The faintest solar coronal hard X-rays observed with FOXSI

Context. Solar nanoflares are small impulsive events releasing magnetic energy in the corona. If a fairly quiet configuration, displaying only one aged non-flaring active region. Using the entire ∼ 6.5 minutes of FOXSI-3 data, we report a 2 σ upper limits of ∼ 10 − 4 photons s − 1 cm − 2 keV − 1 for the 5-10 keV energy range. Conclusions. FOXSI-3’s upper limits on quiet Sun emission are similar to that reported by Hannah et al. (2010), but FOXSI-3 achieved these results with only 5 minutes of observations or about 1 / 2600 less time than RHESSI. A possible future spacecraft using hard X-ray focusing optics like FOXSI’s concept would allow enough observation time to constrain the current HXR quiet Sun limits further or perhaps even make direct detections. This is the first report of quiet Sun HXR limits from FOXSI and the first science paper using FOXSI-3 observations.