A Seven-Day Multi-Wavelength Flare Campaign on AU Mic III: Quiescent and Flaring Properties of the X-ray Spectra and Chromospheric lines

Abstract

We present the X-ray quiescent and flaring properties from a unique, 7-day multiwavelength observing campaign on the M1 flare star AU Mic. Combining the XMM-Newton X-ray spectra with the chromospheric line and broadband NUV and optical continuum observations provides a dataset that is one of the most comprehensive to date. We analyze the sample of 38 X-ray flares and study in detail the X-ray flare temperature ($T$) and emission measure (EM) evolutions of three largest flares with the X-ray flare energies of $>10^{33}$ erg. The $T-\mathrm{EM}$ evolution tracks and multi-wavelength emission evolutions of the largest-amplitude Neupert-type flare reveal that the so-called ``Flare H-R diagram" is consistent with thermal coronal flare emission evolution. The two other more gradual and longer duration X-ray flares are interpreted as having larger size scales. None of the 17 H$\alpha$ and H$\beta$ flares show clear blue/red wing asymmetries, including the ones associated with the potential X-ray dimming event previously reported. The above largest-amplitude Neupert flare shows clear symmetric H$\alpha$ and H$\beta$ broadenings with roughly $\pm$400 and $\pm$600 km s${}^{-1}$, respectively, which are synchronized with the optical/NUV continuum emission evolution. Radiative hydrodynamic modeling results suggest that electron beam heating parameters that have been used to reproduce M-dwarf flare NUV/optical continuum emissions can reproduce these large broadenings of H$\alpha$ and H$\beta$ lines. These results suggest that these most energetic M-dwarf flares are associated with stronger magnetic field flux densities and larger size scales than solar flares but can be interpreted in terms of the standard flare model.

77 pages, 44 figures and 7 tables. The online figure set will be available in the published journal. Accepted for publication in The Astrophysical Journal (on September 8, 2025)

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• Young M dwarfs flare activity model: Towards better exoplanetary atmospheric characterisation