The Sun as an X-ray star V.: A new method to retrieve coronal filling factors
Wilhelmina Maryann Joseph, Beate Stelzer, Salvatore Orlando, Moritz Klawin
TL;DR
The paper introduces a new SaXS implementation that converts solar emission measure distributions into XSPEC multi-temperature components for four coronal region types and fits broad-band X-ray spectra to recover filling factors. Applied to two DAXSS solar spectra, it finds that quiescent emission is AR-dominated with $ff_{AR}\approx$22%, while a flare requires AR, CO, and FL-AVG with $ff_{AR}\approx$47.5%, $ff_{CO}\approx$4.1%, and $ff_{FL-AVG}\approx$0.062%, with validation against Hinode/XRT imaging showing general consistency. Limitations include a 0.7 keV low-energy cutoff and a small, nonuniform EMD sample, motivating future refinements of EMDs, abundances, and potential non-equilibrium effects. The approach provides a physically grounded framework to extract coronal structure distributions from unresolved stellar X-ray spectra, offering an interpretable alternative to traditional few-temperature fits and enabling robust inferences about stellar activity.
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 test whether broad-band X-ray spectra alone can recover their filling factors. Methods. We built XSPEC multi-temperature spectral models for four solar region types (background/quiet corona, active regions, cores, and flares) by using EMDs derived from Yohkoh/SXT data and translating each EMD bin into an isothermal apec component. These models were fit (using PyXspec) to two one-hour DAXSS spectra representative of quiescent (2022-06-29) and flaring (2022-04-25) states. Best-fit normalizations were converted into projected areas and filling factors and compared with near-coincident Hinode/XRT full-disk images. Results. Using the Yohkoh/SXT EMDs, the quiescent Sun spectrum is dominated by active region emission (filling factor ~22%), with the background corona poorly constrained. The flaring Sun spectrum is best described by a combination of active regions, cores, and flares with filling factors of ~47.5%, ~4.1%, and ~0.062%, respectively. The dominant components match spatial features seen in Hinode/XRT images. Limitations include the DAXSS low-energy cutoff (~0.7 keV) and the small, non-uniform Yohkoh EMD sample. Conclusions. Our SaXS implementation enables direct retrieval of coronal filling factors from broad-band X-ray spectra and provides a physically motivated alternative to ad hoc few-temperature fits, suitable for stellar X-ray analyses.