New Insights into the Large 1992 July 15–17 Flare on AU Microscopii: The First Detection of Posteruptive Energy Release on a Red Dwarf Star

Abstract

Extreme Ultraviolet Explorer (EUVE) observations of the long-duration 1992 July 15-17 flaring event on the red dwarf star AU Mic are discussed. The flare was detected in the EUVE Deep Survey (DS) Lexan/B band (65-190 Å) and was made up of a large impulsive event corresponding to an increase in the extreme ultraviolet (EUV) emission by a factor of 15 relative to quiescence, followed by a slow exponential decay interspersed with another smaller impulsive event and further decay. As observed in the EUV, the flare lasted for more than half a day. A similar exponential decay of the flux in the Fe XVIII λ93.9 line was detected in the EUVE short wavelength (SW) spectrometer following the first powerful impulsive phase. The flare decay in the 65-190 Å band and in the Fe XVIII line during this prolonged event lasted a factor of 10 or more longer than would have been the case had the plasma cooling been due only to radiative losses at typical flare plasma densities. An analysis of the simultaneous DS Lexan/B data and the Fe XVIII line flux has allowed us to determine the approximate temporal behavior of the flaring plasma temperature (d log T/dt ≈ -0.5 day-1) and of the emission measure of the EUV source. The total energy emitted in the 1-2000 Å range during the first 12 hours or so of the flaring event was 3 × 1035 ergs. Solar analogs of processes providing similar long-duration soft X-ray radiation are discussed: (1) a coronal mass ejection (CME); (2) a system of giant postflare arches, which can form in powerful solar flares after a CME; (3) typical soft X-ray flaring loops. A general expression describing the energy balance in the EUV source is derived for both CMEs and giant arch systems. As a result, we conclude that the long-duration EUV emission in the AU Mic flare most resembles the solar scenario of emission from a system of giant coronal arches forming after a CME. Such a system on AU Mic would have a size of 1-2 stellar radii and would be energized by reconnection in a vertical current sheet. In this picture, a substantial part of the radiative loss is compensated for by additional postflare energy input, in a similar way as is thought to take place in the posteruptive phases of the most powerful solar flares. Apparently, we are faced here with a new kind of surface activity on late-type stars, which is intermediate between impulsive flares on red dwarf stars and the long-duration, powerful events seen on subgiant components of the RS CVn binaries.