On the interaction between two Kerr black holes

The double-Kerr solution is generated using both a Backlund transformation and the Belinskii-Zakharov inverse-scattering technique. We build a dictionary between the parametrisations naturally obtained in the two methods and show their equivalence. We then focus on the asymptotically flat double-Kerr system obeying the axis condition which is Z_2^\phi invariant; for this system there is an exact formula for the force between the two black holes, in terms of their physical quantities and the coordinate distance. We then show that 1) the angular velocity of the two black holes decreases from the usual Kerr value at infinite distance to zero in the touching limit; 2) the extremal limit of the two black holes is given by |J|=cM^2, where c depends on the distance and varies from one to infinity as the distance decreases; 3) for sufficiently large angular momentum the temperature of the black holes attains a maximum at a certain finite coordinate distance. All of these results are interpreted in terms of the dragging effects of the system.Comment: 19 pages, 4 figures. v2: changed statement about thermodynamical equilibrium in section 3; minor changes; added references. v3: added references to previous relevant work; removed one equation (see note added); other minor corrections; final version to be published in JHE

Thermodynamical description of stationary, asymptotically flat solutions with conical singularities

We examine the thermodynamical properties of a number of asymptotically flat, stationary (but not static) solutions having conical singularities, with both connected and non-connected event horizons, using the thermodynamical description recently proposed in arXiv:0912.3386 [gr-qc]. The examples considered are the double-Kerr solution, the black ring rotating in either S^2 or S^1 and the black Saturn, where the balance condition is not imposed for the latter two solutions. We show that not only the Bekenstein-Hawking area law is recovered from the thermodynamical description but also the thermodynamical angular momentum is the ADM angular momentum. We also analyse the thermodynamical stability and show that, for all these solutions, either the isothermal moment of inertia or the specific heat at constant angular momentum is negative, at any point in parameter space. Therefore, all these solutions are thermodynamically unstable in the grand canonical ensemble.Comment: 19 pages, 12 figure

Estudo paramétrico de coeficientes de comportamento de pórticos metálicos com ligações semi-rígidas

Nesta comunicação são apresentados alguns resultados relativos ao estudo paramétrico de coeficientes de comportamento de pórticos metálicos. Os modelos numéricos usados incluem a análise geometricamente não-linear dos pórticos e as relações constitutivas não-lineares das ligações. No estudo são consideradas ligações viga-coluna semi-rígidas caracterizadas por relações bi-lineares momento-rotação definidas pela sua rigidez inicial e pós-elástica, resistência e capacidade de deformação. São considerados três níveis de performance ou estados limites, Estado Limite de Serviço (SLS), Estado Limite de Acumulação de Danos (DLS) e Estado Limite Último (ULS)

Investigação, inserção profissional e espírito empresarial

As universidades encontram na sua relação com o meio envolvente o seu maior desafio estratégico, não sendo desprezível, nem independente deste, os esforços dirigidos à investigação, pura ou aplicada, às saídas profissionais e ao desenvolvimento do espírito empresarial entre os estudantes, em todos os estudantes e não só naqueles que frequentam cursos de MBA

Behaviour factors of MR steel frames with semi-rigid connections: a parametric study

In this paper same results are presented, related to the parametric study of the q-factor of steel moment resistant frames. The study considers semi-rigid beam-to-column joints with standard behaviour, characterized by their stiffness, resistance and deformation capacity. Three performance levels or limit states are considered, Serviceability Limit State (SLS), Damageability Limit State (DLS) and Ultimate Limit State (ULS)

A Double Myers-Perry Black Hole in Five Dimensions

Using the inverse scattering method we construct a six-parameter family of exact, stationary, asymptotically flat solutions of the 4+1 dimensional vacuum Einstein equations, with U(1)^2 rotational symmetry. It describes the superposition of two Myers-Perry black holes, each with a single angular momentum parameter, both in the same plane. The black holes live in a background geometry which is the Euclidean C-metric with an extra flat time direction. This background possesses conical singularities in two adjacent compact regions, each corresponding to a set of fixed points of one of the U(1) actions in the Cartan sub-algebra of SO(4). We discuss several aspects of the black holes geometry, including the conical singularities arising from force imbalance, and the torsion singularity arising from torque imbalance. The double Myers-Perry solution presented herein is considerably simpler than the four dimensional double Kerr solution and might be of interest in studying spin-spin interactions in five dimensional general relativity.Comment: 23 pages, 7 figures, LaTeX. v2: minor changes, references added; version published in JHE