Authors: Juan Camilo Buitrago-Casas (UC Berkeley), Lindsay Glesener (University of Minnesota), Steven Christe (NASA GSFC), Säm Krucker (UC Berkeley), Juliana Vievering (Applied Physics Laboratory Johns Hopkins), P.S. Athiray (University of Alabama in Huntsville), Sophie Musset (ESA), Lance Davis (University of Minnesota), Brian Ramsey (NASA MSFC), Stephen Bongiorno (NASA MSFC), Daniel Ryan (FHNW), Tadayuki Takahashi (The University of Tokyo), Kento Furukawa (The University of Tokyo), Noriyuki Narukage (National Astronomical Observatory of Japan), Shin-nosuke Ishikawa (Rikkyo University), Jessie Duncan (University of Minnesota), Yixian Zhang (University of Minnesota), and Stuart D. Bale (UC Berkeley)

Observing faint HXR emission is challenging because it demands instruments with high sensitivity and dynamic range. RHESSI has insufficient sensitivity to detect such faint sources, especially in the form of a large, diffuse signal rather than the bright, compact signals for which RHESSI was designed. The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket experiment excels in these two attributes. FOXSI achieves such a superior performance by pairing nested grazing-incidence Wolter-I mirrors with low-noise semiconductor detectors optimized for high energies. FOXSI’s direct focusing capabilities allow quiet regions of the corona to be isolated to look for the presence of HXR sources.

This work constrains the quiet Sun emission in the 5-10 keV energy range using FOXSI observations from the second and third rocket flights (FOXSI-2 and -3). To fully characterize FOXSI’s sensitivity, this work presents a ray-tracing simulation to assess ghost ray backgrounds generated by sources outside of the telescope field of view. This work demonstrates a Bayesian approach to provide upper thresholds of quiet Sun HXR emissions and probability distributions for the expected flux of a quiet-Sun HXR source when it is assumed to exist. For FOXSI-2 and -3, such upper limits are $4.5\times 10^{-2}$ photons s$^{-1}$ cm$^{-2}$ keV$^{-1}$ and  $6.0\times10^{-4}$ photons s$^{-1}$ cm$^{-2}$ keV$^{-1}$, respectively (both in the 5-10 keV energy range). These two limits are similar to that of RHESSI in the 6-12 keV energy band ($9.5\times 10^{-4}$ photons s$^{-1}$ cm$^{-2}$ keV$^{-1}$) but with an important difference: it took $\sim$1/2600 less time for FOXSI to get enough statistics to yield these equivalent limits. The FOXSI-2 limit presented here is the first-ever quiet Sun upper threshold in HXR estimated from observations performed during a period of high solar activity. The FOXSI-3 quiet Sun HXR analyses are the first scientific results that use the $\sim6.5$ minutes of the FOXSI-3 rocket observations. A possible future spacecraft using FOXSI’s concept would allow enough observation time to constrain the current HXR quiet Sun limits further or perhaps even make direct detections.