a) Difference of χ² of the best fit of epoch I with frozen CRSF parameters (Fig. 2k) and the χ² of the best fit without CRSF. The centroid energy was varied in steps of 1 keV. The red dashed-dotted curve displays the χ² of the best fit when using τ_CRSF and σ_CRSF from Heindl et al. (2001). For the light blue solid curve the parameters from this work were used. The dark blue dashed-double-dotted curve shows this difference when the width as well as the depth (the upper limit has been set to 3) of the CRSF are allowed to vary, i.e., the same situation as in Fig. 2. The two additional minima in χ² are discussed in Sect. 3.2. The horizontal dashed line indicates Δχ² = 0, and the horizontal dotted line seperates the logarithmic and linear scaling in y-direction. The vertical dashed lines shows the position of the CRSF derived in Heindl et al. (2001) and this work. b), c) Results for the optical depth and width of the CRSF, respectively, when these parameters were allowed to vary. The plateau of the curve displayed in panel b at  ~60–70 keV is caused by the upper limit for τ_CRSF of 3.