Hard X-Ray Emission in AU Mic Flares: A Minor Contributor to Planetary Atmospheric Escape
Creators
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1.
California Institute of Technology
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2.
Imperial College London
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3.
Osservatorio Astronomico di Palermo
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4.
National Astronomical Observatories
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5.
Jet Propulsion Lab
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6.
University of Chinese Academy of Sciences
Abstract
Stellar flares are potent drivers of atmospheric evolution on orbiting exoplanets, primarily through extreme ultraviolet (EUV) and soft X-ray irradiation. However, the contribution of hard X-rays (HXR; 3–20 keV), which penetrate deeper into planetary atmospheres, to mass loss and particle acceleration has remained poorly understood. To quantify the HXR share of the total radiative budget, we conducted quasi-simultaneous observations of the active M-dwarf AU Mic using NuSTAR, Swift, and the Einstein Probe. Our analysis detected two major flares, and we performed an empirical check by deriving a quiescent-phase soft X-ray (SXR; 0.3–3 keV) to HXR relation and then applying it to the flares. By combining this with the quiescent coronal SXR–EUV relation conversion of J. Sanz-Forcada et al, we computed the total high-energy flux (EUV + SXR + HXR) and assessed the relative role of HXR in atmospheric escape. We find that HXR accounts for only a few percent of the total radiative energy budget during both quiescent and flaring states. While a high-energy spectral tail is detected in the second flare, time-resolved spectroscopy reveals a dominant chromospheric-evaporation signature, indicating that the flare energetics are primarily thermal.
Copyright and License
© 2025. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Acknowledgement
We are grateful to the anonymous referee and to the editor, Allen W. Shafter, for the careful reading of and useful comments on the manuscript. Y.H. is deeply grateful to Prof. Giovanna Tinetti for her valuable support and the insightful suggestions that significantly improved the discussion section. This work 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 (NASA). We thank the NuSTAR Operations, Software, and Calibration teams for their support with the execution and analysis of these observations. This research has made use of the NuSTAR Data Analysis Software, jointly developed by the ASI Science Data Center (Italy) and the California Institute of Technology (USA). It also utilized data and software provided by the High Energy Astrophysics Science Archive Research Center, a service of the Astrophysics Science Division at NASA’s Goddard Space Flight Center. This work also made use of data provided by NASA through the Swift mission. We thank Dr. James Delaunay, Dr. Kim Page, and the Swift Observatory Duty Scientist for their assistance in coordinating observations with NuSTAR. We also acknowledge the use of data from the Einstein Probe (EP) mission and sincerely thank the EP mission team for supporting our analysis. We are especially grateful to the coordinators and schedulers from Einstein Probe, NuSTAR, and Swift for their generous help in planning and executing the joint observations. We gratefully acknowledge the Principal Investigators of EP, NuSTAR, and Swift for approving our DDT and ToO requests. This project was supported by the Caltech SURF program and NASA grants FAH.NUSBCD-1-NASA.NS. Yuk L. Yung was supported by NASA Grant P2794428 to Caltech.
Facilities
NuSTAR - The NuSTAR (Nuclear Spectroscopic Telescope Array) mission, Swift - Swift Gamma-Ray Burst Mission, EP. -
Software References
XSPEC (K. A. Arnaud 1996), HEASoft, Python.
Files
Hu_2025_ApJ_995_189.pdf
Additional details
Additional titles
- Alternative title
- Hard X-ray Contribution in AU Mic Flares and Its Minor Role in Atmospheric Escape
Related works
- Is new version of
- Discussion Paper: arXiv:2508.15107 (arXiv)
Funding
- California Institute of Technology
- National Aeronautics and Space Administration
- NuSTAR FAH.NUSBCD-1-NASA.NS
- National Aeronautics and Space Administration
- P2794428
Dates
- Submitted
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2025-08-21
- Accepted
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2025-10-28
- Available
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2025-12-16Published
Caltech Custom Metadata
- Caltech groups
- Astronomy Department , NuSTAR , Space Radiation Laboratory , Division of Geological and Planetary Sciences (GPS) , Division of Physics, Mathematics and Astronomy (PMA) , Physics Department
- Publication Status
- Published