kT_bb − L_X diagram for soft X-ray sources in the galaxies of our sample. A cut of 20 net counts was applied. The luminosities are in the 0.3−2.0 keV band and are not corrected for absorption. All plotted sources have no statistically significant flux above 2 keV (open circles). Sources with median energy Ẽ ≤ 1.2 keV are shown with solid orange circles. Sources with Gaia matches of measured parallax or proper motion are marked by black diamonds. Solid red curves show kT_bb − L_X relations at the lower boundary of stable nuclear burning. Along these curves, the WD mass increases from 0.51 M_⊙ at the lower left end to 1.34 M_⊙ at the upper right end. The dotted red line for Centaurus A shows the location of this boundary in the dust lanes of this galaxy. Green lines correspond to the emission spectrum of the general relativistic accretion disk model around a Schwarzschild BH (Ebisawa et al. 1991) for BH masses of 10, 20, 30, and 50 M_⊙. The accretion rate changes along the curves from 0.02 Ṁ_Edd to 1 Ṁ_Edd. Horizontally oriented solid black curves show the luminosity corresponding to 20 net counts. The dashed blue line on each plot shows the 20 counts luminosity limit for M 51 corrected for the difference in absorption column density between M 51 and the galaxy under consideration. All model curves show absorbed luminosities. Errors are at 1σ confidence. See Sect. 5.2 for details of calculations.