Unveiling the soft X-ray source population toward the inner Galactic disk with XMM-Newton

¹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
² Max-Planck-Institut für Extraterrestriche Physik, Gießenbachstraße 1, 85748 Garching, Germany
³ Como Lake Center for Astrophysics (CLAP), DiSAT, Università degli Studi dell’Insubria, via Valleggio 11, 22100 Como, Italy
⁴ Department of Astronomy, University of Illinois, 1002 W. Green St., Urbana, IL 61801, USA
⁵ Department of Physics and Astronomy, University of California, Los Angeles, CA 90095-1547, USA
⁶ Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA
⁷ School of Astronomy and Space Science, Nanjing University, Nanjing 210046, China

^★ Corresponding author: tong.bao@inaf.it

Received: 4 August 2025
Accepted: 26 October 2025

Abstract

Context. Across the Galactic disk lies a diverse population of X-ray sources, including coronally active stars and accreting compact objects. While high-luminosity sources are well characterized, the fainter end of the population remains poorly understood due to sensitivity limitations in previous X-ray surveys.

Aims. We aim to classify and characterize faint X-ray sources detected in the eROSITA All-Sky Survey (eRASS1) toward the inner Galactic disk using deeper XMM-Newton observations. By combining X-ray spectral analysis with Gaia counterparts, we assessed the representativeness of the eRASS1 catalog for the broader Galactic X-ray population.

Methods. We analyzed 189 eRASS1 X-ray sources toward the inner Galactic disk using deep XMM-Newton observations (typical exposure of 20 ks) that cover the region 350° < l < 360° and −1° < b < 1°. Source classification was carried out by combining X-ray spectral fitting in the 0.2-10 keV band with Gaia astrometric and photometric data.

Results. Approximately 74% of X-ray sources detected by eROSITA toward the inner Galactic disk are coronal sources (primarily active stars and binaries), while 8% are wind-powered massive stars and 18% are accreting compact objects. We propose an empirical hardness-ratio cut (HR > -0.2, with HR defined using the 0.5-2 and 2-8 keV bands) to efficiently identify non-coronal sources within the eRASS1 catalog. By stacking classified sources and comparing with the Galactic ridge X-ray emission (GRXE), we estimate that ~6% of the GRXE flux can be resolved into point sources in the 0.5-2.0 keV band above the eRASS1 flux limit of ~5× 10⁻¹⁴ erg cm⁻² s⁻¹, with soft-band emission dominated by active stars and hard-band flux primarily from X-ray binaries.

Conclusions. Our results demonstrate that the eRASS1 catalog toward the inner Galactic disk is dominated by coronal sources but retains a non-negligible population of massive stars and accreting compact objects that can be effectively identified with X-ray color selection.