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

Abstract

Across the Galactic disk lies a diverse population of X-ray sources, with the fainter end remaining poorly understood due to past survey sensitivity limits. We aim to classify and characterize faint X-ray sources detected in the eROSITA All-Sky Survey (eRASS1) towards the inner Galactic disk ($350^{\circ }<l<360^{\circ }$, $-1^{\circ }<b<1^{\circ }$) using deeper XMM-Newton observations (typical exposure of $\sim 20\text{ks}$). We analyzed 189 eRASS1 sources, combining X-ray spectral fitting ($0.2$--$10\text{keV}$) with Gaia astrometric and photometric data for robust classification. Our results show that the eRASS1 catalog towards the Galactic disk is overwhelmingly dominated by coronal sources ($\sim 74\%$), primarily active stars and binaries, with $\sim 8\%$ being wind-powered massive stars and $\sim 18\%$ being accreting compact objects. We propose an empirical hardness-ratio cut ($\text{HR}>-0.2$) to efficiently isolate these non-coronal sources. By stacking the classified population and comparing with the Galactic Ridge X-ray Emission (GRXE), we estimate that $\sim 6\%$ of the GRXE flux in the $0.5$--$2.0\text{keV}$ band is resolved into point sources above the eRASS1 flux limit ($\sim 5\times 10^{-14}\text{erg}{\text{cm}}^{-2}{\text{s}}^{-1}$). This resolved soft-band emission is dominated by active stars, while hard-band flux originates primarily from X-ray binaries. We conclude that the eRASS1 catalog retains a non-negligible population of compact objects that can be effectively distinguished using X-ray color selection.

19 pages, 12 figures, accepted for publication on Astronomy & Astrophysics

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