Correlation between the width (sigma) and the centroid energy of cyclotron lines for different X-ray binaries. The blue rhombs are data from the compilation in Staubert et al. (2019), the filled squares (with uncertainties) are from Coburn et al. (2002). The red line is a fit to these two data sets defining σ = 0.18 E_cyc. The individual sources are GX 304−1 (Klochkov et al. 2015), Swift J1626.6−5156 (DeCesar et al. 2013), Vela X-1 (La Parola et al. 2016), and Her X-1 (same as Fig. 4).