A survey of long-term X-ray variability in cool stars

X-ray variability in cool stars can be indicative of coronal magnetic field changes and reconfiguration from a variety of phenomena, including flare events (typical timescales of minutes – hours), active-region evolution (hours – days – weeks), rotational modulation (hours – days – weeks), and activity cycles (years – decades). As part of the EXTraS project (Exploring the X-ray transient and variable sky - http://www.extras-fp7.eu/ ), we are performing a survey of ‘long-term’ X-ray variability using the ~decade-long public database of XMM-Newton observations. We are thus focussing here on timescales from ~a day to ~a decade, using average flux values from individual XMM-Newton observations. Though the resulting sampling is often highly non-uniform in time, the light-curves can provide valuable insights into the magnetic activity outside of shorter-term flaring episodes. We have taken a number of stellar samples (Hipparcos-Tycho, Simbad …) and are evaluating the statistical properties of the flux distributions, and comparing these across, for example spectral type, and with previously-published estimates. We are also examining the potential effects of flare events on the apparent long-term variability estimates. We give a preliminary report both on the overall variability distributions and extreme cases, distinguishing between serendipitously-observed stars (yielding, in some sense an unbiased sample) and XMM-Newton target objects (a number of them already reported by other authors).