The average X-ray spectrum of the volume-complete M-, F-, G-, and K-type star sample within 10 pc of the Sun

¹ Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching bei München, Germany
² INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
³ Como Lake centre for AstroPhysics (CLAP), DiSAT, Università dell’Insubria, via Valleggio 11, 22100 Como, Italy
⁴ Institut für Astronomie und Astrophysik, Eberhard Karls Universität Tübingen, Sand 1, 72076 Tübingen, Germany
⁵ Dr. Karl Remeis Observatory, Erlangen Centre for Astroparticle Physics, Friedrich-Alexander-Universität Erlangen-Nürnberg Sternwartstraße 7, 96049 Bamberg, Germany
⁶ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
⁷ Department of Astronomy, University of Science and Technology of China, Hefei 230026, China
⁸ School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China
⁹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁰ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85748 Garching, Germany

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Received: 28 October 2025
Accepted: 23 February 2026

Abstract

Context. The F-, G-, K-, and M-type stars are the most abundant stellar population in the Milky Way and are expected to contribute to its diffuse X-ray emission. Yet their intrinsic average X-ray spectrum remains poorly constrained due to their faint X-ray luminosities, leaving their collective role in the X-ray background of the Milky Way uncertain.

Aims. We aim to derive a distance-normalised average X-ray spectrum for nearby M dwarfs and F, G, and K stars and to characterise their ensemble spectral properties.

Methods. We analysed the volume-complete sample of M dwarfs (M0-M6) and F, G, and K stars within 10 pc of the Sun using data from the eROSITA all-sky survey aboard the Spectrum-Roentgen-Gamma (SRG) mission (eRASS:4). Individual spectra were normalised by exposure and distance and then stacked to produce representative averages. From these spectra, we derived characteristic X-ray luminosities, which are helpful for estimating the contribution of late-type stars to the soft, diffuse X-ray emission of the Galaxy. Results. The distance-normalised emission measures yield an average X-ray luminosity of (2.6 ± 0.1) × 10²⁷ erg/s for M-type stars, and (15 ± 3) × 10²⁷ erg/s for F-, G-, and K-type stars in 0.2-2.0 keV. The 10-pc average spectra could be well described by a sum of three and two thermal models. The fitted temperatures and abundances remain consistent across M-star sub-groups, while early-M stars are, surprisingly, less luminous on average than mid-to-late-M types. These results offer new insights into the collective X-ray properties of nearby stars in the volume-complete context, and provide motivation to further explore the link with the unresolved soft X-ray background of the Galaxy.