Close view of the properties of the soft X-ray flare detected around MJD 57 200, derived fitting the source spectra with Comptonization models (nthcomp and compps). Panel a) time evolution of the source flux, F_x, in the 20–200 keV energy range (× 10^-8 erg cm^-2 s^-1) in comparison with the N_H evolution, where N_H was derived by leaving it as a free parameter in our fits, as described in Sect. 3.4. During the flare, when N_H displayed lower values, it could not be constrained by our fits. Panels b)–e) time evolution of the spectral parameters derived for this period. Panels f)–h) relations between the various nthcomp spectral parameters during this period. Panels i)–k) relations between the various compps spectral parameters during this period. For reference, we also show in panels f)–k) the complete dataset analysed in this work (grey points). During the flare rise and decay we observed parameter correlations characteristic of the soft flaring branch (see panels f)–k), Sect. 3.3). Around the peak of the flare, the spectra softened above Γ ~ 2.4 and T_e was constrained using nthcomp. Spectral fits to these data using compps provided un-constrained T_e values (T_e≳ 100 keV), and the lowest τ values found in this work (τ ≲ 1). These spectra occupy a region in the Γ–F_x diagram (panel f)) reminiscent of the ultra-luminous state in the BHB HID. These spectra also occupy a separate branch in the compps F_x–T_e diagram (panel i)). After the peak of the flare, as F_x decreased and the spectrum hardened below Γ ~ 2.4 the system returned to the soft flaring branch. Around MJD 57 199.85 a dramatic drop in flux was observed (from F_x ~ 50 to F_x ~ 15 × 10^-8 erg cm^-2 s^-1 in about half an hour) and the system entered the hard branch. After this transition, the system started the decay to quiescence, following the hard branch characteristic correlations, but for a decreasing flux.