Evolution of the local absorption column over time for NGC 5408 X-1 (green), NGC 7793 P13 (blue), Holmberg IX X-1 (red), and NGC 1313 X-2 (black) shifted by 0.2 × 10²² cm⁻² for clarity. All spectra have been fitted with a model consisting of an absorbed dual diskbb model in the 0.3−8 keV band. Errors are shown at 3σ confidence level. Circles and squares correspond to XMM-Newton and Chandra data respectively.