Relative-observer definition of the Simon tensor

The definition of Simon tensor, originally given only in the Kerr spacetime and associated with the static family of observers, is generalized to any spacetime and to any possible observer family. Such generalization is obtained by a standard "3+1" splitting of the Bianchi identities, which are rewritten here as a "balance equation" between various spatial fields, associated with the kinematical properties of the observer congruence and representing the spacetime curvature.