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

Abstract

We present the first observations of quiescent active regions (ARs) using 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 $1-8\times 10^{46}$~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 $\sim 10^{46}$cm${}^{-3}$ and $\sim 10^{43}$ cm${}^{-3}$, 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.

arXiv comment7 page, 5 figures, accepted for publication in ApJL

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• Long-Exposure NuSTAR Constraints on Decaying Dark Matter in the Galactic Halo
• Microflare Heating of a Solar Active Region Observed with NuSTAR, Hinode/XRT, and SDO/AIA
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• The First Focused Hard X-ray Images of the Sun with NuSTAR
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• NuSTAR Observation of a Minuscule Microflare in a Solar Active Region
• Hard X-Ray Constraints on Small-Scale Coronal Heating Events
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• Achievements and Lessons Learned from Successful Small Satellite Missions for Space Weather-Oriented Research
• Evidence of Significant Energy Input in the Late Phase of a Solar Flare from NuSTAR X-Ray Observations
• FOXSI-2 Solar Microflares II: Hard X-ray Imaging Spectroscopy and Flare Energetics
• Estimate of the Upper Limit on Hot Plasma Differential Emission Measure (DEM) in Non-Flaring Active Regions and Nanoflare Frequency Based on the Mg XII Spectroheliograph Data from CORONAS-F/SPIRIT
• Fine-pitch CdTe detector for hard X-ray imaging and spectroscopy of the Sun with the FOXSI rocket experiment
• First NuSTAR Limits on Quiet Sun Hard X-Ray Transient Events
• Quantifying the Influence of Key Physical Processes on the Formation of Emission Lines Observed by IRIS: I. Non-Equilibrium Ionization and Density-Dependent Rates
• Hot plasma in a quiescent solar active region as measured by RHESSI, XRT, and AIA
• Simulation of Quiet-Sun Hard X-rays Related to Solar Wind Superhalo Electrons
• How Nanoflares Produce Kinetic Waves, Nano-Type III Radio Bursts, and Non-Thermal Electrons in the Solar Wind
• First joint X-ray solar microflare observations with NuSTAR and Solar Orbiter/STIX
• A new view of the solar interface region from the Interface Region Imaging Spectrograph (IRIS)
• Thermal Evolution of an Active Region through Quiet and Flaring Phases as Observed by NuSTAR XRT, and AIA
• Detecting non-thermal emission in a solar microflare using nested sampling
• Synthesising Solar Radio Images From Atmospheric Imaging Assembly Extreme-Ultraviolet Data
• The Quiet-Sun DEM Under Kappa: Diagnostic Degeneracy and the Failure of the Conductive Closure