Simulated NuSTAR spectrum based on the polcap model. Top panel: Simulated rebinned spectra in blue and teal for the two NuSTAR focal plane modules, respectively. The red line shows the polcap model, on which the simulated spectra are based. The input parameters are given in the lower left corner. Consistent with our treatment in Sect. 2, we base spectral fits only on data points sufficiently well below the CRSF energy, as the doublehump model does not account for CRSFs; data points not used spectral fitting are shown in gray. The fit doublehump model is shown in orange; its two cutoffpl components are shown as dashed orange lines. The cyclotron energy in observer’s rest frame is indicated by the black dashed line. Bottom panel: χ residuals of doublehump fit.