Spectral parameter as a function of the 3–79 keV luminosity based on our orbit-by-orbit spectroscopy results (dark blue diamonds represent observation I, bright red stars observation IIa, and dark red stars observation IIb) together with kilosecond-integrated spectral fits from Fig. 7 in Fürst et al. (2014b) (grey circles). Upper panel: photon index (Γ). The dot-dashed black line is a linear regression through all data points, meant to guide the eye. Middle panel: energy of the harmonic CRSF (E_CRSF, H). The solid black line is the theoretical prediction for Λ = 1, E_NS = 30 keV, and M_NS = 1.9 M_⊙. The dot-dashed lines above and below the solid line are for M_NS = 1.7 M_⊙ and M_NS = 2.1 M_⊙, respectively. Lower panel: energy of the fundamental line (E_CRSF, F).