Angular power spectra over angular quantum number ℓ. Panel a) shows the inferred spectra of the diffuse photon flux φ^(s) in each energy band and a power-law fit thereof, cf. legend. Panel b) shows the direct realization spectra of the diffuse signal s = log (φ^(s)/φ₀) and the inferred, fitted, and averaged spectra, cf. legend. The colors (from red to violet to blue) indicate energy (from band 1 to 9) in both panels, and a white noise spectrum with arbitrary normalization is included for comparison.