A survey of long-term X-ray variability in cool stars
Abstract
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 have performed a systematic survey of long-term X-ray variability using the ∼decade-long public database of XMM-Newton observations (3XMM). 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 and evaluated statistical properties of the flux distributions, and compared these across, for example spectral type, and with previously-published estimates. We have also examined the potential effects of flare events on the apparent long-term variability estimates. We report here on overall variability distributions and extreme cases, focussing on serendipitously-observed stars (yielding,in some sense an unbiased sample).
- Publication:
-
The X-ray Universe 2017
- Pub Date:
- October 2017
- Bibcode:
- 2017xru..conf..319P