The Sun as an X-ray star: Active region evolution, rotational modulation, and implications for stellar X-ray variability

We study the contribution of an active region and its core to the luminosity and the spectrum of the Sun in the X-ray band and to the relevant solar emission measure vs. temperature distribution, EM(T). We also study the relevant changes in the course of four solar rotations, and the solar rotational modulation due to this active region, the only one present at that time. To this end, we have used a large sample of full-disk Yohkoh/SXT observations taken between July and October 1996, covering most of the active region evolution. From the Yohkoh/SXT data we have synthesized the X-ray spectra of the whole solar corona, and the focal plane data as they would be collected with Rosat/PSPC, XMM-Newton/EPIC and Chandra/ACIS. This work is part of a project to study the Sun as an X-ray star, using the solar data as a guide and a template for stellar observations. We found that the active region contributes significantly to the X-ray spectrum of the Sun mainly during the first month of its evolution. The rotational modulation due to the active region causes a significant variability of the average X-ray flux, with only moderate spectral variation, in the pass-bands of Rosat/PSPC, XMM-Newton/EPIC and Chandra/ACIS. We investigated the characteristics of the X-ray variability due to the rotational modulation, to the solar cycle, and to flares together with the possible implications on stellar X-ray variability. We derived the diagram of X-ray surface flux vs. spectral hardness ratio in the Rosat/PSPC band; we studied the variability due to the solar cycle and to the rotational modulation and we found that both lead to the same correlation between F_pspc and HR_pspc with a very steep slope. The variability due to the evolution of flares again produces a correlation between F_pspc and HR_pspc but with a much flatter slope than in the other two cases. Analogous results have been found when analyzing data in the format of XMM-Newton/EPIC and Chandra/ACIS.