The faintest solar coronal hard X-rays observed with FOXSI
Authors: Buitrago-Casas Juan Camilo , Glesener Lindsay , Christe Steven , Krucker Säm , Vievering Juliana , Athiray P. S. , Musset Sophie , Davis Lance , Courtade Sasha , Dalton Gregory , Turin Paul , Turin Zoe , Ramsey Brian , Bongiorno Stephen , Ryan Daniel , Takahashi Tadayuki , Furukawa Kento , Watanabe Shin , Narukage Noriyuki , Ishikawa Shin-nosuke , Mitsuishi Ikuyuki , Hagino Kouichi , Shourt Van , Duncan Jessie , Zhang Yixian , Bale Stuart D. 
Published: July 2022

Context.Solar nanoflares are small impulsive events releasing magnetic energy in the corona. If nanoflares follow the same physics as their larger counterparts, they should emit hard X-rays (HXRs) but with a rather faint intensity. A copious and continuous presence of nanoflares would result in a sustained HXR emission. These nanoflares could deliver enormous amounts of energy into the solar corona, possibly accounting for its high temperatures. To date, there has not been any direct observation of such persistent HXRs from the quiescent Sun. However, the quiet-Sun HXR emission was constrained in 2010 using almost 12 days of quiescent solar off-pointing observations by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). These observations set 2σupper limits at 3.4 × 10−2photons s−1cm−2keV−1and 9.5 × 10−4photons s−1cm−2keV−1for the 3–6 keV and 6–12 keV energy ranges, respectively.Aims.Observing faint HXR emission is challenging because it demands high sensitivity and dynamic range instruments. The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket experiment excels in these two attributes when compared with RHESSI. FOXSI completed its second and third successful flights (FOXSI-2 and -3) on December 11, 2014, and September 7, 2018, respectively. This paper aims to constrain the quiet-Sun emission in the 5–10 keV energy range using FOXSI-2 and -3 observations.Methods.To fully characterize the sensitivity of FOXSI, we assessed ghost ray backgrounds generated by sources outside of the field of view via a ray-tracing algorithm. We used a Bayesian approach to provide upper thresholds of quiet-Sun HXR emission and probability distributions for the expected flux when a quiet-Sun HXR source is assumed to exist.Results.We found a FOXSI-2 upper limit of 4.5 × 10−2photons s−1cm−2keV−1with a 2σconfidence level in the 5–10 keV energy range. This limit is the first-ever quiet-Sun upper threshold in HXR reported using ∼1 min observations during a period of high solar activity. RHESSI was unable to measure the quiet-Sun emission during active times due to its limited dynamic range. During the FOXSI-3 flight, the Sun exhibited a fairly quiet configuration, displaying only one aged nonflaring active region. Using the entire ∼6.5 min of FOXSI-3 data, we report a 2σupper limit of ∼10−4photons s−1cm−2keV−1for the 5–10 keV energy range.Conclusions.The FOXSI-3 upper limits on quiet-Sun emission are similar to that previously reported, but FOXSI-3 achieved these results with only 5 min of observations or about 1/2600 less time than RHESSI. A possible future spacecraft using hard X-ray focusing optics like those in the FOXSI 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.