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O. B. Zaslavskii

Publications and source records attributed to O. B. Zaslavskii.

At least 73 records · Page 4Linked to original sources
arXiv2015

General limitations on trajectories suitable for super-Penrose process

Collisions of particles near a rotating black hole can lead to unbound energies $E_{c.m.}$ in their centre of mass frame. There are indications that the Killing energy of debris at infinity can also be unbound for some scenarios of collisions near the extremal black hole horizon (so-called super-Penrose process). They include participation of a particle that (i) has generic (not fine-tuned) parameters and (ii) moves away from a black hole before collision. We show that for any finite particle's mass, such a particle cannot be obtained as a result of the preceding collision. However, this can be done if one of initial infalling particles has the mass of the order $N^{-2}$ that generalizes previous observations made in literature for radial infall in the Kerr background.

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arXiv2015

Innermost stable circular orbit near dirty black holes in magnetic field and ultra-high energy particle collisions

We consider the behavior of the innermost stable circular orbit (ISCO) in the magnetic field near "dirty" (surrounded by matter) axially-symmetric black holes. The cases of near-extremal, extremal and nonextremal black holes are analyzed. For nonrotating black holes, in the strong magnetic field ISCO approaches the horizon (when backreaction of the field on geometry is neglected). Rotation destroys this phenomenon. The angular momentum and radius of ISCO look model-independent in the main approximation. We also study the collisions between two particles that results in the ultra-high energy $E_{c.m.}$ in the centre of mass frame. Two scenarios are considered - when one particle moves on the near-horizon ISCO or when collision occurs on the horizon, one particle having the energy and angular momentum typical of ISCO. If the magnetic field is strong enough and a black hole is slow rotating, $E_{c.m.}$ can become arbitrarily large. Kinematics of high-energy collision is discussed. As an example, we consider the magnetized Schwarzschild black hole for an arbitrary strength of the field (the Ernst solution). It is shown that backreaction of the magnetic field on the geometry can bound the growth of $E_{c.m.}$.

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arXiv2015

Near-horizon circular orbits and extremal limit for dirty rotating black holes

We consider generic rotating axially symmetric "dirty" (surrounded by matter) black holes. Near-horizon circular equatorial orbits are examined in two different cases of near-extremal (small surface gravity $κ$) and exactly extremal black holes. This has a number of qualitative distinctions. In the first case, it is shown that such orbits can lie as close to the horizon as one wishes on suitably chosen slices of space-time when $κ\rightarrow 0$. This generalizes observation of T.\ Jacobson Class. Quantum Grav. 28 187001 (2011) made for the Kerr metric. If a black hole is extremal ($κ=0$), circular on-horizon orbits are impossible for massive particles but, in general, are possible in its vicinity. The corresponding black hole parameters determine also the rate with which a fine-tuned particle on the noncircular near-horizon orbit asymptotically approaches the horizon. Properties of orbits under discussion are also related to the Ba% ñados-Silk-West effect of high energy collisions near black holes. Impossibility of the on-horizon orbits in question is manifestation of kinematic censorship that forbids infinite energies in collisions.

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arXiv2015

Unbounded energies of debris from head-on particle collisions near black holes

If two particles move toward a black hole and collide near the horizon, the energy E_{c.m.} in the centre of mass can grow unbounded. This is a so-called Bañados-Silk-West (BSW) effect. One of problems creating obstacles to the possibility of its observation consists in that individual energy E of a fragment at infinity remains finite because of redshift. We show that in the case of head-on collision, debris may have unbounded energy E. An essential ingredient of this scenario is a particle moving away from a black hole in the near-horizon region. It can appear due to precedent collision that implies multiple scattering.

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arXiv2015

Quasi-black holes: general features and purely field configurations

Objects that are on the threshold of forming the horizon but never collapse are called quasi-black holes (QBHs). We discuss the properties of the general spherically symmetric QBH metric without addressing its material source, including its limiting cases as the corresponding small parameter tends to zero. We then show that QBHs can exist among self-gravitating configurations of electromagnetic and dilatonic scalar fields without matter. These general results are illustrated by explicit examples of exact solutions.

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arXiv2015

Ultra-high energy collisions in static space-times: single versus multi-black hole cases

We consider collision of two particles near static electrically charged extremal black holes and elucidate the conditions under which the energy in the centre of mass frame E_{c.m.} grows unbounded. For a single black hole, we generalize the results obtained earlier for the Reissner-Nordström metric, to distorted black holes. In the multi-black hole space-time, qualitatively new features appear. If the point of collision is close to at least two horizons simultaneously, unbounded E_{c.m.} are possible (i) without fine-tuning of particles' parameters, (ii) for an arbitrary mutual orientation of two velocities. Such a combination of properties (i) and (ii) has no analogues in the single black hole case and facilitates the condition of getting unbounded E_{c.m.}. Collisions in the electro-vacuum Majumdar-Papapetrou metric (several extremal black holes in equilibrium) is analyzed explicitly.

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arXiv2015

Kinematics of ultra-high energy particle collisions near black holes in the magnetic field

There are different versions of collisions of two particles near black holes with unbound energy $E_{c.m.}$ in the centre of mass frame. The so-called BSW effect arises when a slow fine-tuned "critical" particle hits a rapid "usual" one. We discuss a scenario of collision in the strong magnetic field for which explanation tuns out to be different. Both particles are rapid but the nonzero angle between their velocities (which are both close to $c$, the speed of light) results in a relative velocity close to $c$ and, hence, big $% E_{c.m.}

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arXiv2014

On geodesics with negative energies in the ergoregions of dirty black holes

We consider behavior of equatorial geodesics with the negative energy in the ergoregion of a generic rotating "dirty" (surrounded by matter) black hole. It is shown that under very simple and generic conditions on the metric coefficients, there are no such circular orbits. This entails that such geodesic must originate and terminate under the event horizon. These results generalize the observation made for the Kerr metric in A. A. Grib, Yu. V. Pavlov, and V. D. Vertogradov, Mod. Phys. Lett. 29, 1450110 (2014) [arXiv:1304.7360].

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arXiv2014

Ultrahigh energy particle collisions near many-dimensional black holes: general approach

If two particles moving towards a black hole collide near the horizon, their energy in the centre of mass frame can grow unbounded. This is the so-called Banados - Silk - West (BSW) effect. Earlier, it was shown that in the 3+1 space-time this effect has a universal nature. We show that for a wide class of many-dimensional black holes (including, say, the Myers-Perry black hole) this is also true. The suggested analysis is general and does not require special properties of the metric like separability of variables for geodesics, etc.

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arXiv2014

Banados-Silk-West effect with nongeodesic particles: Nonextremal horizons

When two particles collide near a black hole, the energy in their center of mass frame can, under certain conditions, grow unbounded. This is the Banados-Silk-West effect. We show that this effect retains its validity even if some force acts on a particle, provided some reasonable and weak restrictions are imposed on this force. In the present work we discuss the case of nonextremal horizons. The result under discussion is similar to that for extremal horizons considered in our previous work.

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arXiv2014

Dilaton gravity, (quasi-) black holes, and scalar charge

We consider static electrically charged dust configurations in the framework of Einstein-Maxwell-dilaton gravity with the interaction term P(χ) F_{mn} F^{mn} in the Lagrangian, where P(χ) is an arbitrary function of the dilaton field χ, and the latter is allowed to be normal or phantom. It is shown that, for any regular P(χ), static configurations are possible with arbitrary functions g_{00} = e^{2γ(x^i)} (i=1,2,3) and χ= χ(γ), without any assumption of spatial symmetry. The corresponding matter, electric charge and scalar charge densities are found from the field equations. Meanwhile, configurations with nontrivial χ(x^i) generically require a nonzero scalar charge density distribution. The classical Majumdar-Papapetrou (MP) system is obtained as a special case where χ= const; there is its scalar analogue in the case F_{mn} = 0, but only with a phantom χfield. Among possible solutions are black-hole (BH) and quasi-black-hole (QBH) ones. Some general results on QBH properties obtained previously for the MP system are here extended to systems with the dilaton. Particular examples of asymptotically flat spherically symmetric BH and QBH solutions are found, some of them being phantom-free, that is, exist with positive energy densities of matter and both scalar and electromagnetic fields.

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arXiv2014

High energy collision of particles in the magnetic field far from black holes

We consider collision of two particles in the axially symmetric black hole metric in the magnetic field. If the value of the angular momentum |L| of one particles grows unbound (but its Killing energy remains fixed) one can achieve unbound energy in the center of mass frame E_{c.m.} In the absence of the magnetic field, collision of this kind is known to happen in the ergoregion. However, if the magnetic field strength B is also large, with the ratio |L|/B being finite, large E_{c.m.} can be achieved even far from a black hole, in the almost flat region. Such an effect also occurs in the metric of a rotating star.

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arXiv2014

Dilaton gravity, charged dust, and (quasi-) black holes

We consider Einstein-Maxwell-dilaton gravity with charged dust and interaction of the form $P(χ) F_{μν} F^{μν}$, where $P(χ)$ is an arbitrary function of the dilaton field $χ$ that can be normal or phantom. For any regular $P(χ)$, static configurations are possible with arbitrary functions $g_{00} = \exp(2γ(x^i))$ ($i=1,2,3$) and $χ= χ(γ)$, without any assumption of spatial symmetry. The classical Majumdar-Papapetrou system is restored by putting $χ=$ const. Among possible solutions are black-hole (BH) and quasi-black-hole (QBH) ones. Some general results on BH and QBH properties are deduced and confirmed by examples. It is found, in particular, that asymptotically flat BHs and QBHs can exist with positive energy densities of matter and both scalar and electromagnetic fields.

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arXiv2014

High-energy collisions inside black holes and their counterpart in flat space-time

Two particles can collide inside a nonextremal black hole in such a way that the energy E_{c.m.} in their centre of mass frame becomes as large as one likes. We show that this effect can be understood with the help of a simple analogy with particle collisions in flat space-time. As the two-dimensional part of near-horizon geometry inside a black hole is described by the flat Milne metric, the results have a general character. Full classification of scenarios with unbound E_{c.m.} is suggested. Some scenarios of this kind require proximity of collision to the bifurcation point, but for some other ones this is not necessary.

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arXiv2013

Acceleration of particles by acceleration horizons

We consider collision of two particles in the vicinity of the extremal acceleration horizon (charged or rotating) that includes the Bertotti-Robinson space-time and the geometry of the Kerr throat. It is shown that the energy in the centre of mass frame E_{c.m.} can become indefinitely large if parameters of one of the particles are fine-tuned, so the Bañados-Silk-West (BSW) effect manifests itself. There exists coordinate transformation which brings the metric into the form free of the horizon. This leads to some paradox since (i) the BSW effect exists due to the horizon, (ii) E_{c.m.} is a scalar and cannot depend on the frame. Careful comparison of near-horizon trajectories in both frames enables us to resolve this paradox. Although globally the space-time structure of the metrics with acceleration horizons and black holes are completely different, locally the vicinity of the extremal black hole horizon can be approximated by the metric of the acceleration one. The energy of one particle from the viewpoint of the Kruskal observer (or the one obtained from it by finite local boost) diverges although in the stationary frame energies of both colliding particles are finite. This suggests a new explanation of the BSW effect for black holes given from the viewpoint of an observer who crosses the horizon. It is complementary to the previously found explanation from the point of view of a static or stationary observer.

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arXiv2013

Bañados-Silk-West effect with nongeodesic particles: extremal horizons

The Bañados-Silk-West (BSW) effect consists in the possibility to obtain arbitrarily large energy $E_{c.m.}$ in the centre of mass frame of two colliding particles near the black hole horizon. One of the common beliefs was that the action of force on these particles (say, due to gravitational radiation) should necessarily restrict the growth of $E_{c.m.}$. We consider extremal horizons and develop a model-independent approach and analyze the conditions for the force to preserve or kill the effect, using the frames attached both to observers orbiting the black hole and to ones crossing the horizon. We argue that the aforementioned expectations are not confirmed. Under rather general assumptions, the BSW effect survives. For equatorial motion it is only required that in the proper frame the radial component of the force be finite, while the azimuthal one tend to zero not too slowly. If the latter condition is violated, we evaluate $E_{c.m.}$, which becomes indeed restricted but remains very large for small forces.

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arXiv2013

What happens to Petrov classification on horizons of axisymmetric dirty black holes

We consider axisymmetric stationary dirty black holes with regular non-extremal or extremal horizons, and compute their on-horizon Petrov types. The Petrov type (PT) in the frame of the observer crossing the horizon can be different from that formally obtained in the usual (but singular in the horizon limit) frame of an observer on a circular orbit. We call this entity the boosted Petrov type (BPT), as the corresponding frame is obtained by a singular boost from the regular one. The PT off-horizon can be more general than PT on-horizon and that can be more general than the BPT on horizon. This is valid for all regular metrics, irrespective of the extremality of the horizon. We analyze and classify the possible relations between the three characteristics and discuss the nature and features of the underlying singular boost. The three Petrov types can be the same only for space-times of PT D and O off-horizon. The mutual alignment of principal null directions and the generator in the vicinity of the horizon is studied in detail. As an example, we also analyze a special class of metrics with utra-extremal horizons (for which the regularity conditions look different from the general case) and compare their off-horizon and on-horizon algebraic structure in both frames.

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