Evolution of the spectral parameters with the 15–50 keV flux, F_PL, of GRO J1008−57. a) The power-law photon index, Γ, and b) the black body flux F_BB (in erg s^-1 cm^-2), with the source’s flux (uncertainties represent 1σ uncertainties with two degrees of freedom). The black data points are the results of our previous RXTE and the black triangle the joined Suzaku RXTE analysis (K13). Their fluxes have been corrected for calibration uncertainties (see the text for details). The new results presented in this paper are the green triangles based on Suzaku and the blue circles based on NuSTAR data. The red lines show the empirical model with (solid) and without (dashed) the breaks at certain luminosity levels as described in the text. The NuSTAR results marked with 1) have been excluded from these fits. The bolometric luminosity for one accretion column has been calculated as described in Sect. 5.