Stellar Activity Levels from X-ray Observations of Solar-Analog Superflare Stars

Soft X-ray observations provide an excellent measurement of stellar magnetic activity, exhibiting cyclic changes from solar minimum to solar maximum. For the Sun, X-ray measurements have been taken every 1-min with the GOES X-ray telescopes providing 40 years and four solar cycles worth of monitoring. The X-ray flux correlates with sunspot number and allows for historical comparison of activity and flare information collected over the past several hundred years (Clette et al. 2014). The last solar superflare occurred in 1859, disturbing the technology of the time (telegraph systems) and producing powerful aurorae. In order to better understand the conditions leading to superflares, which are extremely rare events with recurrence of 500-5000 years (Maehara et al., 2012), we must study superflares from large statistical samples of solar-analogs. From recent Kepler studies, superflares are identified in only a few slowly rotating, solar-analogs. Here we present XMM-Newton data for three of these slowly-rotating, high orbital period, Kepler stars. Stellar magnetic activity levels derived from the X-ray observations of these known superflare solar-analog stars are then compared with historical magnetic activity in the Sun.