected by the amplification of the incoming radiation intensity due to polarization by it of the C₆₀ electron shell. Between 4d and 5p thresholds, the effect of 4d is becoming stronger while own resonances of 5p and 5s are becoming much less important.     

Three particle correlation function of metal ions in tetrahedral coordination determined by XANES

The three particle correlation function of local atomic distribution at metal ion sites in solutions has been extracted from XANES (X-ray absorption near edge structure) spectra of [CrO₄]²⁻ and [MnO₄]⁻ ions measured by using synchrotron radiation. The absorption cross section for 1s core level excitation is calculated in the real space multiple scattering approach using the Hedin and Lundqvist energy dependent potential. We show that in these systems the expansion of the total absorption cross section in terms of contributions of higher order scattering processes is possible over a large energy range. This gives a unified theory of XANES and EXAFS and allows the third order correlation function to be extracted from experimental data.