Dependence of the energy of the fundamental cyclotron line on luminosity. The two positive and three negative correlations clearly seen in the figure are derived from the pulse-to-pulse analysis (blue crosses). The red and magenta crosses are values obtained from the pulse-averaged spectra of the 2015 giant outburst and the 2016 outburst. The observations with negative p2p-correlations comprise Group 1, while Group 2 consists of two observations with positive p2p-correlations, labeled (1) and (2), respectively. The gray line reflects the “hysteresis” behavior observed with Swift/BAT (Cusumano et al. 2016).