QPE spectral evolution during XMM4. In the upper panel we show L_bol as a function of blackbody rest-frame temperature. The colour-code is that defined in Fig. 17. The evolution starts at R1 (lowest L_bol at ≃83 eV, red data point) and proceeds counter-clockwise. The two dotted lines are L_bol ∝ T⁴ relations normalised to the extremal of the QPE evolution (R1 and D2). The lower panel shows the estimated radius of the blackbody emitting region as a function of QPE phase. The re-scaled QPE profile is also shown for reference (light grey).