First NuSTAR Limits on Quiet Sun Hard X-Ray Transient Events

Abstract

We present the first results of a search for transient hard X-ray (HXR) emission in the quiet solar corona with the \textit{Nuclear Spectroscopic Telescope Array} (\textit{NuSTAR}) satellite. While \textit{NuSTAR} was designed as an astrophysics mission, it can observe the Sun above 2~keV with unprecedented sensitivity due to its pioneering use of focusing optics. \textit{NuSTAR} first observed quiet Sun regions on 2014 November 1, although out-of-view active regions contributed a notable amount of background in the form of single-bounce (unfocused) X-rays. We conducted a search for quiet Sun transient brightenings on time scales of 100 s and set upper limits on emission in two energy bands. We set 2.5--4~keV limits on brightenings with time scales of 100 s, expressed as the temperature T and emission measure EM of a thermal plasma. We also set 10--20~keV limits on brightenings with time scales of 30, 60, and 100 s, expressed as model-independent photon fluxes. The limits in both bands are well below previous HXR microflare detections, though not low enough to detect events of equivalent T and EM as quiet Sun brightenings seen in soft X-ray observations. We expect future observations during solar minimum to increase the \textit{NuSTAR} sensitivity by over two orders of magnitude due to higher instrument livetime and reduced solar background.

arXiv comment11 pages, 7 figures; accepted for publication in The Astrophysical Journal

References in corpus (15)

• The Nuclear Spectroscopic Telescope Array (NuSTAR) Mission
• Global Energetics of Thirty-Eight Large Solar Eruptive Events
• Calibration of the NuSTAR High Energy Focusing X-ray Telescope
• Key Aspects of Coronal Heating
• NuSTAR Observations of the Bullet Cluster: Constraints on Inverse Compton Emission
• Temporal aspects and frequency distributions of solar soft X-ray flares
• RHESSI Microflare Statistics II. X-ray Imaging, Spectroscopy & Energy Distributions
• Microflares and the Statistics of X-ray Flares
• Are Heating Events in the Quiet Solar Corona Small Flares? - Multiwavelength Observations of Individual Events
• Microflare Heating of a Solar Active Region Observed with NuSTAR, Hinode/XRT, and SDO/AIA
• The First Focused Hard X-ray Images of the Sun with NuSTAR
• The first X-ray imaging spectroscopy of quiescent solar active regions with NuSTAR
• Constraining the hard X-ray properties of the quiet Sun with new RHESSI observations
• NuSTAR hard X-ray observation of a sub-A class solar flare
• Evidence of Significant Energy Input in the Late Phase of a Solar Flare from NuSTAR X-Ray Observations

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• A New Component from the Quiet Sun: Synchrotron Radiation from Galactic Cosmic-Ray Electrons
• On the faintest solar coronal hard X-rays observed with FOXSI