AN EXCEPTIONAL X-RAY FLARE ON THE DME STAR EQ1839.6+8002

A large impulsive flare was detected serendipitously during observatio ns of the radio galaxy 3C390.3 made with the Ginga satellite on 1991 F ebruary 14, between 22:30 and 23:00 (UT). The dMe star EQ1839.6+8002 i s the most likely source of this flare. The rise of the flare was obse rved and the spectra obtained are of sufficient quality to determine t he temperature and emission measure as a function of time. The flare i s exceptional in its high peak temperature (T-e similar to 10(8) K) an d emission measure (similar to 9 x 10(53) cm(-3)), the largest recorde d for a flare on a dMe star, and comparable with those for flares in R S CVn systems. The normalized X-ray luminosity is also very large (L(x )/L(bol)similar to 0.25). The flare geometry and plasma parameters are derived by making various assumptions concerning the dominant terms i n 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 similar to 1.7 x 10( 12) cm(-3), and the hot plasma has a length similar to 10(10) cm. The flare plasma then cools initially by 'evaporative conduction'. Two alt ernative simple models are made of the flare decay beyond this time (c ooling with constant mass or constant volume). Both require continued heating, and the latter gives results similar to those expected in qua si-static conditions. The evolution of the flare temperature and densi ty broadly resembles that predicted by numerical simulations. Observat ions with the Einstein IPC, the EXOSAT LE and the ROSAT PSPC instrumen ts show flaring and quiescent properties similar to those of the flare stars discussed by Pallavicini et al.