Variability of the cyclotron line energy with luminosity for different sources. The blue dashed line represents the critical luminosity, plotted by setting L_X = L_crit and E_cyc = E_∗, where L_crit is evaluated using Eq. (55). The red dashed line represents the Coulomb stopping luminosity, plotted by setting L_X = L_coul and E_cyc = E_∗, where L_coul is evaluated using Eq. (59). Left: observations on a pulse-to-pulse timescale. Right: observations on longer timescales.