  In the paper we apply asymptotic technique based on the method of stationary
phase and obtain the approximate analytical description of thermal motions
caused by a source on an isotopic defect of an arbitrary mass in a 1D harmonic
crystal. It is well known that localized oscillation is possible in this system
in the case of a light defect. We consider the unsteady heat propagation and
obtain formulae, which provide continualization (everywhere excepting a
neighbourhood of a defect) and asymptotic uncoupling of the thermal motion into
the sum of the slow and fast components. The slow motion is related with
ballistic heat transport, whereas the fast motion is energy oscillation related
with transformation of the kinetic energy into the potential one and in the
opposite direction. To obtain the propagating component of the fast and slow
motions we estimate the exact solution in the integral form at a moving point
of observation. We demonstrate that the propagating parts of the slow and the
fast motions are "anti-localized" near the defect. The physical meaning of the
anti-localization is a tendency for the unsteady propagating wave-field to
avoid a neighbourhood of a defect. The effect of anti-localization increases
with the absolute value of the difference between the alternated mass and the
mass of a regular particle, and, therefore, more energy concentrates just
behind the leading wave-front of the propagating component. The obtained
solution is valid in a wide range of a spatial co-ordinate (i.e., a particle
number), everywhere excepting a neighbourhood of the leading wavefront.
