Best-fit functions and 1σ confidence contours for the XMM and LAT spectra of PSR J2021+4026. The parameters for the XMM spectral models are those reported in Sect. 3.4. LAT spectral models are taken from the 4FGL-DR2 catalogue. Power laws were extrapolated down to 1 eV to highlight the observed discontinuity in the high-energy spectral index. The optical upper limits are reported in blue. The dotted vertical lines indicate the validity energy bands of the X-ray (0.3–12 keV) and γ-ray (0.1–300 GeV) best-fit functions.