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

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 minutes) with respect to their ultraviolet counterparts, in agreement with the Neupert effect. Using X-ray spectral fits, we estimated the parameters of the emitting plasma. The results indicate the presence of a hot multi-thermal corona with average temperatures in the range of ∼7–15 MK and emission measure of ∼(2.9–4.5) × 1052 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 (∼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 ∼1031–1032 erg; the energy-duration relations indicate the presence of magnetic fields stronger than in typical solar flares.