Efficiency of the Keplerian accretion in braneworld Kerr-Newman spacetimes and mining instability of some naked singularity spacetimes

We show that the braneworld rotating Kerr-Newman black hole and naked singularity spacetimes with both positive and negative braneworld tidal charge parameters can be separated into 14 classes according to the properties of circular geodesics governing the Keplerian accretion. We determine the efficiency of the Keplerian accretion disks for all braneworld Kerr-Newman spacetimes. We demonstrate the occurrence of an infinitely deep gravitational potential in Kerr-Newman naked singularity spacetimes having the braneworld dimensionless tidal charge $b\ensuremath{\in}(1/4,1)$ and the dimensionless spin $a\ensuremath{\in}(2\sqrt{b}\ensuremath{-}\sqrt{b(4b\ensuremath{-}1)}$, $2\sqrt{b}+\sqrt{b(4b\ensuremath{-}1)})$, implying unbound efficiency of the Keplerian accretion and the possibility of extracting the whole naked singularity mass. Therefore, we call them braneworld ``mining-unstable'' Kerr-Newman naked singularity spacetimes. Fundamental restriction on the relevance of the extraordinary---but fully classical---phenomenon of the mining instability is given by validity of the assumption of geodesic motion of the accreting matter.

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