On the Bel radiative gravitational fields
  We analyze the concept of intrinsic radiative gravitational fields defined by Bel and we show that the three radiative types, N, III and II, correspond with the three following different physical situations: {\it pure radiation}, {\it asymptotic pure radiation} and {\it generic} (non pure, non asymptotic pure) {\it radiation}. We introduce the concept of {\em observer at rest} with respect to the gravitational field and that of {\em proper super-energy} of the gravitational field and we show that, for non radiative fields, the minimum value of the relative super-energy density is the proper super-energy density, which is acquired by the observers at rest with respect to the field. Several {\it super-energy inequalities} are also examined. 
Joan Josep Ferrando and Juan Antonio S\'aez
gr-qc