Example of the spectra analysed in this work. Panel a) Comptonized spectra showing a cut-off at high energies within the IBIS/ISGRI energy range. We classify these spectra in two groups: hard (Γ ≤ 1.7; dark blue) and soft (Γ > 1.7; light blue). The hardness selection is based on the Γ values derived using the nthcomp model. From top to bottom, the effective integration times for these spectra are 36, 170 s (Γ > 1.7 spectra) and 91, 1285 s (Γ ≤ 1.7 spectra). Panel b) Comptonized spectra for which T_e cannot be constrained by our data. We fixed the electron temperature to the value T_e = 999 keV in these spectral fits. We note the range in fluxes and hardness presented by these spectra. The effective integration times for these spectra are 8, 403, 1730 and 2251 s (top to bottom). Panel c) spectra for which a Comptonized model was not statistically favoured by our model selection criteria (p–value <0.05). These spectra are predominantly found during the X-ray plateaus detected at fluxes F_x ~ 5 × 10^-8 erg cm^-2 s^-1, as explained in the text. The effective integration times for these spectra are 297, 579 and 912 s (top to bottom).