Composite R-band light curve and X-ray 1 keV light curve for GRB 210731A. For each rising and decaying segment of the optical light curve, we show the power-law slope (t^α) as an indicator of the steepness of the light curve between each pair of adjacent vertical dotted lines. We also show the fit comprising the sum of three BPL components and a constant term equal to the r′-band host galaxy flux measured by GROND at 285 days (solid red line), along with each individual BPL component (dashed blue lines). For the X-ray light curve, we indicate the steepness of the WT-mode segment and we show the BPL fit to the PC-mode data as a dashed-dotted red line. The results of the X-ray and optical fits are presented in Table 1. The ratio of observed flux to fitted flux is shown in the lower panel.