The faintest solar coronal hard X-rays observed with FOXSI

Juan Camilo Buitrago-Casas, Lindsay Glesener, Steven Christe, Säm Krucker, Juliana Vievering, Subramania Athiray Panchapakesan, Sophie Musset, Lance A Davis, Sasha Courtade, Gregory Dalton, Paul Turin, Zoe Turin, Brian Ramsey, Stephen Bongiorno, Daniel Ryan, Tadayuki Takahashi, Kento Furukawa, Shin Watanabe, Noriyuki Narukage, Shin Nosuke IshikawaIkuyuki Mitsuishi, Kouichi Hagino, Van Shourt, Jessie M Duncan, Yixian Zhang, Stuart D. Bale

Research output: Contribution to journalArticlepeer-review

7 Scopus citations

Abstract

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.

Original languageEnglish (US)
Article numberA103
JournalAstronomy and Astrophysics
Volume665
DOIs
StatePublished - Sep 1 2022

Bibliographical note

Funding Information:
The first author of this paper is funded by the NASA FINESST grant 80NSSC19K1438. The FOXSI sounding rocket experiment is funded by NASA grants NNX11AB75G, NNX16AL60G, and 80NSSC21K0030. The Univeristy of Minnesota team is supported by an NSF Faculty Development Grant (AGS-1429512), an NSF CAREER award (NSF-AGS-1752268), and the SolFER DRIVE center (80NSSC20K0627). This work was also supported by JSPS KAKENHI Grant Numbers 22H00134, 21KK0052, JP18H03724, JP18H05463, JP17H04832, JP16H02170, JP16H03966, JP15H03647, JP24244021, JP21540251, JP20244017, and World Premier International Research Center Initiative (WPI), MEXT, Japan. The FOXSI team is grateful to the NSROC teams at WSMR and Wallops for the excellent operation of their systems. Furthermore, the authors would like to acknowledge the contributions of each member of the FOXSI experiment team to the project, particularly our team members at ISAS and Kavli IPMU for the provision of Si and CdTe detectors and at MSFC for the fabrication of the focusing optics.

Funding Information:
The first author of this paper is funded by the NASA FINESST grant 80NSSC19K1438. The FOXSI sounding rocket experiment is funded by NASA grants NNX11AB75G, NNX16AL60G, and 80NSSC21K0030. The Univeristy of Minnesota team is supported by an NSF Faculty Development Grant (AGS-1429512), an NSF CAREER award (NSF-AGS-1752268), and the SolFER DRIVE center (80NSSC20K0627). This work was also supported by JSPS KAKENHI Grant Numbers 22H00134, 21KK0052, JP18H03724, JP18H05463, JP17H04832, JP16H02170, JP16H03966, JP15H03647, JP24244021, JP21540251, JP20244017, and World Premier International Research Center Initiative (WPI), MEXT, Japan. The FOXSI team is grateful to the NSROC teams at WSMR and Wallops for the excellent operation of their systems. Furthermore, the authors would like to acknowledge the contributions of each member of the FOXSI experiment team to the project, particularly our team members at ISAS and Kavli IPMU for the provision of Si and CdTe detectors and at MSFC for the fabrication of the focusing optics

Publisher Copyright:
© 2022 Published by EDP Sciences.

Keywords

  • Gamma rays
  • Methods: statistical
  • Sun: X-rays
  • Sun: activity
  • Sun: corona
  • X-rays: diffuse background

Fingerprint

Dive into the research topics of 'The faintest solar coronal hard X-rays observed with FOXSI'. Together they form a unique fingerprint.

Cite this