An exceptional X-ray flare on the dMe star EQ1839.6+8002

A large impulsive flare was detected serendipitously during observations of the radio galaxy 3C390.3 made with the Ginga satellite on 1991 February 14, between 22:30 and 23:00 (UT). The dMe star EQ1839.6+8002 is the most likely source of this flare. The rise of the flare was observed and the spectra obtained are of sufficient quality to determine the temperature and emission measure as a function of time. The flare is exceptional in its high peak temperature (T_e~10^8K) and emission measure (~9x10^53 cm^-3), the largest recorded for a flare on a dMe star, and comparable with those for flares in RS CVn systems. The normalized X-ray luminosity is also very large (L_X/L_bol~0.25). The flare geometry and plasma parameters are derived by making various assumptions concerning the dominant terms in the internal energy equation, which takes account of a varying mass and volume. The initial heating appears to be proportional to the gas pressure. At the peak T_e the electron density is ~1.7x10^12 cm^-3, and the hot plasma has a length ~10^10 cm. The flare plasma then cools initially by `evaporative conduction'. Two alternative simple models are made of the flare decay beyond this time (cooling with constant mass or constant volume). Both require continued heating, and the latter gives results similar to those expected in quasi-static conditions. The evolution of the flare temperature and density broadly resembles that predicted by numerical simulations. Observations with the Einstein IPC, the EXOSAT LE and the ROSAT PSPC instruments show flaring and quiescent properties similar to those of the flare stars discussed by Pallavicini et al.