X-ray Variability of dM Stars Observed by EXOSAT

Abstract

We have studied the quiescent X-ray emission from 6 dM stars observed by EXOSAT, searching for variability on time scales in the range 50 – 6000 sec. Excluding from the analysis obvious large flares we analysed the apparently “quiescent” emission using an optimized χ ² test which allows a determination of the variability amplitude and time scale or upper limits thereof.

None of the 6 stars in our sample exhibits significant variability in its “quiescent” emission. The X-ray emission from the other stars in the sample is consistent with the emission from a constant source governed by Poisson noise. Since the upper limits to the variability during quiescent periods are much lower then the variability observed during large flares, our analysis rules out the possibility of a continuous flare distribution with frequent low energy events.