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The Spectrometer/Telescope for Imaging X-rays (STIX) is the HXR instrumentonboard Solar Orbiter designed to observe solar flares over a broad range offlare sizes, between 4-150 keV. We report the first STIX observations ofmicroflares recorded during the instrument commissioning phase in order toinvestigate the STIX performance at its detection limit. This first resultpaper focuses on the temporal and spectral evolution of STIX microflaresoccuring in the AR12765 in June 2020, and compares the STIX measurements withGOES/XRS, SDO/AIA, and Hinode/XRT. For the observed microflares of the GOES Aand B class, the STIX peak time at lowest energies is located in the impulsivephase of the flares, well before the GOES peak time. Such a behavior can eitherbe explained by the higher sensitivity of STIX to higher temperatures comparedto GOES, or due to the existence of a nonthermal component reaching down to lowenergies. The interpretation is inconclusive due to limited counting statisticsfor all but the largest flare in our sample. For this largest flare, thelow-energy peak time is clearly due to thermal emission, and the nonthermalcomponent seen at higher energies occurs even earlier. This suggests that theclassic thermal explanation might also be favored for the majority of thesmaller flares. In combination with EUV and SXR observations, STIX corroboratesearlier findings that an isothermal assumption is of limited validity. Futurediagnostic efforts should focus on multi-wavelength studies to derivedifferential emission measure distributions over a wide range of temperaturesto accurately describe the energetics of solar flares. Commissioningobservations confirm that STIX is working as designed. As a rule of thumb, STIXdetects flares as small as the GOES A class. For flares above the GOES B class,detailed spectral and imaging analyses can be performed.

STIX X-ray microflare observations during the Solar Orbiter commissioning phase