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Fig. 7.

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Upper limits of the quiet-Sun photon flux spectrum. The thresholds in gray are taken from Hannah et al. (2010). They correspond to 2σ limits calculated based on the analysis of 11.9 days of solar off-pointing observations with RHESSI during solar quiescent conditions. The three upper limits found using FOXSI-2 and -3 data are overlapping, all three in the 5–10 keV energy range. The limit in orange (4.5 × 10−2 s−1 cm−2 keV−1) is calculated by implementing the ON/OFF Li-Ma method over an area free of ghost rays during ∼1 min of FOXSI-2 observations. The threshold in red (9.6 × 10−4 s−1 cm−2 keV−1) corresponds to the upper limit obtained by combining FOXSI-3 measurements of only one detector that also had background measurements from the FOXSI-2 flight. This limit was also computed by implementing the ON/OFF Li-Ma method. The blue bar (6.0 × 10−4 s−1 cm−2 keV−1) is the upper threshold estimated from the 6.49 min of observations with three FOXSI-3 silicon detectors combined. For this last threshold (blue), the Gehrels (1986) approach was used to set the upper expected rate of a hypothetical source of solar origin.

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