Cyclotron line energy as a function of the Swift count rate. For the data points from this work we use the CRSF line energies as obtained from the phase-averaged analysis in Sect. 3.1. Where required, the ASM count rate was converted as Swift count rate as described in Appendix A. The uncertainties are given on a 90% level. The semi-transparent data points with dashed error bars do not satisfy the criteria described in Sect. 5.4, as they are either too wide or the line energy is too poorly constrained. The two gray areas represent regions where we find the cyclotron line energies to be believable. Data are from Ding et al. (2021), Nabizadeh et al. (2019), Fürst et al. (2018), Suchy et al. (2012), Doroshenko et al. (2010), La Barbera et al. (2005), Coburn et al. (2002), and Orlandini & Fiume (2001).