2025A&A...698A.206M

Context. The origin of the unresolved X-ray emission towards the Galactic Centre and the Galactic disc is unclear. Previous studies suggest that the Galactic diffuse X-ray emission is composed of unresolved point sources, primarily magnetic cataclysmic variables (mCVs). However, mCVs have a much lower 6.7 keV line equivalent width (EW_6.7) and a much higher line intensity ratio between Fe XXVI and Fe XXV (I_6.9/I_6.7) compared to the diffuse X-ray emission. Therefore, the primary contributors to the unresolved X-ray emission remain unclear.
Aims. We performed a deep X-ray scan of the inner Galactic disc using XMM-Newton observations covering 350^° < l < 7° and -0.8° < b < +0.8°. We aim to characterise the different populations of point sources that contribute to the Galactic diffuse X-ray emission by estimating the X-ray spectral slope Γ in the 0.2-10 keV band, EW_6.7, and I_6.9/I_6.7.
Methods. We performed source detection in the 6.5-7 keV band. We then extracted the spectra of the X-ray point sources and performed spectral modelling using XSPEC to estimate the X-ray spectral index Γ, equivalent width, and intensities of the iron 6.7 and 6.9 keV lines.
Results. We detected a total of 859 sources in the 6.5-7 keV band, of which 72 sources show significant iron line emission at 6.7 keV. The distribution of spectral index Γ for these 72 sources is bimodal, with peaks at Γ = 0.5 ± 0.4 and 1.8 ± 0.3, suggesting two populations of sources. The soft X-ray sources (Γ ∼ 1.8) have significantly larger EW_6.7 than the hard X-ray sources (Γ ∼ 0.5). Furthermore, 18 of the 32 hard sources are associated with previously known CVs. We identify CV candidates in our sample as those with spectral index Γ < 1.25. The line ratio, 2-10 keV luminosity, and previous detection of spin period suggest that most of these CVs are magnetic. The distribution of the EW_6.7 line for the combined sample of previously identified and candidate CVs has a mean value of ⟨EW_6.7⟩ = 415 ± 39 eV. Furthermore, we computed the stacked spectra of all sources detected in the 6.5-7 keV band for different flux groups, and we find evidence in the stacked spectra of hard sources that the EW_6.7 increases with decreasing flux. The soft X-ray sources have ⟨EW_6.7⟩ = 1.1 ± 0.1 keV. We identified 13 of the 30 soft sources associated with active stars, young stellar objects, and active binaries of RS CVn type.
Conclusions. The Fe XXV line-emitting sources towards the Galactic disc can be broadly categorised into two types: soft and hard X-ray sources. The ⟨EW_6.7⟩ of our CV candidate sample is more than twice as large as the typical EW_6.7 found in mCVs within 500 pc, and the ⟨EW_6.7⟩ of our CV candidate sample is close to the EW_6.7 value of Galactic diffuse X-ray emission. In our survey, the detection sensitivity for point sources in the 6.5-7 keV band is ∼10^–14 erg s^–1 cm^–2. Therefore, up to a flux limit of ∼10^–14 erg s^–1 cm^–2 or higher, nearly 50% of Galactic diffuse X-ray emission in the 6.5-7 keV band originates from mCVs. The large ⟨EW_6.7⟩ of the soft source sample indicates that these sources also contribute significantly to the Galactic diffuse X-ray emission, as well as from CVs.