Efficiency of the Keplerian accretion in the 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 parameter can be separated into fourteen classes according to properties of circular geodesics governing the~Keplerian accretion. We determine efficiency of the~Keplerian accretion disks for all braneworld Kerr--Newman spacetimes. We demonstrate occurrence of an~infinitely deep gravitational potential in Kerr--Newman naked singularity spacetimes having the~braneworld dimensionless tidal charge $b \in (1/4,1)$ and the~dimensionless spin $a \in (2\sqrt{b}-\sqrt{b(4b-1)},2\sqrt{b}+\sqrt{b(4b-1)})$, implying unbound efficiency of the~Keplerian accretion and possibility to extract 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. 
Martin Blaschke and Zden\v{e}k Stuchl\'ik
gr-qc