Observed cyclotron-line energy variation of the source V0332+53 with luminosity. The blue points correspond to data reported in Tsygankov et al. (2010). The solid black line is the optimal fit to the data, using our theoretical prediction. The dashed green line is the expected variation only due to the magnetic-field strength (see Eq. (3) and Basko & Sunyaev 1976), using as E_c, * and a the parameters obtained through the fit. The dash-dotted magenta line is the one accounting for the magnetic field variation and gravitational redshift, but not the Doppler effect (see Sect. 2.2.1). The vertical red line denotes the critical luminosity, and the horizontal blue line represents the cyclotron line energy on the NS’s surface.