Left: Joint fit of the J1231 data at five epochs (colour–coded) using the fit function constant*TBabs*slimdz. The EPIC MOS data are excluded for plot clarity only. The solid, dashed, and dotted lines represent the total model, the slim disc, and the background components, respectively. The bottom panel shows the ratio between observed counts per spectral bin (data) and the predicted counts from the best-fit model (solid lines in the top panel). The spectral evolution is well explained by a slim disc with only its ṁ varying between epochs (Table D.1). Right: Same as the left plot, but here the spectral evolution across the three XMM-Newton epochs is explained by a varying θ while the ṁ remains constant.