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

The Spectrometer/Telescope for Imaging X-rays (STIX) is the HXR instrument onboard Solar Orbiter designed to observe solar flares over a broad range of flare sizes, between 4-150 keV. We report first STIX observations microflares recorded during commissioning phase in order investigate performance at its detection limit. This result paper focuses on temporal and spectral evolution occuring AR12765 June 2020, compares measurements with GOES/XRS, SDO/AIA, Hinode/XRT. For observed GOES A B class, peak time lowest energies located impulsive flares, well before time. Such behavior can either be explained by higher sensitivity temperatures compared GOES, or due existence nonthermal component reaching down low energies. interpretation inconclusive limited counting statistics all but largest our sample. this flare, low-energy clearly thermal emission, seen occurs even earlier. suggests that classic explanation might also favored majority smaller flares. In combination EUV SXR observations, corroborates earlier findings an isothermal assumption validity. Future diagnostic efforts should focus multi-wavelength studies derive differential emission measure distributions wide accurately describe energetics Commissioning confirm working as designed. As rule thumb, detects small class. above detailed imaging analyses performed.