Index theory and dynamical symmetry enhancement of M-horizons

We show that near-horizon geometries of 11-dimensional supergravity preserve an even number of supersymmetries. The proof follows from Lichnerowicz type theorems for two horizon Dirac operators, the field equations and Bianchi identities, and the vanishing of the index of a Dirac operator on the 9-dimensional horizon sections. As a consequence of this, we also prove that all M-horizons with non-vanishing fluxes admit a sl(2,R) subalgebra of symmetries.Comment: Minor typos corrected. 22 pages, latex. Repeats equations and descriptions from arXiv:1207.708

On supersymmetric AdS6 solutions in 10 and 11 dimensions

We prove a non-existence theorem for smooth, supersymmetric, warped AdS6 solutions with connected, compact without boundary internal space in D=11 and (massive) IIA supergravities. In IIB supergravity we show that if such AdS6 solutions exist, then the NSNS and RR 3-form fluxes must be linearly independent and certain spinor bi-linears must be appropriately restricted. Moreover we demonstrate that the internal space admits an so(3) action which leaves all the fields invariant and for smooth solutions the principal orbits must have co-dimension two. We also describe the topology and geometry of internal spaces that admit such a so(3) action and show that there are no solutions for which the internal space has topology F * S^2, where F is an oriented surface.Comment: 26 pages, late

Gravitational Instantons and Euclidean Supersymmetry

Supersymmetric instanton solutions in four dimensional Euclidean ungauged Einstein-Maxwell theory are analysed and classified according to the fraction of supersymmetry they preserve, using spinorial geometry techniques.Comment: 10 pages, late

Concentric Black Rings

We present new supersymmetric solutions of five-dimensional minimal supergravity that describe concentric black rings with an optional black hole at the common centre. Configurations of two black rings are found which have the same conserved charges as a single rotating black hole; these black rings can have a total horizon area less than, equal to, or greater than the black hole with the same charges. A numerical investigation of these particular black ring solutions suggests that they do not have closed timelike curves.Comment: 7 pages, minor alterations, typos corrected. Version to be published in PR

On Non-extremal Instantons and Black Holes

We consider a general analysis and a specific ansatz for the study of non-supersymmetric solutions in arbitrary dimensions and various metric signatures. In all cases, we find that the conditions on the solutions can be written in terms of quadratic forms involving the gauge coupling of the theory and constants of integration associated with the scalar fields. Depending on the signature of the metric, our analysis should provide a general framework for finding non-extremal black holes, instantons, branes and S-branes.Comment: 19 page

Null Half-Supersymmetric Solutions in Five-Dimensional Supergravity

We classify half-supersymmetric solutions of gauged N=2, D=5 supergravity coupled to an arbitrary number of abelian vector multiplets for which all of the Killing spinors generate null Killing vectors. We show that there are four classes of solutions, and in each class we find the metric, scalars and gauge field strengths. When the scalar manifold is symmetric, the solutions correspond to a class of local near horizon geometries recently found by Kunduri and Lucietti.Comment: 46 pages, typos corrected and reference added. Section 7.1 has been added: it is shown that the solutions found here correspond to a class of solutions found in arXiv:0708.3695. Uses JHEP3.cl