ROSAT All-Sky Survey X-ray and EUV observations of YY Gem and AU Mic

Abstract

Simultaneous X-ray and extreme ultraviolet (EUV) (ROSAT XRT and WFC All-Sky Survey) observations of the highly active dMe flare stars YY Gem and AU Mic show that the two stars displayed an unusual type of flaring behaviour. We detect several X-ray and EUV flares superimposed on an enhanced and smoothly varying quiescent background. The two large impulsive-type X-ray flares on YY Gem reach peak X-ray luminosities of and we estimate that they had similar integrated luminosities (∼6–8×1033 erg). AU Mic also produced several X-ray and EUV flares, with one very impulsive flare producing a 10-fold increase in XRT count rate. This flare was even larger than the YY Gem flares (peak LX of and integrated LX of The ratio for both stars is at the ‘saturation’ limit found in rapidly rotating dwarfs and the most active RS CVn stars. We suggest that the gradually varying components are the result of a period of continuous, unresolved flaring activity. Alternatively, they may be the result of the emergence and subsequent decay of a new magnetic active region on the stellar surface of these stars.