Population synthesis of DA white dwarfs: constraints on soft X-ray spectra evolution

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 [1]. 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 [2] TX-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 x 10(-12) pc(-3) yr(-1) T-max = 4.1 x 10(4) K. These values are in good correspondence with values obtained by [3, 2]. 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.