  Einstein's general relativity predicts that pressure, in general stresses,
play a similar role to energy density in generating gravity. The source of
gravitational field, the active gravitational mass density, sometimes referred
to as Whittaker's mass density, is not conserved, hence its changes can
propagate as monopole gravitational waves. Such waves can be generated only by
astrophysical sources with varying gravitational mass. Here we show that
relativistic fireballs, considered in modelling gamma-ray burst phenomena, are
likely to radiate monopole gravitational waves from high-pressure plasma with
varying Whittaker's mass. Also, ejection of a significant amount of initial
mass-energy of the progenitor contributes to the monopole gravitational
radiation. We identify monopole waves with h^11+h^22 waves of Eddington's
classification which propagate (in the z-direction) together with the energy
carried by massless fields. We show that the monopole waves satisfy Einstein's
equations, with a common stress-energy tensor for massless fields. The
polarization mode of monopole waves is Phi_22, i.e. these are perpendicular
waves which induce changes of the radius of a circle of test particles only
(breathing mode). The astrophysical importance of monopole gravitational waves
is discussed.
