X-ray luminosity per unit SFR as a function of the oxygen abundance of the galaxies in our sample. Black filled squares correspond to those galaxies with measured X-ray luminosities, while blue triangles represent the detection limits for single sources. These can be regarded as upper limits for the galaxy luminosity in the case it had only one source. The red short-dashed line represents the value found by Mineo et al. (2012) for solar-metallicity galaxies (), while the gray dotted line is the best linear fit to our sample, . A clear trend of decreasing with oxygen abundance is observable, which is a direct evidence of the existence of metallicity effects in HMXB populations. Stochasticity effects are also evident, and require a more refined treatment to make a robust prediction of the variation of the ratio with metallicity. The cyan and green long-dashed lines above and below 12 + log (O / H) = 8 respectively, represent our best models for the high- and low-metallicity subsamples, with the 68% confidence bands for the parameter Q (small-dotted gray lines).
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