Rindler approximation to Kerr black hole

Abstract: We show that the Rindler approximation to the time-radial part of the Kerr and Kerr-Newman metrics near their external $h_+$ and internal $h_-$ horizons {\bf only} holds {\bf outside} $h_+$ and {\bf inside} $h_-$, so respectively inside and outside the external and internal ergospheres, regions where, in Boyer-Lindquist coordinates, both $g_{tt}$ and $g_{rr}$ are negative, but preserving the Lorentzian character of the metric, and $r>0$ i.e. outside the region $r<0$ where closed timelike curves exist. At each point, the choice of Rindler coordinates is not trivial, but depends on the polar angle $\theta$. The approximation, as is known, automatically gives the absolute values of the surface gravities $\kappa_\pm$ as the corresponding proper accelerations, and therefore the Hawking temperatures $\tau_\pm$ at $h_\pm$.