a: X-ray spectrum of Vela X-1 as measured with NuSTAR (ObsID 30002007003). Data from the two instruments of NuSTAR, FPMA and FPMB, are shown in red and blue, respectively. The best-fit model as presented by Fürst et al. (2014) is shown in black. The dashed green lines indicates the continuum model without the CRSF components. b: residuals in terms of χ² between the data and the best-fit model. c: residuals between the data and the model without the CRSF components. The harmonic line around 55 keV is much more prominent than the fundamental line around 25 keV.