Observed number of HMXBs with luminosities higher than 1038 erg s-1, per unit SFR, as a function of the oxygen abundance. Black circles are the data points, with error bars representing Poissonian uncertainties. Large error bars reaching the lower end of the plot represent galaxies with no detected sources; their upper ends correspond to N38 = 1. Our best model for low-metallicity galaxies (12 + log (O / H) < 8, green solid line, Q = 3.13, σextra = 0) predicts ten times more HMXBs than the corresponding best model for high-metallicity galaxies (cyan dot-dashed line, Q = 2.14, σextra = 22). For both models, the 68% confidence bands for the parameter Q and the dispersion predicted for the data are shown (gray dashed lines and shaded regions, respectively). The mean Q = 2.28 for high-metallicity galaxies with σextra = 0 is also plotted (blue solid line). The difference between low- and high-metallicity galaxies cannot be explained by statistical effects.
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