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Published November 10, 2017 | Accepted Version + Published
Journal Article Open

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


We present the first results of a search for transient hard X-ray (HXR) emission in the quiet solar corona with the Nuclear Spectroscopic Telescope Array (NuSTAR) satellite. While 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. 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 timescales of 100 s and set upper limits on emission in two energy bands. We set 2.5–4 keV limits on brightenings with timescales 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 timescales 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 NuSTAR sensitivity by over two orders of magnitude due to higher instrument livetime and reduced solar background.

Additional Information

© 2017 The American Astronomical Society. Received 2017 May 13; revised 2017 October 2; accepted 2017 October 2; published 2017 November 8. This paper made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. We thank the NuSTAR Operations, Software and Calibration teams for support with the execution and analysis of these observations. This research has made use of the NuSTAR Data Analysis Software (NUSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). Some of the figures within this paper were produced using IDL color-blind-friendly color tables (Wright 2017). The authors would like to thank Albert Shih for helpful comments and suggestions. A.M. was supported by NASA Earth and Space Science Fellowship award NNX13AM41H. I.G.H. acknowledges support from a Royal Society University Research Fellowship. S.K. acknowledges support from the Swiss National Science Foundation (200021-140308). A.C. was supported by NASA grants NNX15AK26G and NNX14AH54G. L.G. was supported by an NSF Faculty Development Grant to UMN (AGS-1429512). P.J.W. was supported by an EPSRC/Royal Society Fellowship Engagement Award (EP/M00371X/1). This work was supported by NASA grants NNX12AJ36G and NNX14AG07G. Facility: NuSTAR - The NuSTAR (Nuclear Spectroscopic Telescope Array) mission.

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Published - Marsh_2017_ApJ_849_131.pdf

Accepted Version - 1711.05385.pdf


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August 19, 2023
October 17, 2023