Phase-resolved spectrum of PSR J2021+4026 fitted with a power law (Γ = 1.0) and the atmosphere model discussed in Sect. 2 (T_p = 1 MK, B_p = 4 × 10¹² G, θ_cap = 20°, χ = 90°, ξ = 25°). Main panel: spectra corresponding to the five phase bins displayed in the inset. Bottom panels: five spectra and their residuals, in units of σ, with respect to the best fit (same color code as before). The lines indicate the two spectral components: The thermal one changes with phase, while the power law is constant.