Temperature and optical depth of the simulated spectra used to validate the correlation found in the NuSTAR data. A thousand NuSTAR spectra were simulated with random values of kT and τ (see details in Sect. 5). The left panel show the values used to simulate the data with average values in bins of temperature (red), while the right panel shows the temperature and optical depth found when fitting the simulated spectra, with the average values in temperature bins.