EXOSAT Observations of Flare Stars and Coronal Heating

Abstract

A program of observation of dMe stars is described that has been undertaken with EXOSAT as part of a collaboration of groups at Armagh, Catania, and JILA (University of Colorado) for multiband coverage of flare stars. The following conclusions were drawn: (1) The LE X-ray flux varies continuously over time scales of 20 seconds to hours with many surges or micro-flares lasting for periods of a fraction of a minute to several minutes; (2) When simultaneous ground-based optical spectroscopy has been possible these micro-flares are seen to be accompanied by increases in Balmer line flux similar to that seen in solar flares; (3) The micro-flares typically have energies of 2 x 10 to the 30th ergs in the 0.1-2-keV range, i.e. very similar to compact solar events (though with decay times sometimes of the order of tens of seconds they can be much more short-lived than solar flares; and (4) The frequency of occurrence of the micro-flares is of the order of several per hour.