The faintest solar coronal hard X-rays observed with FOXSI

被引：0

作者：

Buitrago-Casas, Juan Camilo ^{[1
,2
]}

Glesener, Lindsay ^{[3
]}

Christe, Steven ^{[4
]}

Krucker, Säm ^{[1
,5
]}

Vievering, Juliana ^{[3
,6
]}

Athiray, P.S. ^{[3
,7
]}

Musset, Sophie ^{[3
,8
]}

Davis, Lance ^{[3
]}

Courtade, Sasha ^{[1
]}

Dalton, Gregory ^{[1
]}

Turin, Paul ^{[9
]}

Turin, Zoe ^{[10
]}

Ramsey, Brian ^{[7
]}

Bongiorno, Stephen ^{[7
]}

Ryan, Daniel ^{[4
,5
]}

Takahashi, Tadayuki ^{[11
,12
]}

Furukawa, Kento ^{[12
]}

Watanabe, Shin ^{[11
,13
]}

Narukage, Noriyuki ^{[14
]}

Ishikawa, Shin-Nosuke ^{[15
]}

Mitsuishi, Ikuyuki ^{[16
]}

Hagino, Kouichi ^{[17
]}

Shourt, Van ^{[1
]}

Duncan, Jessie ^{[3
]}

Zhang, Yixian ^{[3
]}

Bale, Stuart D. ^{[1
,2
]}

机构：

[1] Space Sciences Laboratory, University of California Berkeley, Berkeley,CA, United States

[2] Physics Department, University of California, Berkeley,CA, United States

[3] University of Minnesota, Physics and Astronomy, Minneapolis,MN, United States

[4] Nasa Goddard Space Flight Center, Greenbelt,MD, United States

[5] University of Applied Sciences and Arts Northwestern Switzerland, Windisch, Switzerland

[6] Johns Hopkins University, Applied Physics Laboratory, Laurel,MD, United States

[7] Nasa Marshall Space Flight Center, Huntsville,AL, United States

[8] Esa, European Space Research and Technology Centre (ESTEC), Noordwijk, Netherlands

[9] Heliospace Corporation, Berkeley,CA, United States

[10] University of Colorado, Boulder,CO, United States

[11] Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo, Tokyo, Japan

[12] Department of Physics, The University of Tokyo, Tokyo, Japan

[13] Institute of Space and Astronautical Science, Kanagawa, Japan

[14] National Astronomical Observatory of Japan, Tokyo, Japan

[15] Rikkyo University, Graduate School of Artificial Intelligence and Science, Tokyo, Japan

[16] Nagoya University, Graduate School of Science, Aichi, Japan

[17] Tokyo University of Science, Shinjuku City, Tokyo, Japan

来源：

Astronomy and Astrophysics | 2022年 / 665卷

关键词：

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 2 upper limits at 3.4-102 photons s1 cm2 keV1 and 9.5-104 photons s1 cm2 keV1 for 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 4.5-102 photons s1 cm2 keV1 with a 2 confidence level in the 510 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 2 upper limit of 104 photons s1 cm2 keV1 for 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. © 2022 Published by EDP Sciences;

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