Reconstructed simulated spectra of GRO J1655-40 for t_exp = 10 ks at a flux level of 1 Crab (top panel). The dotted and dashed lines represent the black-body and the power-law components, respectively. The spectra are rebinned for clarity reasons. Bottom panel: log ξ vs. log N_H confidence contours at the 1, 2, and 3σ levels. The black cross shows the input values used for the simulations. The inset shows the typical precision (at lσ) that can be achieved using current facilities. We plot the 3σ contours obtained with the X-IFU, for comparison.