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

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

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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.
Original languageEnglish
Article number131
JournalAstrophysical Journal Supplement Series
Issue number2
Number of pages8
Publication statusPublished - 2017


  • Sun: flares - sun: X-rays
  • Gamma rays

Cite this

Marsh, A. J., Smith, D. M., Glesener, L., Hannah, I. G., Grefenstette, B. W., Caspi, A., Krucker, S., Hudson, H. S., Madsen, K. K., White, S. M., Kuhar, M., Wright, P. J., Boggs, S. E., Christensen, F. E., Craig, W. W., Hailey, C. J., Harrison, F. A., Stern, D., & Zhang, W. W. (2017). First NuSTAR Limits on Quiet Sun Hard X-Ray Transient Events. Astrophysical Journal Supplement Series, 849(2), [131].