Unveiling the soft X-ray source population towards the inner Galactic disk with XMM-Newton
Tong Bao, Gabriele Ponti, Frank Haberl, Samaresh Mondal, Mark R. Morris, Kaya Mori, Shifra Mandel, Xiao-jie Xu
TL;DR
This work targets the nature of faint X-ray sources detected by eRASS1 toward the inner Galactic disk by leveraging deeper XMM-Newton observations and Gaia DR3 data. Using X-ray spectral analysis in $0.2$--$10\, ext{keV}$, Gaia astrometry, and cross-matches with the HamStar framework, the authors classify 158 eRASS1 sources within the XMM footprint, finding that ~74% are coronal sources, ~8% wind-powered massive stars, and ~18% accreting compact objects, with an empirical hardness-ratio cut $HR > -0.2$ helping to isolate non-coronal sources. Stacking the classified population and comparing to the GRXE suggests that about 6% of the GRXE flux in $0.5$--$2.0\, ext{keV}$ is resolved into point sources above the eRASS1 flux limit of ~$5\times10^{-14}\, ext{erg cm}^{-2} \, ext{s}^{-1}$, dominated in soft band by active stars and in hard band by X-ray binaries. The results demonstrate that the eRASS1 catalog toward the inner Galactic disk is primarily coronal yet retains a non-negligible compact-object population identifiable via X-ray color selection, informing the composition of the GRXE and guiding future population studies in crowded disk fields.
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.