The first X-ray imaging spectroscopy of quiescent solar active regions with NuSTAR

Iain G. Hannah, Brian W. Grefenstette, David M. Smith, Lindsay Glesener, Säm Krucker, Hugh S. Hudson, Kristin K. Madsen, Andrew Marsh, Stephen M. White, Amir Caspi, Albert Y. Shih, Fiona A. Harrison, Daniel Stern, Steven E. Boggs, Finn Erland Christensen, William W. Craig, Charles J. Hailey, William W. Zhang

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We present the first observations of quiescent active regions (ARs) using the Nuclear Spectroscopic Telescope Array (NuSTAR), a focusing hard X-ray telescope capable of studying faint solar emission from high-temperature and non-thermal sources. We analyze the first directly imaged and spectrally resolved X-rays above 2 keV from non-flaring ARs, observed near the west limb on 2014 November 1. The NuSTAR X-ray images match bright features seen in extreme ultraviolet and soft X-rays. The NuSTAR imaging spectroscopy is consistent with isothermal emission of temperatures 3.1-4.4 MK and emission measures 1-8 × 1046 cm−3. We do not observe emission above 5 MK, but our short effective exposure times restrict the spectral dynamic range. With few counts above 6 keV, we can place constraints on the presence of an additional hotter component between 5 and 12 MK of∼1046 cm−3 and ∼1043cm−3, respectively, at least an order of magnitude stricter than previous limits. With longer duration observations and a weakening solar cycle (resulting in an increased livetime), future NuSTAR observations will have sensitivity to a wider range of temperatures as well as possible non-thermal emission.
Original languageEnglish
Article numberL14
JournalThe Astrophysical Journal Letters
Issue number1
Number of pages7
Publication statusPublished - 2016


  • Sun: activity
  • Sun: corona
  • Sun: X-rays, gamma rays


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