Broadband X-ray spectra of APM 08279 collected by XMM-Newton and NuSTAR in 2019. Panel a: observed-frame spectra and best-fit model (axill model). Panels b to e: observed-frame residuals. Vertical black dashed lines mark the energy of the Fe Kα emission line. Spectra in each panel were rebinned to 4σ (with setplot rebin 4 100 in Xspec) for showing purposes. Data are color-coded as follows: XMM 301 EPIC-pn is shown in black, XMM 301 EPIC-MOS1 in red, XMM 301 EPIC-MOS2 in green, XMM 101 EPIC-pn in blue, Nu02 FPMA in cyan and FPMB in magenta, Nu04 FMPA in yellow, and FPMB in orange. Model from top to bottom: (a) axill model, (b) pl model, (c) acutpl model, (d) acutpl+Kα model, (e) axill model. All models are modified by Galactic absorption (N_H = 4.2 × 10²⁰ cm⁻²). Best-fit parameters are summarized in Table 2.