  Modulo conventional scale factors, the Simon and Simon-Mars tensors are
defined for stationary vacuum spacetimes so that their equality follows from
the Bianchi identities of the second kind. In the nonvacuum case one can absorb
additional source terms into a redefinition of the Simon tensor so that this
equality is maintained. Among the electrovacuum class of solutions of the
Einstein-Maxwell equations, the expression for the Simon tensor in the
Kerr-Newman-Taub-NUT spacetime in terms of the Ernst potential is formally the
same as in the vacuum case (modulo a scale factor), and its vanishing
guarantees the simultaneous alignment of the principal null directions of the
Weyl tensor, the Papapetrou field associated with the timelike Killing vector
field, the electromagnetic field of the spacetime and even the Killing-Yano
tensor.
