Ergosphere Geometry and Thermodynamic Properties of Boosted Kerr-Taub-NUT Solutions in Kaluza-Klein Theory

Abstract

We investigate rotating black holes obtained by applying a Kaluza-Klein boost to the Kerr-Taub-NUT spacetime and study the resulting four-dimensional geometry and thermodynamics after dimensional reduction. The boost along the compact direction generates an Einstein-Maxwell-Dilaton black hole in which the electric charge originates purely from higher-dimensional momentum rather than from an independent matter source. We demonstrate that the coordinate location of the stationary limit surface, defined by the condition $g_{tt}=0$ in the Einstein frame, is invariant under the Kaluza-Klein boost. Nevertheless, the boost induces a substantial enlargement of the \emph{physical} ergoregion, as measured by the proper spatial volume on constant-time hypersurfaces, through its modification of the induced spatial metric. We further verify the first law of black-hole thermodynamics with both the electric and magnetic Kaluza-Klein work terms included -- the latter being a genuinely dyonic feature generated by the interplay of the boost with the NUT charge -- and carefully distinguish the seed mass parameter from the asymptotic ADM mass and from the horizon Komar mass. Our results establish a clear separation between boost-invariant horizon thermodynamics and boost-dependent global geometric properties. In particular, higher-dimensional momentum enhances the effective inertial-frame rotation measured by ZAMOs and ergoregion volume without altering the horizon radius, entropy, or temperature, providing a clean geometric signature of extra dimensions in rotating black hole spacetimes.