Strong gravitational lensing by Schwarzschild black holes

The properties of the relativistic rings which show up in images of a source when a black hole lies between the source and observer are examined. The impact parameters are calculated, along with the distances of closest approach of the rays which form a relativistic ring, their angular sizes, and their “magnification” factors, which are much less than unity. __________ Translated from Astrofizika, Vol. 51, No. 1, pp. 125–138 (February 2008).     

Dark matter accretion wakes of high-redshift black holes

Anisotropic emission of gravitational waves during the merger of black holes induces a recoil velocity on the centre of mass of the binary and the final merger product can then be ejected from its host galaxy. We consider ejected black holes which stay on bound orbits around their host haloes. A recoiled black hole which moves on an almost radial orbit outside the virial radius of its central galaxy, in the cold dark matter background, reaches its apapsis in a finite time. Due to small dark matter velocity dispersion at high redshifts and also the small black hole velocity near the apapsis passage a high-density wake forms around these black hole. Gamma-ray emission can result from the enhancement of dark matter annihilation in these wakes. The diffuse high-energy gamma-ray background from the ensemble of such black holes in the Hubble volume is also evaluated.     

Bounds for mass and rotation of black holes in the observable universe

The bounds for the energy change rate of a Maxwell field in the outer space of a rotating black hole, calculated by de Vries (1994) and de Vries (1995), enable us to deduce limits for the rotation and the mass of black holes. For this purpose we assume that a certain part of the anisotropies of the cosmic background radiation is due to absorption or superradiance of black holes (Teukolsky, 1973, Starobinskii and Churilov, 1973). The knowledge of these anisotropies yields bounds for rotation and mass of the black holes in the observable universe.     

Gravitational wave bursts from collisions of primordial black holes in clusters

The rate of gravitational wave bursts from the mergers of massive primordial black holes in clusters is calculated. Such clusters of black holes can be formed through phase transitions in the early Universe. The central black holes in clusters can serve as the embryos of supermassive black holes in galactic nuclei. The expected burst detection rate by the LISA gravitational wave detector is estimated.     

Extreme gravitational lensing near rotating black holes

We describe a new approach to calculating photon trajectories and gravitational lensing effects in the strong gravitational field of the Kerr black hole. These techniques are applied to explore both the imaging and spectral properties of photons emitted from an accretion disc, which perform multiple orbits of the central mass before escaping to infinity. Viewed at large inclinations, these higher-order photons contribute ∼20 per cent of the total luminosity of the system for a Schwarzschild hole, while for an extreme Kerr black hole this fraction rises to ∼60 per cent. In more realistic models, these photons will be reabsorbed by the disc at large distances from the hole, but this returning radiation could provide a physical mechanism to resolve the discrepancy between the predicted and observed optical/ultraviolet colours in active galactic nuclei. Conversely, at low inclinations, higher-order images reintercept the disc plane close to the black hole, so need not be absorbed by the disc if this is within the plunging region. These photons form a bright ring carrying approximately 10 per cent of the total disc luminosity for a Schwarzschild black hole. The spatial separation between the inner edge of the disc and the ring is similar to the size of the event horizon. This is resolvable for supermassive black holes with proposed X-ray interferometery missions such as the Microarcsecond X-ray Imaging Mission (MAXIM), and so has the potential to provide an observational test of strong field gravity.     

Gravitational waves: The future of black hole physics

The new millennium will witness the operation of several long-baseline ground-based interferometric detectors, possibly a space-based detector too, which will make it possible to directly observe black holes by catching gravitational waves emitted by them during their formation or when they are perturbed or when a binary consisting of black holes in-spirals due to radiation reaction. Such observations will help us not only to test some of the fundamental predictions of Einstein's general relativity but will also give us the unique opportunity to map black hole spacetimes, to measure the masses and spins of black holes and their population, etc.     

Supermassive binary black holes among cosmic gamma-ray sources

Supermassive binary black holes (SBBHs) are a natural outcome of galaxy mergers. Here we show that low-frequency (f≤10⁻⁶ Hz) quasi-periodic variability observed from cosmic blazar sources can provide substantial inductive support for the presence of close (d≲0.1 pc) SBBHs at their centers. It is argued on physical grounds that such close binary systems are likely to give rise to different (although not independent) periodicities in the radio, optical and X-ray/TeV regime, and, hence that detection of appropriate period ratios significantly corroborates the SBBH interpretation. This is illustrated for a binary model where optical longterm periodicity is related to accretion disk interactions, radio periodicity to Newtonian jet precession, and periodicities in the high energy bands to the orbital motion of the jet. We use the observed periodicities to constrain the properties for a sample of SBBH candidates including OJ 287 and AO 0235+16, and discuss the results within the context of jet activity and binary evolution.     

The influence of central black holes on gravitational lenses

Recent observations indicate that many if not all galaxies host massive central black holes. In this paper we explore the influence of black holes on the lensing properties. We model the lens as an isothermal ellipsoid with a finite core radius plus a central black hole. We show that the presence of the black hole substantially changes the critical curves and caustics. If the black hole mass is above a critical value, then it will completely suppress the central images for all source positions. Realistic central black holes are likely to have masses below this critical value. Even in such subcritical cases, the black hole can suppress the central image when the source is inside a zone of influence, which depends on the core radius and black hole mass. In the subcritical cases, an additional image may be created by the black hole in some regions, which for some radio lenses may be detectable with high-resolution and large dynamic range VLBI maps. The presence of central black holes should also be taken into account when one constrains the core radius from the lack of central images in gravitational lenses.     

From white dwarfs to black holes (commemorating the 70th anniversary of the theory of compact objects)

We discuss basic ideas which were fundamental for the black hole concept. The major goal of the historical part is an attempt to explain the long way to the birth of the black hole concept, since the black hole solution was already found in 1916 by K. Schwarzschild, but the black hole concept was only introduced in 1967 by J.A. Wheeler. We discuss the basic notations of the black hole theory and observational manifestations of black holes. We analyse the possibility to interpret the very peculiar distortion of the Fe K_α‐line in such a way.     

Neutrino radiation of the AGN black holes

In the framework of ‘microscopic’ theory of black holes (J. Phys. Soc. Jpn. Suppl. B 70, 84, 2001; Astrophys. USSR 4, 659, 1996; 35, 335, 1991, 33, 143, 1990, 31, 345, 1989a; Astrophys. Space Sci. 1, 1992; Dokl. Akad. Nauk USSR 309, 97, 1989b), and references therein, we address the ‘pre-radiation time’ (PRT) of neutrinos from black holes, which implies the lapse of time from black hole’s birth till radiation of an extremely high energy neutrinos. For post-PRT lifetime, the black hole no longer holds as a region of spacetime that cannot communicate with the external universe. We study main features of spherical accretion onto central BH and infer a mass accretion rate onto it, and, further, calculate the resulting PRT versus bolometric luminosity due to accretion onto black hole. We estimate the PRTs of AGN black holes, with the well-determined masses and bolometric luminosities, collected from the literature by Woo Jong-Hak and Urry (Astrophys. J. 579, 530, 2002) on which this paper is partially based. The simulations for the black holes of masses M _BH ≃(1.1⋅10⁶ ÷4.2⋅10⁹) M _⊙ give the values of PRTs varying in the range of about T _BH ≃(4.3⋅10⁵ ÷5.6⋅10¹¹) yr. The derived PRTs for the 60 AGN black holes are longer than the age of the universe (∼13.7 Gyr) favored today. At present, some of remaining 174 BHs may radiate neutrinos. However, these results would be underestimated if the reservoir of gas for accretion in the galaxy center is quite modest, and no obvious way to feed the BHs with substantial accretion.     

Parameters of the early Universe and primordial black holes

We consider the possibility of low-mass primordial black holes being formed in terms of the inflationary theory of the early Universe. We found a condition on the reheating temperature under which the relic remnants of primordial black holes had been formed by now. These relic remnants may account for a part of the dark matter in our Universe.     

Variable and broad iron lines around black holes

“Variable and Broad Iron Lines around Black Holes” was the topic of the first scientific workshop organized by the XMMNewton Science Operations Centre. This foreword gives a brief introduction to the history of black hole research, which motivated the workshop's scientific topic. The organizing committees are listed and the meeting is shortly outlined. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Black holes and rotation

In this article, we first consider briefly the basic properties of the non-rotating Schwarzschild black hole and the rotating Kerr black hole Rotational effects are then described in static and stationary spacetimes with arial symmetry by studying inertial forces, gyroscopic precession and gravi-electromagnetism. The results are applied to the black hole spacetimes.     

Gravitational collapse, black holes and naked singularities

This article gives an elementary review of gravitational collapse and the cosmic censorship hypothesis. Known models of collapse resulting in the formation of black holes and naked singularities are summarized. These models, when taken together, suggest that the censorship hypothesis may not hold in classical general relativity. The nature of the quantum processes that take place near a naked singularity, and their possible implication for observations, is briefly discussed.     

Formation and evolution of galactic nuclei, black holes

The formation and evolution of galactic nuclei is discussed, mainly from the viewpoint of the dense gaseous and stellar system in the central portions of young galactic nuclei. A review is given on models of galactic nuclei containing one or more black holes and how the standard historic Rees-Spitzer-Stone scenario of isolated nuclei formation has been changed by the cosmological structure formation scenario. The dynamical interaction of two or more black holes with their stellar environment and their merging possibilities is presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

Power calculation for gravitational radiation: oversimplification and the importance of time scale

A simplified formula for gravitational‐radiation power is examined. It is shown to give completely erroneous answers in three situations, making it useless even for rough estimates. It is emphasised that short timescales, as well as fast speeds, make classical approximations to relativistic calculations untenable (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Clusters of black holes as point-like gamma-ray sources

The possibility of identifying some of Galactic gamma-ray sources as clusters of primordial black holes is discussed. The known scenarios of supermassive black hole formation indicate the multiple formation of lower-mass black holes. Our analysis demonstrates that due to Hawking evaporation the cluster of black holes with masses about 10¹⁵ g could be observed as a gamma-ray source. The total mass of typical cluster is ∼10 M_⊙. Detailed calculations have been performed on the basis of specific model of primordial black hole formation.     

Why do we see so few black holes in massive binaries?

We offer a simple explanation for the small number of black holes observed in pairs with massive stars. In detached massive binaries, spherically symmetric accretion takes place. This accretion could result in effective energy release in the hard band only if the equipartition of the gravitational and magnetic energy of plasma is established (Shvartsman’s theorem). However, we show that due to the magnetic exhaust effect this equilibrium is virtually never established for the actual magnetic fields observed on massive stars: Shvartsman’s theorem does not work. As a result, it is virtually impossible to detect black holes in detached massive binaries by currently available means (mainly, through X-ray observations).     

A simple model for the evaporation of black holes at final stages

We present a simple model for the evaporation of primordial black holes at final stages with the formation of a relic remnant with a mass of 1–10³ m _P1. The model takes into account the conservation of energy and the impossibility of passing through the state with the minimum possible mass. These relic remnants may account for a substantial fraction of dark matter in the Universe.     

The revival of white holes as Small Bangs

Black holes are extremely dense and compact objects from which light cannot escape. There is an overall consensus that black holes exist and many astronomical objects are identified with black holes. White holes were understood as the exact time reversal of black holes, therefore they should continuously throw away material. It is accepted, however, that a persistent ejection of mass leads to gravitational pressure, the formation of a black hole and thus to the “death of while holes”. So far, no astronomical source has been successfully tagged a white hole. The only known white hole is the Big Bang which was instantaneous rather than continuous or long-lasting. We thus suggest that the emergence of a white hole, which we name a ‘Small Bang’, is spontaneous - all the matter is ejected at a single pulse. Thus, unlike black holes, white holes cannot be continuously observed rather their effect can only be detected around the event itself. γ-ray bursts are the most energetic explosions in the universe. Long γ-ray bursts were connected with supernova eruptions. There is a new group of γ-ray bursts, which are relatively close to Earth, but surprisingly lack any supernova emission. We propose identifying these bursts with white holes. White holes seem like the best explanation of γ-ray bursts that appear in voids. We also predict the detection of rare gigantic γ-ray bursts with energies much higher than typically observed.