2025ApJ...995..189H

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.