X-Ray and Ultraviolet Flares on AT Microscopii Observed by AstroSat

TL;DR

This study reports multi-wavelength observations of flares on the active M-dwarf binary AT Mic, revealing hot coronal plasma, elemental abundance variations, and magnetic field strengths exceeding those of typical solar flares.

Contribution

First simultaneous UV and X-ray observations of AT Mic's flares, providing detailed plasma diagnostics and insights into stellar magnetic activity.

Findings

X-ray flares lag UV flares by 5-6 minutes, consistent with the Neupert effect.

Coronal temperatures ranged from 7 to 15 MK, increasing during flares.

Flares had energies of 10^{31} to 10^{32} erg, indicating strong magnetic fields.

Abstract

We present observations of the active M-dwarf binary AT Mic (dM4.5e+dM4.5e) obtained with the orbital observatory AstroSat. During 20 ks of observations, in the far ultraviolet ($130-180$ nm) and soft X-ray ($0.3-7$ keV) spectral ranges, we detected both quiescent emission and at least five flares on different components of the binary. The X-ray flares were typically longer than and delayed (by $5-6$ min) with respect to their ultraviolet counterparts, in agreement with the Neupert effect. Using X-ray spectral fits, we have estimated the parameters of the emitting plasma. The results indicate the presence of a hot multi-thermal corona with the average temperatures in the range of $\sim 7-15$ MK and the emission measure of $\sim (2.9-4.5)\times 10^{52}$ $\textrm{cm}^{-3}$; both the temperature and the emission measure increased during the flares. The estimated abundance of heavy elements in the corona of AT Mic is considerably lower than at the Sun ($\sim 0.18-0.34$ of the solar photospheric value); the coronal abundance increased during the flares due to chromospheric evaporation. The detected flares had the energies of $\sim 10^{31}-10^{32}$ erg; the energy-duration relations indicate the presence of magnetic fields stronger than in typical solar flares.