Populations of super-soft X-ray sources in galaxies of different
  morphological types
  We study populations of soft and super-soft X-ray sources in nearby galaxies
of various morphological types with the special emphasis on characterizing
populations of stable nuclear burning accreting WDs. Analysing the content of
Chandra archive we assembled a sample of nearby galaxies suitable for studying
populations of super-soft X-ray sources. Our sample includes 4 spiral galaxies,
2 lenticular galaxies and 3 ellipticals with stellar mass exceeding $10^{10}$
$M_\odot$ and X-ray sensitivity of the order of a ${\rm few}\times 10^{36}$
erg/s. We used combination of hardness ratio and median energy to pre-select
X-ray sources with soft spectra, and temperature - X-ray luminosity diagram to
identify super-soft X-ray sources - likely nuclear burning accreting white
dwarfs. For spiral galaxies, there is a distinct and rare population of
super-soft sources, largely detached from the rest of sources on the
$kT_{bb}-L_X$ plane. The boundary between these sources and the much more
numerous population of harder (but still soft) sources is consistent with the
boundary of stable hydrogen burning on the white dwarf surface. Combined
spectrum of soft sources located outside this boundary, shows clear emission
lines of Mg and S, which equivalent width is similar to that in the combined
spectrum of a large number of confirmed supernova remnants in M83. This
confirms earlier suggestions that the vast majority of so called quasi-soft
sources are supernova remnants. In early-type galaxies, populations of
super-soft sources are about a factor of $\approx 8$ less abundant, in broad
agreement with the population synthesis calculations. Specific frequencies of
super-soft sources are: (2.08$\pm$0.46)$\times10^{-10}$ M$_{\odot}^{-1}$ in
spiral galaxies and (2.47$\pm$1.34)$\times10^{-11}$ M$_{\odot}^{-1}$ in
lenticular and elliptical galaxies, with the ratio of the latter to the former
of $0.12\pm0.05$.
