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P.C.H. Martens (Montana State University), P. Chattterjee (Indian Institute of Science), S.H. Saar (Center for Astrophysics), L.W.A. Acton (Montana State University)
A decade worth of data from the Soft X-ray Telescope onboard the Japan/US/UK mission Yohkoh shows that the Sun has a variation of non-flaring disk-integrated soft X-ray emission (0.4 - 2.4 keV) of about a factor 30 over its sunspot cycle. To date no cyclic variation of that magnitude has been observed in other late-type stars.
We show that this negative result is partly explained by the inclusion of EUV emission in stellar observations done with ROSAT: we calculated what the solar signal would be if observed in the ROSAT passband and found a cycle amplitude of a factor two to three. That leaves open the question of the cycle amplitude in soft X-rays for solar-type stars.
To adress this we analysed ROSAT data for the energy band above the C-K edge, but found no sufficiently frequent observations of individual stars. The next best approach is to compare the measured soft X-ray flux for singly observed stars with the average flux predicted from the star's Rossby number: if cycles with amplitudes as large as that of the Sun exist, the residual fluxes should be statistically significant. From a sample of about 15 single and cyclic late-type stars (as evidenced from Ca-K data) we find that with 99.6% certainty the residual fluxes are due to X-ray cycles that are similar to or greater than that of the Sun.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.