The X-ray luminosity function of stars observed with SRG/eROSITA

¹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
² Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching bei München, Germany
³ Como Lake Center for Astrophysics (CLAP), DiSAT, Università degli Studi dell’Insubria, via Valleggio 11, 22100 Como, Italy
⁴ Institut für Astronomie & Astrophysik, Eberhard Karls Universität Tübingen, Sand 1, 72076 Tübingen, Germany

^★ Corresponding author: nicola.locatelli@inaf.it

Received: 15 July 2025
Accepted: 24 September 2025

Abstract

Background diffuse X-ray emission is contributed in large part by the emission of point sources that are not individually resolved. While this has been established for decades regarding the contribution of quasars to the diffuse emission above 1 keV energies, the possible contribution of undetected stars in the Milky Way to the softer band background emission (0.5−1 keV) is still poorly constrained. The overall unresolved X-ray flux from the stars is the product of the stellar spatial distribution in the Milky Way and the underlying X-ray luminosity function (XLF). In this work we determined the XLF of the stars and studied its structure with respect to a set of main-sequence spectral types (F, G, K, and M) and evolutionary stages (giants and white dwarfs). We built the XLF in volume-limited subsamples of increasing maximum distance and corresponding minimum X-ray luminosity using the HamStar catalog of X-ray-emitting stars detected in the western Galactic hemisphere with the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Spectrum-Roentgen-Gamma (SRG) space observatory. We built an extinction-corrected Gaia color-magnitude diagram and decomposed the XLF into the relative contribution of different groups of stars. We provide an empirical polynomial description of the XLF for the total sample and for the different stellar subgroups that can be used to estimate the unresolved contribution from the stars to the soft X-ray background of the Milky Way in a flux-limited survey.