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arXiv · 2508.15107

Hard X-ray Emission in AU Mic Flares: A Minor Contributor to Planetary Atmospheric Escape

Yifan Hu·Murray Brightman·Fabio Favata·Haiwu Pan·Brian Grefenstette·Fiona A. Harrison·Daniel Stern·Weimin Yuan·Yuk L. Yung·Xiurui Zhao
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Abstract

Stellar flares are potent drivers of atmospheric evolution on orbiting exoplanets, primarily through extreme ultraviolet (EUV) and soft X-ray (XUV) 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)-HXR relation and then applying it to the flares. By combining this with the quiescent coronal SXR-EUV relations conversion of J. Sanz-Forcada et al. (2011), 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.

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Yifan Hu, Murray Brightman, Fabio Favata, Haiwu Pan, Brian Grefenstette, Fiona A. Harrison, Daniel Stern, Weimin Yuan, Yuk L. Yung, Xiurui Zhao. 2025-08-20. Hard X-ray Emission in AU Mic Flares: A Minor Contributor to Planetary Atmospheric Escape. https://arxiv.org/abs/2508.15107

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