Variation in spectral parameters of RX J0440.9+4431 at various luminosity epochs of the giant outburst. An absorbed cutoff power-law model (i.e. cutoffpl) with additional components such as (1) a Gaussian component at 6.4 keV, (2) a cyclotron absorption component varying in 33.6–41.6 keV, (3) a soft blackbody component near 0.3 keV, and sometimes a second blackbody component near 2.5 keV, was used to obtain the best-fitted continuum model.