arXiv subjects

Dmitri V. Gal'tsov

Publications and source records attributed to Dmitri V. Gal'tsov.

At least 19 recordsLinked to original sources

arXiv2026

Precision tests of analytical tail-term approximations for radiation reaction in Schwarzschild spacetime

We investigate the consistency and precision of approximate analytical expressions for the electromagnetic self-force acting on a charged particle in Schwarzschild spacetime endowed with weak electromagnetic fields. A fundamental requirement of relativistic particle dynamics is the preservation of the four-velocity normalization ($u^μu_μ=-1$), which implies that the total self-force must remain orthogonal to the particle's four-velocity. We introduce a covariant diagnostic based on the orthogonality condition ($u_μF^μ_{\text{tail}}=0$), which provides a quantitative measure of the internal consistency of approximate tail-term models used in radiation-reaction calculations. We apply this diagnostic to two widely used analytical approximations for the electromagnetic tail force: the conservative component derived by Smith and Will and the dissipative component derived by Gal'tsov. The analysis is performed for several physical configurations, including pure Schwarzschild spacetime, a weakly electrically charged Schwarzschild black hole, and a Schwarzschild black hole immersed in a weak external magnetic field. We find that the conservative Smith--Will term alone leads to small but measurable deviations from the orthogonality condition, while inclusion of the dissipative Gal'tsov contribution suppresses these deviations by many orders of magnitude. For realistic radiation-reaction parameters, the violation becomes extremely small. The proposed orthogonality diagnostic offers a simple and covariant tool for validating approximate self-force models in curved spacetime and may be useful for future studies of radiation-reaction dynamics near compact objects.

gr-qc↗

arXiv2026

Electromagnetic radiation-reaction near black holes: orbital widening and the role of the tail

We investigate the orbital evolution of a classical charged particle around a Schwarzschild black hole immersed in an external, uniform magnetic field, taking into full account both local radiation-reaction and the nonlocal tail self-force arising in curved spacetime. Starting from the DeWitt-Brehme equation and its Landau-Lifshitz reduction, we derive analytic expressions for the conservative and dissipative components of the electromagnetic self-force in both the weak-field (Newtonian) and strong-field regimes. By implementing backward-in-time integration of the third-order DeWitt-Brehme equation alongside the second-order Landau-Lifshitz equation, we demonstrate that the so-called orbital widening effect persists even when the tail term is included, and that for astrophysically realistic charge-to-mass ratios the tail contribution to the trajectory is negligible. We further show that this widening is directly controlled by the product of the magnetic field and radiation-reaction parameters and can be captured in the Newtonian limit. Finally, we identify a scaling symmetry showing that simulations with moderate parameter values can accurately represent the dynamics in realistic astrophysical conditions, confirming that orbital widening is a robust phenomenon that can persist even in astrophysical black hole environments.

gr-qc↗

arXiv2018

Radiation reaction of charged particles orbiting magnetized Schwarzschild black hole

In many astrophysically relevant situations radiation reaction force acting upon a charge can not be neglected and the question arises about the location and stability of circular orbits in such regime. Motion of point charge with radiation reaction in flat spacetime is described by Lorenz-Dirac (LD) equation, while in curved spacetime -- by DeWitt-Brehme (DWB) equation containing the Ricci term and the tail term. We show that for the motion of elementary particles in vacuum metrics the DWB equation can be reduced to the covariant form of the LD equation which we use here. Generically, the LD equation is plagued by runaway solutions, so we discuss computational ways to avoid this problem in constructing numerical solutions. We also use the first iteration of the covariant LD equation which is the covariant Landau-Lifshitz equation, comparing results of these two approaches and showing smallness of the third-order Schott term in the ultrarelativistic case. We calculate the corresponding energy and angular momentum loss of a particle and study the damping of charged particle oscillations around an equilibrium radius. We find that depending on the orientation of the Lorentz force, the oscillating charged particle either spirals down to the black hole, or stabilizes the circular orbit by decaying its oscillations. The later case leads to an interesting new result of shifting of the particle orbit outwards from the black hole. We also discuss the astrophysical relevance of the presented approach and provide estimations of the main parameters of the model.

gr-qc↗

arXiv2017

Higher-$n$ triangular dilatonic black holes

Dilaton gravity with the form fields is known to possess dyon solutions with two horizons for the discrete ("triangular") values of the dilaton coupling constant $a = \sqrt{n (n + 1)/2}$. From this sequence only $n = 1,\, 2$ members were known analytically so far. We present two new $n = 3,\, 5$ triangular solutions for the theory with different dilaton couplings $a,\, b$ in electric and magnetic sectors in which case the quantization condition reads $a b = n (n + 1)/2$. These are derived via the Toda chains for $B_2$ and $G_2$ Lie algebras. Solutions are found in the closed form in general $D$ space-time dimensions. They satisfy the entropy product rules and have negative binding energy in the extremal case.

hep-th↗

arXiv2013

Bertotti-Robinson solutions of D=5 Einstein-Maxwell-Chern-Simons-Lambda theory

We present a series of new solutions in five-dimensional Einstein-Maxwell-Chern-Simons theory with an arbitrary Chern-Simons coupling $γ$ and a cosmological constant $Λ$. For general $γ$ and $Λ$ we give various generalizations of the Bertotti-Robinson solutions supported by electric and magnetic fluxes, some of which presumably describe the near-horizon regions of black strings or black rings. Among them there is a solution which could apply to the horizon of a topological AdS black ring in gauged minimal supergravity. Others are horizonless and geodesically complete. We also construct extremal asymptotically flat multi-string solutions for $Λ=0$ and arbitrary $γ$.

gr-qc↗

arXiv2013

New coset matrix for D=6 self-dual supergravity

Toroidal reduction of minimal six-dimensional supergravity, minimal five-dimensional supergravity and four-dimensional Einstein-Maxwell gravity to three dimensions gives rise to a sequence of cosets $O(4,3)/(O(4)\times O(3))\supset G_{2(2)}/(SU(2)\times SU(2))\supset SU(2,1)/S(U(2)\times U(1))$ which are invariant subspaces of each other. The known matrix representations of these cosets, however, are not suitable to realize these embeddings which could be useful for solution generation. We construct a new representation of the largest coset in terms of $7\times 7$ real symmetric matrices and show how to select invariant subspaces corresponding to lower cosets by algebraic constraints. The new matrix representative may be also directly applied to minimal five-dimensional supergravity. Due to full O(4,3) covariance it is simpler than the one derived by us previously for the coset $G_{2(2)}/(SU(2)\times SU(2))$.

hep-th↗

arXiv2013

Oxidation of D=3 cosets and Bonnor dualities in $D\leq 6$

Bonnor's map in General Relativity is duality between (dimensionally reduced) vacuum gravity and static truncation of electro-vacuum theory. It was used as a tool to generate an exact solution of electro-vacuum from some vacuum solution. It can be expected that similar dualities will be useful for solution generation in higher-dimensional theories too. Here we study such maps within a class of theories in dimensions $D\leq 6$ using oxidation of D=3 cosets and consistent truncation of the corresponding theories. Our class includes those theories whose D=3 symmetries are subgroups of $G=O(5,4)$. It contains six-dimensional minimal supergravity, five-dimensional minimal and $U(1)^3$ supergravities and a number of four-dimensional theories which attracted attention recently in the search of exact solutions. We give explicit reduction/truncation formulas relating different theories in dimensions $D=4, 5, 6$ in terms of metrics and matter fields and discuss various alternative duality chains between them.

hep-th↗

arXiv2011

All extremal instantons in Einstein-Maxwell-dilaton-axion theory

We construct explicitly all extremal instanton solutions to $\mathcal{N}=4,\, D=4$ supergravity truncated to one vector field (Einstein-Maxwell-dilaton-axion (EMDA) theory). These correspond to null geodesics of the target space of the sigma-model $G/H=Sp(4,\mathbb{R})/GL(2,\mathbb{R})$ obtained by compactification of four-dimensional Euclidean EMDA on a circle. They satisfy a no-force condition in terms of the asymptotic charges and part of them (corresponding to nilpotent orbits of the $Sp(4,\mathbb{R})$ U-duality) are presumably supersymmetric. The space of finite action solutions is found to be unexpectedly large and includes, besides the Euclidean versions of known Lorentzian solutions, a number of new asymptotically locally flat (ALF) instantons endowed with electric, magnetic, dilaton and axion charges. We also describe new classes of charged asymptotically locally Euclidean (ALE) instantons as well as some exceptional solutions. Our classification scheme is based on the algebraic classification of matrix generators according to their rank, according to the nature of the charge vectors and according to the number of independent harmonic functions with unequal charges. Besides the nilpotent orbits of $G$, we find solutions which satisfy the asymptotic no-force condition, but are not supersymmetric. The renormalized on-shell action for instantons is calculated using the method of matched background subtraction.

hep-th↗

arXiv2010

Transplanckian bremsstrahlung and black hole production

Classical gravitational bremsstrahlung in particle collisions at transplanckian energies is studied in ${\mathcal M}_4\times {\mathcal T}^d$. The radiation efficiency $ε\equiv E_{\rm rad}/E_{\rm initial}$ is computed in terms of the Schwarzschild radius $r_S(\sqrt{s})$, the impact parameter $b$ and the Lorentz factor $γ_{\rm cm}$ and found to be $ε=C_d (r_S/b)^{3d+3} γ_{\rm cm}^{2d+1}$, larger than previous estimates by many powers of $γ_{\rm cm}\gg 1$. The result is reliable for impact parameters in the overlap of $r_S λ_C$, with $b_c$ marking (for $d\neq 0$) the loss of the notion of classical trajectories and $λ_C\equiv \hbar/mc$ the Compton length of the scattered particles. The condition on $s$ and $m$ for extreme radiation damping and (presumably) no black hole production is also derived.

hep-ph↗

arXiv2010

Lorentz-covariant perturbation theory for relativistic gravitational bremsstrahlung

We formulate Lorentz-covariant classical perturbation theory to deal with relativistic bremsstrahlung under gravitational scattering. Our approach is a version of the fast motion approximation scheme, the main novelty being the use of the momentum space representation. Using it we calculate in a closed form the spectrum of scalar, electromagnetic and gravitational radiation. Our results for the total emitted energy agree with those by Thorne and Kovacs. We also explain why the method of equivalent gravitons fails to produce the correct result for the spectral-angular distribution of emitted radiation under gravitational scattering, contrary to the case of Weizsäcker-Williams approximation in quantum electrodynamics.

gr-qc↗

arXiv2010

Radiation reaction in curved even-dimensional spacetime

We develop a new method of computing radiation reaction for a point particle interacting with massless scalar and vector fields in curved space-time. It is based on the analysis of field Green's functions with both points lying on the particle world-line and does not require integration of the field stresses outside the world line as was used in the DeWitt-Brehme approach, thus leading to a substantial simplification of the problem. We start with space-time of an arbitrary dimension and show that the Hadamard expansion of the massless scalar and vector Green's functions contain only integer inverse powers of the Synge world function in even dimensions and only half-integer in the odd dimensions. The even-dimensional case then is treated in detail. We analyze divergencies, calculate higher-derivative counterterms, and find a recurrent formula for the local parts of the reaction force in neighboring dimensions. Higher-dimensional curved space counterterms are not simply the covariant generalizations of the flat ones, but contain additional curvature-dependent terms. We illustrate our formalism in four and six dimensions. In the first case we rederive the results of DeWitt-Brehme-Hobbs in a simpler way, in the second case we give a covariant generalization of the Kosyakov equation. The local part of the reaction force is found to contain a term proportional to the Riemann tensor which is absent in four dimensions.

gr-qc↗

arXiv2009

Global solutions for higher-dimensional stretched small black holes

Small black holes in heterotic string theory have vanishing horizon area at the supergravity level, but the horizon is stretched to the finite radius $AdS_2 \times S^{D-2}$ geometry once higher curvature corrections are turned on. This has been demonstrated to give good agreement with microscopic entropy counting. Previous considerations, however, were based on the classical local solutions valid only in the vicinity of the event horizon. Here we address the question of global existence of extremal black holes in the $D$-dimensional Einstein-Maxwell-Dilaton theory with the Gauss-Bonnet term introducing a variable dilaton coupling $a$ as a parameter. We show that asymptotically flat black holes exist only in a bounded region of the dilaton couplings $0 < a < a_{\rm cr}$ where $a_{\rm cr}$ depends on $D$. For $D \geq 5$ (but not for $D = 4$) the allowed range of $a$ includes the heterotic string values. For $a > a_{\rm cr}$ numerical solutions meet weak naked singularities at finite radii $r = r_{\rm cusp}$ (spherical cusps), where the scalar curvature diverges as $|r - r_{\rm cusp}|^{-1/2}$. For $D \geq 7$ cusps are met in pairs, so that solutions can be formally extended to asymptotically flat infinity choosing a suitable integration variable. We show, however, that radial geodesics cannot be continued through the cusp singularities, so such a continuation is unphysical.

hep-th↗

arXiv2009

Three-charge 2J black ring

Using recently proposed new solution generating technique, we construct the charged version of Pomeranski-Senkov doubly rotating black ring in the $U(1)^3$ five-dimensional supergravity. For arbitrary values of charges the solution is unbalanced, but the Dirac-Misner string is removed when two of the charges are set to zero. In this particular case our solution can be uplifted to some solution of six-dimensional vacuum gravity.

hep-th↗

arXiv2008

Non-Abelian condensates as alternative for dark energy

We review basic features of cosmological models with large-scale classical non-Abelian Yang-Mills (YM) condensates. There exists a unique SU(2) YM configuration (generalizable to larger gauge groups) compatible with homogeneity and isotropy of the three-space which is parameterized by a single scalar field. In the past various aspects of Einstein-Yang-Mills (EYM) cosmology were discussed in the context of the Early Universe. Due to conformal invariance, solvable EYM FRW models exist both on the classical and quantum levels. To develop the YM model for dark energy one has to find mechanisms of the conformal symmetry breaking. We discuss the Born-Infeld generalization and some phenomenological models motivated by quantum corrections exploring possibility of transient DE and phantom regimes.

gr-qc↗

arXiv2008

Generating solutions via sigma-models

We review recent development of solution-generating techniques for four and five-dimensional Einstein equations coupled to vector and scalar fields. This includes D=4 Einstein-Maxwell-dilaton-axion theory with multiple vector fields, D=5 Einstein-Maxwell gravity with the Chern-Simons term (minimal five-dimensional supergravity), and some other models which attracted attention in connection with black rings. The method is based on reduction to three-dimensional gravity coupled sigma-models with symmetric target spaces. Our recent results open a way to construct the general charged black rings in five-dimensional supergravity possibly coupled to vector multiplets.

gr-qc↗

arXiv2008

Curvature-corrected dilatonic black holes and black hole -- string transition

We investigate extremal charged black hole solutions in the four-dimensional string frame Gauss-Bonnet gravity with the Maxwell field and the dilaton. Without curvature corrections, the extremal electrically charged dilatonic black holes have singular horizon and zero Bekenstein entropy. When the Gauss-Bonnet term is switched on, the horizon radius expands to a finite value provided curvature corrections are strong enough. Below a certain threshold value of the Gauss-Bonnet coupling the extremal black hole solutions cease to exist. Since decreasing Gauss-Bonnet coupling corresponds to decreasing string coupling $g_s$, the situation can tentatively be interpreted as classical indication on the black hole -- string transition. Previously the extremal dilaton black holes were studied in the Einstein-frame version of the Gauss-Bonnet gravity. Here we work in the string frame version of this theory with the S-duality symmetric dilaton function as required by the heterotic string theory.

hep-th↗

arXiv2008

Improved generating technique for D=5 supergravities and squashed Kaluza-Klein Black Holes

Recently we suggested a solution generating technique for five-dimensional supergravity with three Abelian vector fields based on the hidden SO(4,4) symmetry of the three-dimensionally reduced theory. This technique generalizes the $G_{2(2)}$ generating technique developed earlier for minimal 5D supergravity (A. Bouchareb, G. Clément, C-M. Chen, D. V. Gal'tsov, N. G. Scherbluk, and Th. Wolf, Phys. Rev. D {\bf 76}, 104032 (2007)) and provides a new matrix representation for cosets forming the corresponding sigma-models in both cases. Here we further improve these methods introducing a matrix-valued dualisation procedure which helps to avoid difficulties associated with solving the dualisation equations in the component form. This new approach is used to generate a five-parametric rotating charged Kaluza-Klein black hole with the squashed horizon adding one parameter more to the recent solution by Tomizawa, Yasui and Morisawa which was constructed using the previous version of the $G_{2(2)}$ generating technique.

hep-th↗

arXiv2008

Extremal dyonic black holes in D=4 Gauss-Bonnet gravity

We investigate extremal dyon black holes in the Einstein-Maxwell-dilaton (EMD) theory with higher curvature corrections in the form of the Gauss-Bonnet density coupled to the dilaton. In the same theory without the Gauss-Bonnet term the extremal dyon solutions exist only for discrete values of the dilaton coupling constant $a$. We show that the Gauss-Bonnet term acts as a dyon hair tonic enlarging the allowed values of $a$ to continuous domains in the plane $(a, q_m)$ the second parameter being the magnetic charge. In the limit of the vanishing curvature coupling (a large magnetic charge) the dyon solutions obtained tend to the Reissner-Nordström solution but not to the extremal dyons of the EMD theory. Both solutions have the same values of the horizon radius as a function of charges. The entropy of new dyonic black holes interpolates between the Bekenstein-Hawking value in the limit of the large magnetic charge (equivalent to the vanishing Gauss-Bonnet coupling) and twice this value for the vanishing magnetic charge. Although an expression for the entropy can be obtained analytically using purely local near-horizon solutions, its interpretation as the black hole entropy is legitimate only once the global black hole solution is known to exist, and we obtain numerically the corresponding conditions on the parameters. Thus, a purely local analysis is insufficient to fully understand the entropy of the curvature corrected black holes. We also find dyon solutions which are not asymptotically flat, but approach the linear dilaton background at infinity. They describe magnetic black holes on the electric linear dilaton background.

hep-th↗