Spectral evolution observed during the 2022–2023 giant outburst. Left panel: spectral evolution and pulse profile of RX J0440.9+4431 during the outburst, based on Insight-HXMT data. The color bar signifies flux intensity, with dark blue denoting periods without observations. White dots represent X-ray luminosity in the 0.3–120 keV energy range, aligned with the right y-axis. Two dashed-dotted lines (MJD 59948; MJD 59981) indicate the positions of ObsID P0514361005 (low state) and ObsID P0514361040 (high state), respectively. Right panel: evolution of the energy ratio over phase from ObsID P0514361040. The solid black line represents the pulse profile for 70–100 keV (Li et al. 2023), corresponding to the right y-axis.