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

TL;DR

This study uses NuSTAR to set the first upper limits on transient hard X-ray emissions from the quiet Sun, improving sensitivity and providing constraints on solar microflare activity in hard X-ray bands.

Contribution

It demonstrates NuSTAR's capability to observe the quiet Sun in hard X-rays and establishes upper limits on transient emissions, advancing solar X-ray observational techniques.

Findings

Set upper limits on quiet Sun hard X-ray emissions in 2.5-4 keV and 10-20 keV bands.

Detected no transient brightenings above the established thresholds.

Projected increased sensitivity during solar minimum with future observations.

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.