Long-term X-ray light curves of the 2020 outburst of 1A 0535+262. Top panel: NICER count rate in the 0.2–12 keV band. Each observation is represented by a purple square. Bottom panel: HXMT light curves from the Core Science Program (P0314316). The LE (1–10 keV), ME (10–35 keV), and HE (27–250 keV) count rates are represented by the black (bottom), green (middle), and red (top) triangles, respectively. Additional observations from proposal P0304099 (PI: P. Reig) are marked with circles. Each data point represents a single observational epoch. The shaded grey region denotes the interval during which the millihertz QPO was most significantly detected at energies above 27 keV (Ma et al. 2022) and selected for cross-spectral analysis.