Spectroscopic observations of flares and superflares on AU Mic

TL;DR

This study conducted extensive spectroscopic monitoring of the active flare star AU Mic, analyzing 24 flares to characterize their properties and identify a rare superflare with energy around 10^33 erg.

Contribution

First detailed spectroscopic analysis of flares on AU Mic, including the detection of a rare chromospheric superflare with high energy.

Findings

24 flares analyzed spectroscopically

Detected a superflare with ~10^33 erg energy

Correlated photometric and spectroscopic flare observations

Abstract

The young active flare star AU~Mic is the planet host star with the highest flare rate from TESS data. Therefore, it represents an ideal target for dedicated ground-based monitoring campaigns with the aim to characterize its numerous flares spectroscopically. We performed such spectroscopic monitoring with the ESO1.52m telescope of the PLATOSpec consortium. In more than 190 hours of observations, we find 24 flares suitable for detailed analysis. We compute their parameters (duration, peak flux, energy) in eight chromospheric lines (H$\alpha$, H$\beta$, H$\gamma$, H$\delta$, Na I D1&D2, He I D3, He I 6678) and investigate their relationships. Furthermore, we obtained simultaneous photometric observations and low-resolution spectroscopy for part of the spectroscopic runs. We detect one flare in the g'-band photometry which is associated with a spectroscopic flare. Additionally, an extreme flare event occurred on 2023-09-16 of which only a time around its possible peak was observed, during which chromospheric line fluxes were raised by up to a factor of three compared to the following night. The estimated energy of this event is around $10^{33}$ erg in H$\alpha$ alone, i.e. a rare chromospheric line superflare.