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

Abstract

Background diffuse X-ray emission is contributed in large part by the emission of point sources not individually resolved. While this is established since decades for 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 (0.5-1 keV) background emission is still poorly constrained. The overall unresolved X-ray flux from the stars is the product between the stellar spatial distribution in the Milky Way and the underlying X-ray luminosity function (XLF). In this work, we build the XLF of the stars and study its structure with respect to a set of main-sequence spectral types (F, G, K, M) and evolutionary stages (giants and white dwarfs). We build 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 build an extinction-corrected Gaia color-magnitude diagram and decompose 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.