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

TL;DR

This study quantifies the contribution of hard X-rays in stellar flares on AU Mic, finding they account for only a small fraction of the energy budget, thus minimally impacting planetary atmospheric escape.

Contribution

It provides the first empirical estimate of the hard X-ray contribution to stellar flare energy budgets using simultaneous multi-instrument observations.

Findings

HXR contributes only a few percent to total flare energy.

Flare energetics are mainly thermal with chromospheric evaporation signatures.

High-energy spectral tail observed in one flare.

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.