Temporal evolution of the thermal X-ray loop-top source of the SOL2021-05-07T18:43 M3.9 flare. a)–d) Source height, volume, and STIX-derived temperature and emission measure. The dotted lines in panel b have the same meaning as in Fig. 5a. The error bars in panels c,d represent the fitting uncertainties to the STIX spectra. e)–h) Source-averaged electron number density, mass, gravitational potential energy, and electron thermal energy derived from panels a–d. The dotted lines represent the uncertainty due to volume alone, and the error bars denote the combined volume and spectral fit uncertainties.