Proof-of-concept Hubble diagram for the BASS sample (grey points) using the relation between X-ray excess variance and the 2–10 keV luminosity. The dark orange points show the mean values obtained by binning the sample into nine equal-sized bins. Cyan points represent the Pantheon+ Supernovae Ia sample (Scolnic et al. 2018), while the black line represents the prediction from a standard flat ΛCDM model. The redshift range and limited statistics of the BASS sample currently undermine the cosmological implementation of the method, but future probes might open new possibilities (see Sect. 6).