SIMBAD references

We have systematically analyzed X-ray variability of dM stars. Our data base is the sample of all dM stars listed in the CNS3 (Gliese & Jahreiss, 1991NSC3..C......0G) catalog which have been observed with the ROSAT PSPC. Our data sample includes 86 pointed observations of 55 distinct stars or multiple systems. A large fraction of stars shows significant variations, regardless of their quiescent flux. Variability is detected on all observable time scales. The amplitudes of these variations are independent of both stellar X-ray and visual luminosity. Compared to solar X-ray variability properties our results suggest that the amplitude distribution of X-ray variability in dM stars is consistent with the analogous distribution for solar flares. We discuss the effect of variability on the spread observed in the X-ray luminosity function of M stars. The comparison of our data with those obtained with Einstein IPC shows that variations on time scales shorter than a few month are more common than long term variations comparable to, e.g., the 11 years solar cycle.