Gravitational energy in stationary spacetimes
  Static observers remain on Killing-vector world lines and measure the rest-mass+kinetic energies of particles moving past them, and the flux of that mechanical energy through space and time. The total mechanical energy is the total flux through a spacelike cut at one time. The difference between the total mass-energy and the total mechanical energy is the total gravitational energy, which we prove to be negative for certain classes of systems. For spherical systems, Misner, Thorne and Wheeler define the total gravitational energy this way.   To obtain the gravitational energy density analogous to that of electromagnetism we first use Einstein's equations with integrations by parts to remove second order derivatives. Next we apply a conformal transformation to reexpress the scalar 3-curvature of the 3-space. The resulting density is non-local.   We repeat the argument for mechanical energies as measured by stationary observers moving orthogonally to constant time slices like the "zero angular momentum" observers of Bardeen who exist even within ergospheres. 
Joseph Katz, Donald Lynden-Bell and Jiri Bicak