The Sun as an X-ray star

V. A new method to retrieve coronal filling factors

¹ Institut für Astronomie & Astrophysik, Eberhard Karls Universität Tübingen, Sand 1, 72076 Tübingen, Germany
² INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 24 October 2025
Accepted: 5 March 2026

Abstract

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 VAPEC component, or, alternatively, to a nonequilibrium–ionization collisional plasma VNEI 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.