The Sun as an X-ray star

Context. Stellar coronae are unresolved in X-rays, so inferences about their structure rely on spectral analysis. The Sun-as-an-X-ray-star (SaXS) approach uses the Sun as a spatially resolved template to interpret stellar spectra, but previous SaXS implementations were indirect and computationally heavy. Aims. We present a new SaXS implementation that converts solar emission measure distributions (EMDs) of distinct coronal region types into XSPEC spectral components, and we aim to test whether broadband X-ray spectra alone can recover the lling factors of those region types. Methods. We built XSPEC multi-temperature spectral models for four solar region types (background or quiet coronae, active regions, cores, and flares) by considering EMDs derived from the analysis of Yohkoh/SXT data and by translating each EMD bin to an isothermal <ns0:sc>VAPEC</ns0:sc> component, or, alternatively, to a nonequilibrium–ionization collisional plasma <ns0:sc>VNEI</ns0:sc> component. These XSPEC models were fit (using PyXspec) to two one-hour DAXSS spectra representative of quiescent (29 June, 2022) and flaring (25 Apr., 2022) states. Best-fit normalizations were converted into projected areas and filling factors and compared with near-coincident Hinode/XRT full-disk images for validation. Results. Using the Yohkoh/SXT EMDs, we found that the spectrum of the quiescent Sun is dominated by active-region emission (filling factor ≈21%), with the background corona poorly constrained, while the spectrum of the flaring Sun is best described by a combination of active regions, cores, and flares with filling factors ≈14%, ≈3%, and ≈0.07%, respectively. We checked that the dominant components qualitatively match spatial features in Hinode/XRT images. Major limitations are the DAXSS low-energy calibration cutoff (∼0.7 keV) and the small, nonuniform Yohkoh EMD sample adopted, which may affect constraints on cool, low-emission regions and on elemental line emission. Conclusions. We demonstrate that our SaXS implementation enables direct retrieval of coronal filling factors from broadband X-ray spectra and provides a physically motivated alternative to ad hoc few-temperature fits. This approach can therefore potentially be routinely applied to stellar X-ray spectra to infer the distribution of coronal structures.