A Statistical Survey of Faint Solar X-Ray Transients Observed by NuSTAR
8 affiliations
- 1University of Minnesota
- 2NASA Marshall Space Flight Center
- 3University of Glasgow
- 4Epic Systems
- 5California Institute of Technology
- 6Air Force Research Laboratory
- 7University of Applied Sciences and Arts Northwestern Switzerland
- 8University of California
Abstract
In this paper, we use a highly sensitive telescope to characterize solar X-ray transients ranging from microflares in active regions down to weakly energetic brightenings in the quiet Sun. X-rays are closely linked to the initial energy release and immediate heating of solar flares, making them invaluable in understanding their driving processes. NuSTAR is the first long-term, direct focusing hard X-ray observatory to have observed the Sun, offering a unique opportunity to search for and characterize X-ray events from inside and outside active regions that would be otherwise unobservable. We present the first statistical survey of NuSTAR solar observations, characterizing the thermal and possibly nonthermal properties of 113 weakly energetic transients down to 10 ²⁶ erg, making this the first study to directly compare events from the quiet Sun to those in active regions. Relative to RHESSI microflares, our NuSTAR transients are generally cooler, dimmer, and have slightly steeper spectra. Thermal energy content of active region transients appears to be independent of the volume of emitting plasma for transients produced by active regions. This is in contrast to those from the quiet corona, which on average have lower energy content, smaller emission volumes, and appear cool but bright rather than hot but dim, suggesting a break in trends from traditional microflares. We found no quiet Sun transients with a thermal energy content above 3 × 10 ²⁷ erg, implying an upper limit on the amount of energy released in plasma above 3 MK by quiescent processes.
Related
- A bright X-ray flaring event from the intermediate supergiant fast X-ray transient IGR J00370+6122 observed with NuSTAR
- Faint and Fast Transients from Double Detonations of Low Mass White Dwarfs
- A bright X-ray flaring event from the intermediate supergiant fast X-ray transient IGR J00370+6122 observed with NuSTAR
- PEARLS: NuSTAR and XMM-Newton Extragalactic Survey of the JWST North Ecliptic Pole Time-Domain Field IV: X-Ray Variability Analysis
- PEARLS: NuSTAR and XMM-Newton Extragalactic Survey of the JWST North Ecliptic Pole Time-Domain Field V: Unraveling X-ray Spectral Variability
BibTeX
@article{masek2026statistical,
title = {{A Statistical Survey of Faint Solar X-Ray Transients Observed by NuSTAR}},
author = {Reed B. Masek and Lindsay Glesener and Jessie Duncan and Kekoa Lasko and Natália Bajnoková and Mary Davenport and Marianne Peterson and Ian Markano and Zasha Avery and Kristopher Cooper and Iain G. Hannah and Brian W. Grefenstette and Stephen M. White and Hugh Hudson and Säm Krucker and David M. Smith and Sarah Paterson},
journal = {The Astrophysical Journal},
year = {2026},
month = oct,
doi = {10.3847/1538-4357/ae9d71},
url = {https://iopscience.iop.org/article/10.3847/1538-4357/ae9d71},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - A Statistical Survey of Faint Solar X-Ray Transients Observed by NuSTAR
AU - Reed B. Masek
AU - Lindsay Glesener
AU - Jessie Duncan
AU - Kekoa Lasko
AU - Natália Bajnoková
AU - Mary Davenport
AU - Marianne Peterson
AU - Ian Markano
AU - Zasha Avery
AU - Kristopher Cooper
AU - Iain G. Hannah
AU - Brian W. Grefenstette
AU - Stephen M. White
AU - Hugh Hudson
AU - Säm Krucker
AU - David M. Smith
AU - Sarah Paterson
PY - 2026
DA - 2026/10/12
JO - The Astrophysical Journal
DO - 10.3847/1538-4357/ae9d71
UR - https://iopscience.iop.org/article/10.3847/1538-4357/ae9d71
AB - In this paper, we use a highly sensitive telescope to characterize solar X-ray transients ranging from microflares in active regions down to weakly energetic brightenings in the quiet Sun. X-rays are closely linked to the initial energy release and immediate heating of solar flares, making them invaluable in understanding their driving processes. NuSTAR is the first long-term, direct focusing hard X-ray observatory to have observed the Sun, offering a unique opportunity to search for and characterize X-ray events from inside and outside active regions that would be otherwise unobservable. We present the first statistical survey of NuSTAR solar observations, characterizing the thermal and possibly nonthermal properties of 113 weakly energetic transients down to 10 ²⁶ erg, making this the first study to directly compare events from the quiet Sun to those in active regions. Relative to RHESSI microflares, our NuSTAR transients are generally cooler, dimmer, and have slightly steeper spectra. Thermal energy content of active region transients appears to be independent of the volume of emitting plasma for transients produced by active regions. This is in contrast to those from the quiet corona, which on average have lower energy content, smaller emission volumes, and appear cool but bright rather than hot but dim, suggesting a break in trends from traditional microflares. We found no quiet Sun transients with a thermal energy content above 3 × 10 ²⁷ erg, implying an upper limit on the amount of energy released in plasma above 3 MK by quiescent processes.
SN - 0004-637X
SN - 1538-4357
ER -