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Abstract

Aims. We study populations of soft and super-soft X-ray sources (SSSs) in nearby galaxies of various morphological types with particular emphasis on characterizing populations of stable nuclear burning, accreting white dwarfs (WDs).
Methods. Analyzing the content of the Chandra archive, we assembled a sample of nearby galaxies suitable for studying populations of SSSs. Our sample includes four spiral galaxies, two lenticular galaxies, and three ellipticals with stellar mass exceeding 10¹⁰ M_⊙ and X-ray sensitivity of the order of a few × 10³⁶ erg s⁻¹. We used a combination of hardness ratio and median energy to pre-select X-ray sources with soft spectra, and used the temperature–X-ray luminosity diagram to identify SSSs, likely nuclear-burning, accreting WDs.
Results. For spiral galaxies, there is a distinct and rare population of super-soft sources that are largely detached from the rest of the 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 surface of WDs. The combined spectrum of soft sources located outside this boundary shows clear emission lines of Mg and S, the equivalent width of which is similar to that in the combined spectrum of a large number of confirmed supernova remnants in M 83. This supports earlier suggestions that the vast majority of the so-called quasi-soft sources are supernova remnants. In early-type galaxies, populations of super-soft sources are about a factor of eight less abundant, in broad agreement with the population synthesis calculations. Specific frequencies of super-soft sources are (2.08 ± 0.46) × 10⁻¹⁰ M_⊙⁻¹ in spiral galaxies and (2.47 ± 1.34) × 10⁻¹¹ M_⊙⁻¹ in lenticular and elliptical galaxies, with the ratio of the latter to the former being 0.12 ± 0.05.