Narrow Results

The semiclassical effects of anti-evaporating black holes can be discussed in the framework of f(R) gravity. In particular, the Bousso–Hawking–Nojiri–Odinstov anti-evaporation instability of degenerate Schwarzschild–de Sitter black holes (the so-called Nariai spacetime) leads to a dynamical increasing of black hole horizon in f(R) gravity. This phenomenon causes the following transition: emitting marginally trapped surfaces (TS) become space-like surfaces before the effective Bekenstein–Hawking emission time. As a consequence, Bousso–Hawking thermal radiation cannot be emitted in an anti-evaporating Nariai black hole. Possible implications in cosmology and black hole physics are also discussed.

In this work, we analyze the Joule–Thomson expansion in an AdS Reissner–Nordström black hole, surrounded by an average cosmological fluid, in Rainbow Gravity. We plot the graphs corresponding to the inversion temperature curves and numerically calculate the ratio between the minimum of the inversion temperature and the critical temperature, with the aim of investigating how Rainbow Gravity alters such behaviors compared to General Relativity.

A perturbative solution describing a two-body system consisting of a Reissner-Nordström black hole and a charged massive particle at rest is presented. The coincidence between such a solution and the linearized form of the recently obtained Belinski-Alekseev exact solution is explicitly shown.

We discuss semiclassical Nariai black holes in the framework of f(T)-gravity. For a diagonal choice of tetrads, stable Nariai metrics can be found, emitting Bekenstein–Hawking radiation in semiclassical limit. However, for a nondiagonal choice of tetrads, evaporation and anti-evaporation instabilities are turned on. In turn, this causes a backreaction effect suppressing the Bekenstein–Hawking radiation. In particular, evaporation instabilities produce a new radiation — different by Bekenstein–Hawking emission — nonviolating unitarity in particle physics sector.

A Gaussian approximation to the bosonic part of M-(atrix) theory with mass deformation is considered at large values of the dimension $d$. From the perspective of the gauge/gravity duality this action reproduces with great accuracy the stringy Hagedorn phase transition from a confinement (black string) phase to a deconfinement (black hole) phase whereas from the perspective of the matrix/geometry approach this action only captures a remnant of the geometric Yang–Mills-to-fuzzy-sphere phase where the fuzzy sphere solution is only manifested as a three-cut configuration termed the “baby fuzzy sphere” configuration. The Yang–Mills phase retains most of its characteristics with two exceptions: (i) the uniform distribution inside a solid ball suffers a crossover at very small values of the gauge coupling constant to a Wigner’s semicircle law, and (ii) the uniform distribution at small values of the temperatures is nonexistent.

A consistent QM/NCG duality is put forward as a model for the ${\mathrm{\text{AdS}}}^2/{\mathrm{\text{CFT}}}_1$ correspondence. This is a duality/correspondence between (1) the dAFF conformal quantum mechanics (QM) on the boundary (which is only “quasi-conformal” in the sense that there is neither an $SO(1,2)$-invariant vacuum state nor there are strictly speaking primary operators), and between (2) the noncommutative geometry of ${\mathrm{\text{AdS}}}_{𝜃}^2$ in the bulk (which is only “quasi-AdS” in the sense of being only asymptotically ${\mathrm{\text{AdS}}}^2$). The Laplacian operators on noncommutative ${\mathrm{\text{AdS}}}_{𝜃}^2$ and commutative ${\mathrm{\text{AdS}}}^2$ have the same spectrum and thus their correlators are conjectured to be identical. These bulk correlation functions are found to be correctly reproduced by appropriately defined boundary quantum observables in the dAFF quantum mechanics. Moreover, these quasi-primary operators on the boundary form a subalgebra of the operator algebra of noncommutative ${\mathrm{\text{AdS}}}_{𝜃}^2$.

In this paper, we present the construction of noncommutative ${\mathrm{\text{AdS}}}_{𝜃}^2$ black hole and its four-dimensional Yang–Mills IKKT-type matrix model which includes two competing Myers term one responsible for the condensation of pure ${\mathrm{\text{AdS}}}_{𝜃}^2$ and the other one responsible for the condensation of the dilaton field. It is argued that the phase diagram of this matrix model features three phases: (1) a gravitational phase (${\mathrm{\text{AdS}}}_{𝜃}^2$ black hole), (2) a geometric phase (${\mathrm{\text{AdS}}}_{𝜃}^2$ background) and (3) a Yang–Mills phase. The Hawking process is therefore seen as an exotic line of discontinuous transitions between the gravitational and geometrical phases. Alternatively, a noncommutative nonlinear sigma model describing the transition of the dilaton field between the gravitational and geometrical phases is also constructed.

The near-horizon noncommutative geometry of black holes, given by ${\mathrm{\text{AdS}}}_{𝜃}^2\times {𝕊}_N^2$, is discussed and the phase structure of the corresponding Yang–Mills matrix models is presented. The dominant phase transition as the system cools down, i.e. as the gauge coupling constant is decreased is an emergent geometry transition between a geometric noncommutative ${\mathrm{\text{AdS}}}_{𝜃}^2\times {𝕊}_N^2$ phase (discrete spectrum) and a Yang–Mills matrix phase (continuous spectrum) with no background geometrical structure. We also find a possibility for topology change transitions in which space or time directions grow or decay as the temperature is varied. Indeed, the noncommutative near-horizon geometry ${\mathrm{\text{AdS}}}_{𝜃}^2\times {𝕊}_N^2$ can evaporate only partially to a fuzzy sphere ${𝕊}_N^2$ (emergence of time) or to a noncommutative anti-de Sitter space–time ${\mathrm{\text{AdS}}}_{𝜃}^2$ (topology change).

The transformation laws for the electric and magnetic parts of the electromagnetic 2-form and the Weyl tensor under a boost are studied using the complex vector approach, which shows the close analogy between the two cases. For a nonnull electromagnetic field, one can always find an observer who sees parallel electric and magnetic fields (vanishing Poynting vector) and also sees a minimum electromagnetic energy density (and minimum electric and magnetic field magnitudes) compared to other observers. For Weyl fields of all Petrov types except III, and boosts along certain directions of the Weyl principal tetrad, the more complicated Weyl transformation closely mimics the electromagnetic boost transformation, allowing one to extend the electromagnetic results directly to the families of boosts along those directions. In particular for black hole spacetimes, the alignment of the electric and magnetic parts of the Weyl tensor (vanishing super-Poynting vector) leads to minimal gravitational super-energy as seen by the Carter observer within the family of all circularly rotating observers at each spacetime point outside the horizon.

The collapse of massive stars have been used to explain many of the largest outbursts known to mankind, from supernovae to hypernovae to gamma-ray bursts. These explosions differ in their level of asymmetry and the spectral energy of the photons they emit. It is likely that such a wide range in the nature of these explosions requires more than one explosion mechanism to extract the gravitational potential energy released during the collapse. Three major classes of mechanisms have been proposed: neutrino-driven, magnetic-field driven, collapsar (black hole accretion disk) driven. This review discusses each mechanism in turn, discussing the current state-of-the-art calculations along with their observational predictions. We conclude with a summary of the current observational constraints on these models.

The mathematical simplicity of black holes, combined with their links to some of the most energetic events in the universe, means that black holes are key objects for fundamental physics and astrophysics. Until recently, it was generally believed that black holes in nature appear in two broad mass ranges: stellar-mass (M~3–20 M_⊙), which are produced by the core collapse of massive stars, and supermassive (M~10⁶–10¹⁰ M_⊙), which are found in the centers of galaxies and are produced by a still uncertain combination of processes. In the last few years, however, evidence has accumulated for an intermediate-mass class of black holes, with M~10²–10⁴ M_⊙. If such objects exist they have important implications for the dynamics of stellar clusters, the formation of supermassive black holes, and the production and detection of gravitational waves. We review the evidence for intermediate-mass black holes and discuss future observational and theoretical work that will help clarify numerous outstanding questions about these objects.

A sample of 39 blazars with well-established rapid variability timescale and bolometric luminosity has been compiled from literature. Based on the assumption that central supermassive black holes (SMBHs) are Kerr black holes, the upper limits of SMBHs were estimated. The masses ranged from 10^7.2M⊙ to 10^9.4M_⊙, showing a distribution of three subclasses: massive flat spectra radio quasars (FSRQs) and smaller mass BL Lacs, occupy separate regions, while medium mass FSRQ and BL Lac object bridge the gap. In addition, we complied a sample of radio galaxies including 9 sources for which their black holes masses have been derived from the M_BH-σ correlation and their optical and infrared data have been well observed. We find that the intrinsic accretion rates are quite different between FSRQs, BL Lacs and the radio galaxies. The diagram of the intrinsic accretion rate–luminosity relations shows that FSRQs occur in the earlier, high luminosity, violent phase of galactic evolution sequence, while BL Lacs occur in the low luminosity, transition phase between quasars and radio galaxies, and the radio galaxies occur in the late stage of the elliptical galaxy evolution sequence. Our theoretical results prove that the evolutionary track of the elliptical galaxy evolutionary sequence is from FSRQs to BL Lacs and then to the radio galaxies. The evolution diagram of blazars derived in this paper seems to be similar to Hertzsprung–Russell diagram of star evolution.

A theoretical attempt to identify the physical process responsible for the afterglow emission of Gamma-Ray Bursts (GRBs) is presented, leading to the occurrence of thermal emission in the comoving frame of the shock wave which gives rise to the bursts. The determination of the luminosities and spectra involves integration over an infinite number of Planckian spectra, weighted by appropriate relativistic transformations, each one corresponding to a different viewing angle in the past light cone of the observer. The relativistic transformations have been computed using the equations of motion of GRBs within our theory, giving special attention to the determination of the equitemporal surfaces. The only free parameter of the present theory is the "effective emitting area" in the shock wave front. A self-consistent model for the observed hard-to-soft transition in GRBs is also presented. When applied to GRB 991216 a precise fit (χ²≃1.078) of the observed luminosity in the 2–10 keV band is obtained. Similarly, detailed estimates of the observed luminosity in the 50–300 keV and in the 10–50 keV bands are obtained.

Low angular momentum accretion flows can have standing and oscillating shock waves. We study the region of the parameter space in which multiple sonic points occur in viscous flows in presence of various cooling effects such as bremsstrahlung and Comptonization. We also quantify the parameter space in which shocks are steady or oscillating. We find that cooling induces effects opposite to heating by viscosity even in modifying the topology of the solutions, though one can never be exactly balanced by the other due to their dissimilar dependence on dynamic and thermodynamic parameters. We show that beyond a critical value of cooling, the flow ceases to contain a shock wave.

It is shown that the concept of a fireball with a definite filamentary structure naturally emerges from the analysis of the spectra of Gamma-Ray Bursts (GRBs). These results, made possible by the recently obtained analytic expressions of the equitemporal surfaces in the GRB afterglow, depend crucially on the single parameter ℛ describing the effective area of the fireball emitting the X-ray and gamma-ray radiation. The X-ray and gamma-ray components of the afterglow radiation are shown to have a thermal spectrum in the co-moving frame of the fireball and originate from a stable shock front described self-consistently by the Rankine–Hugoniot equations. Precise predictions are presented on a correlation between spectral changes and intensity variations in the prompt radiation verifiable, e.g., by the Swift and future missions. The highly variable optical and radio emission depends instead on the parameters of the surrounding medium. The GRB 991216 is used as a prototype for this model.

The new results of spectrophotometry, and X-ray imaging spectroscopy observations for the high-polarization, radio-loud and Gev gamma-ray-loud blazar PKS 1510-089 have been observed. The main results show that: (1) A double-horned broad H_β line in the spectrum of PKS 1510-089 has been discovered. The H_β double-peaked emission profiles are impressive with the line asymmetry that the red peak seems to be higher than the blue peak; (2) The observation of X-rays, performed with the ACIS-S detector aboard the Chandra X-ray observatory, led to the discovery of a bending X-ray jet, coincident with the radio arcsecond jet of PKS 1510-089; (3) According to the reverberation mapping method and the empirical relation between the broad line region (BLR) size and the optical continuum luminosity at 5100 Å (the rest frame), we obtained the Virial mass of the central primary black hole, which coincides with our previous result by extremely rapid optical variability method. In addition, based on these new observational results mentioned above and the other previous observations, we have discussed the theoretical models for PKS 1510-089.

We compute the effects of the centrifugal pressure supported shock waves on the emitted spectrum from an accretion disk primarily consisting of low angular momentum matter. Electrons are very efficiently accelerated by the accretion shock and acquire power-law distribution. The accelerated particles in turn emit synchrotron radiation in the presence of a stochastic magnetic field in equipartition with the gas. Efficient cooling of the electrons by these soft photons reduces its temperature in comparison to the protons. We explore the nature of the broadband spectra by using Comptonization, bremsstrahlung and synchrotron emission. We then show that there could be two crossing points in a broadband spectrum, one near ~10 keV and the other ~300–400 keV.

We use a restricted sample of elliptical galaxies, whose kinematical parameters inside the semi-major axis were calculated correcting for the effect of the integration of the light along the line of sight, in order to analyze a possible relationship between the mass of a supermassive black hole (SMBH) and the kinetic energy of random motions in the host galaxy. We find M_BH ∝ (M_G σ²)^α with 0.87 ≤ α ≤ 1 depending on the different fitting methods and samples used. This result could be interpreted as a new fundamental relationship or as a new way to explain the old M_BH - σ law. In fact, the relations of the velocity dispersion both with the mass of the SMBH (M_BH ∝ σ^4.12) and with the mass of the host galaxy (M_G ∝ σ^2.16) induce us to infer an almost direct proportionality: M_BH ∝ M_Gσ². A similar relationship is found for the total kinetic energy involving the rotation velocity too.

Non-linearities such as shock waves are common in accretion flows around compact objects. Exact quantification of these non-linearities will help testing time-dependent numerical codes. In this paper, we study the detailed properties of these non-linear waves in a steady accretion or wind flows around a rotating black hole. We use a pseudo-Kerr geometry for this purpose. In the context of energy preserving standing shocks, we find that there are two shock locations for a given pair of conserved flow parameters, such as specific energy and angular momentum. We also show that as the Kerr parameter is increased, the shock location moves closer to the black hole. We discuss the astrophysical implications of such solutions.

We support, with new fitting instruments and the analysis of more recent experimental data, the proposal of a relationship between the mass of a Supermassive Black Hole (SMBH) and the kinetic energy of random motions in the host elliptical galaxy. The first results obtained in a previous paper with 13 elliptical galaxies are now confirmed by the new data and an enlarged sample. We find M_BH ∝ (M_Gσ²/c²)^β with 0.8 ≤ β ≤ 1 depending on the different fitting methods and samples used. The meaningful case β = 1 is carefully analyzed. Furthermore, we test the robustness of our relationship, including in the sample also lenticular and spiral galaxies and we show that the result does not change. Finally, we find a stronger correlation between the mass of the galaxy and the corresponding velocity dispersion that allows us to connect our relationship to the M_BH ∝ σ^α law. With respect to this law, our relationship has the advantage of having a smaller scatter.