Screening an extremal black hole with a thin shell of exotic matter

Abstract

We study the possibility of shielding a regular extremal black hole by means of a matter thin-shell. While the surface energy density $\sigma$ on the static shell is negative, the tangential pressures $p$ are positive, both of them being finite when the shell approaches the black hole horizon. The Darmois-Israel junction conditions are used to find $\sigma$ and $p$ in terms of the radius $a$ of the shell. The surface gravitational energy $E_{S}$ is computed, keeping track of the pressure contribution. The stability conditions are briefly investigated.