(a): Unfolded spectra of RX J0440.9+4431 in 2–120 keV obtained from three example observations during the late 2022–2023 giant outburst. The spectrum in the left was obtained from ObsID: P0514361025 (near MJD 59965.37) before the outburst peak. The spectrum shown in the middle was obtained from the ObsID: P0514361038, taken at the peak of the outburst (MJD: 59977.804). The spectrum shown on the right side was obtained from the declining phase of the giant outburst at MJD 600005.769 (ObsID: P0514361057). The spectral residuals shown in the panels (b) in each figure were obtained from the BW model without any cyclotron line component, whereas the residuals shown in panels (c) are for the best-fitting BW model that required a cyclotron line component (in all of the three mentioned observations) and a second cyclotron line component (for the ObsID: P0514361038) in hard X-rays. A Gaussian component near 6.4 keV was added to the spectra to account for the emission from neutral Fe Kα line near the pulsar in RX J0440.9+4431.