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

TL;DR

This study characterizes the average X-ray spectra of nearby M, F, G, and K stars within 10 parsecs, revealing their collective contribution to the Milky Way's X-ray background using eROSITA data.

Contribution

It provides the first constrained average X-ray spectra for a volume-complete sample of nearby stars across multiple spectral types, using stacked eROSITA observations.

Findings

Average X-ray luminosity for M-type stars is (2.6 ± 0.1)×10^{27} erg/s.

F, G, and K stars have an average X-ray luminosity of (15 ± 3)×10^{27} erg/s.

Spectral fits suggest the emission can be modeled with multiple thermal components.

Abstract

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. We analysed the volume-complete sample of M- (M0--M6) and FGK-type stars within 10 pc of the Sun using data from eROSITA all-sky survey aboard the Spectrum-Roentgen-Gamma (SRG) mission (eRASS:4). Individual spectra were normalized by exposure and distance and stacked to produce representative averages. The distance-normalized emission measures yield an average X-ray luminosity of $(2.6 \pm0.1)\times 10^{27}$ erg/s for M-type stars, and $(15\pm3)\times 10^{27}$ erg/s for F, G and K-type stars in 0.2--2.0 keV. The average spectra could be well described by a sum of three and two thermal models. Fitted temperatures and abundances remain consistent across M-star subgroups, while early-M stars are surprisingly on average less luminous than mid/late-M types. These results offer new insights into the collective X-ray properties of nearby stars, and provide motivation to explore the link with the unresolved soft X-ray background of the Galaxy.