The Sun as an X‐Ray Star. II. Using theYohkoh/Soft X‐Ray Telescope–derived Solar Emission Measure versus Temperature to Interpret Stellar X‐Ray Observations

Abstract

This paper is the second of a project dedicated to using solar Yohkoh/SXT data as a guide and a template to interpret data on stellar coronae. In the light of the large differences in scope and approach between solar and stellar studies, we have developed a method to translate Yohkoh/SXT data of the whole solar corona into stellar-like data, i.e., to put them in the same format and context as the stellar ones. First from the Yohkoh/SXT images we derive the whole-Sun X-ray emission measure versus temperature [EM(T)], in the range 10^5.5-10⁸ K, during the specific observation. Then, we synthesize the solar X-ray spectrum; finally, we fold the spectrum through the instrumental response of nonsolar X-ray observatories, for instance, ROSAT/PSPC and ASCA/SIS. Finally, we analyze such solar coronal data in the same band and with the same methods used for stellar observations, allowing a direct and homogeneous comparison with them. In this paper we present in detail our method and, as an example of results, we show and discuss EM(T) and stellar-like spectra for three phases of the solar cycle: maximum, intermediate phase, and minimum. The total amount and the distribution of the emission measure change dramatically during the cycle, in particular at temperatures above 10⁶ K. We also show the EM(T) of the whole solar corona during a large flare. The ROSAT/PSPC- and ASCA/SIS-like X-ray spectra of the Sun as a star that we obtain are discussed in the context of stellar coronal physics. The Sun's coronal total luminosity in the ROSAT/PSPC band ranges from ≈2.7 × 10²⁶ ergs s^-1 (at minimum) to ≈4.7 × 10²⁷ ergs s^-1 (at maximum). We discuss future developments and possible applications of our method.