GBM observations of RX J0440 during 2023 outburst. Black dots (with gray error bars) show the GBM (12–50 keV) three-day average pulsed flux. The horizontal magenta dashed line indicates the critical luminosity at 2.8 × 10³⁷ erg s⁻¹ (Salganik et al. 2023) as related to the right y-axis units. The nonlinearity in the right y-axis is due to a variable conversion factor at different outburst stages (see text). The light green vertical continuous lines mark the interval when the source is showing QPO episodes.