Power-law photon index as a function of X-ray luminosity. Open circles represent the DB (supercritical regime), filled circles the HB (subcritical regime). Orange data points in XTE J0658–073 correspond to the flaring episode during the peak of the outburst (see Fig. 1). In 4U 0115+63, red squares represent the results from the “bump” model (see text). The Eddington luminosity for a neutron star was taken to be 1.7 × 10³⁸ erg s^-1.