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In this paper we show that an isolated Kerr black hole can be interpreted as arising from a pair of identical counter-rotating NUT objects placed on the symmetry axis at an appropriate distance from each other.PACS numbers: 04.20.Jb, 04.70.Bw, 97.60.Lf As has been recently shown [1], the well-known NUT solution [2] represents a stringy object with two counterrotating semi-infinite massive singularities attached to the poles of a central non-rotating mass. This solution is the simplest possible one among the equatorially antisymmetric spacetimes the notion of which has been introduced in [3]. Although the usual NUT object is endowed with some unphysical properties, e.g., the presence of the regions with negative mass and closed time-like curves, in the present paper we will demonstrate that configurations of several NUT constituents can give rise to physically significant models without the pathologies of a single NUT spacetime. Concretely, we shall consider a simple but convincing example: emergence of a Kerr black hole from two interacting NUT objects. The consideration will be carried out within the framework of an exact solution describing a non-linear superposition of two NUT sources.We remind that the stationary axisymmetric vacuum problem reduces to finding the complex function E satisfying the Ernst equation [4] (E +Ē)(E ,ρ,ρ + ρwhere ρ and z are the Weyl-Papapetrou cylindrical coordinates, a comma denotes partial differentiation with respect to the coordinate that follows it, and a bar over a symbol means complex conjugation. Using Sibgatullin's method [5], the potential E satisfying (1) can be constructed from its value on the upper part of the symmetry axis. For z > √ m 2 + ν 2 , the Ernst complex potential of a single NUT solution has the form [1]where m is the total mass of the stringy NUT object, while ν represents the average angular momentum per unit length of the semi-infinite NUT singularity. The non-linear superposition of two NUT solutions with equal masses and opposite angular momenta is formally defined by the axis datathe real parameter k representing a displacement of each NUT constituent along the z-axis. We point out that whereas the axis data (2) define the equatorially antisymmetric solution because of the characteristic relation e(z)e(−z) = 1 fulfilled in this case [3], the axis data (3) satisfy the relation e(z)ē(−z) = 1 and hence define already the equatorially symmetric spacetime [6,7]. Below we give the final form of the Ernst potential E constructed from the data (3) with the aid of Sibgatullin's method, and the form of all the corresponding metric functions f , γ and ω from the Papapetrou stationary axisymmetric line elementomitting the intermediate steps (we refer the reader to the paper [8] for details):, e 2γ = AĀ − BB 64d 4 α 2 + α 2

Abstract. Two new equatorially antisymmetric solutions recently published by Ernst et al are studied. For both solutions the full set of metric functions is derived in explicit analytic form and the behavior of the solutions on the symmetry axis is analyzed. It is shown in particular that two counter-rotating equal Kerr-Newman-NUT objects will be in equilibrium when the condition m 2 + ν 2 = q 2 + b 2 is verified, whereas two counter-rotating equal masses endowed with arbitrary magnetic and electric dipole moments cannot reach equilibrium under any choice of the parameters, so that a massless strut between them will always be present.

Abstract. A simple formula, invariant under the duality rotation Φ → e iα Φ, is obtained for the Poynting vector within the framework of the Ernst formalism, and its application to the known exact solutions for a charged massive magnetic dipole is considered.

Exact axisymmetric stationary solution of the Einstein equations describing a system of two counter-rotating identical Kerr black holes is worked out in a physical parametrization within the framework of the Ernst formalism and analytically extended double-Kerr solution. The derivation of the limiting case of extreme constituents is also discussed.