Exploration of the background and its impact in the high-energy spectrum of Apep. Top:NuSTAR image in the 20–35 keV energy range for FPMB (2022a). We indicate the position of Apep with a 30″ circle, the background extraction region used in the analysis (bkg1), and the alternative backgrounds used as independent checks (bkg2–bkg5). We also show with cyan contours the emission from the supernova remnant G330.2+1.0 obtained in the 2.5–8 keV range with XMM-Newton. The image is in linear scale (instead of logarithmic scale, as in Fig. 3), and it was smoothed in ds9 using a Gaussian with Radius=5. Bottom: Background-subtracted spectra (crosses) and background spectra (open circles) of Apep for the different background regions considered in Appendix B. In all cases, the whole data-set was co-added and rebinned to increase the S/N and to facilitate the visual comparison.