  Extending the population synthesis method to isolated young cooling white
dwarfs we are able to confront our model assumptions with observations made in
ROSAT All-Sky Survey (Fleming et al., 1996). This allows us to check model
parameters such as evolution of spectra and separation of heavy elements in DA
WD envelopes. It seems like X-ray spectrum temperature of these objects is
given by the formula T_{X-ray} = min(T_eff, T_max). We have obtained DA WD's
birth rate and upper limit of the X-ray spectrum temperature: DA birth rate $=
0.61\times 10^{-12}$ in cubic parsec per year and T_max = 41000 K. These values
are in good correspondence with values obtained by other authors (Liebert et
al., 2004; Wolff et al., 1996). From this fact we also conclude that our
population synthesis method is applicable to the population of close-by
isolated cooling white dwarfs as well as to the population of the isolated
cooling neutron stars.
