Joint spectral characterization of GRB 221009A across multiple time periods. (a)–(c) Analysis for the high-flux peak phase (T_ref + 228–230 s) utilizing a Band + BB model. (d)–(f) Spectral analysis for the late decay phase (T_ref + 246–256 s) utilizing a multicomponent Band + Gaussian model. Panels (a) and (d) show observed count rate spectra compared with the folded best-fit models (solid lines). Data are from GECAM-C (high-gain in green; low-gain in orange), Fermi/GBM bismuth germanate (BGO) detectors (b0 in purple; b1 in blue) and HEPP-H (gray). Panels (b) and (e) show the νF_ν spectral distribution (units of keV² · photons · cm⁻² · s⁻¹ · keV⁻¹). For the peak phase (b), the displayed spectrum is fitted with the Band + BB model. For the late phase (e), the model includes a Band tail and a Gaussian line feature centered near 37 MeV. In both time periods, HEPP-H data (gray) span the 2–255 MeV energy range. Panels (c) and (f) show residuals in units of standard deviation (σ) with 1σ error bars.