The Newman-Janis Ansatz was used first to obtain the stationary Kerr metric
from the static Schwarzschild metric. Many works have been devoted to
investigate the physical significance of this Ansatz, but no definite answer
has been given so far. We show that this Ansatz can be applied in general to
conformastatic vacuum metrics, and leads to stationary generalizations which,
however, do not preserve the conformal symmetry. We investigate also the
particular case when the seed solution is given by the Schwarzschild spacetime
and show that the resulting rotating configuration does not correspond to a
vacuum solution, even in the limiting case of slow rotation. In fact, it
describes in general a relativistic fluid with anisotropic pressure and heat
flux. This implies that the Newman-Janis Ansatz strongly depends on the choice
of representation for the seed solution. We interpret this result as as a
further indication of its applicability limitations.Comment: 8 page
