A sample of ionised Fe line-emitting X-ray sources in the inner Galactic disc

¹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
² Department of Astronomy, University of Illinois, 1002 W. Green St., Urbana, IL 61801, USA
³ Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany
⁴ Department of Physics and Astronomy, University of California, Los Angeles, CA 90095-1547, USA
⁵ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Barcelona, Spain
⁶ Institut d'Estudis Espacials de Catalunya (IEEC), Carrer Gran Capità 2–4, 08034 Barcelona, Spain

^★ Corresponding author: samaresh.astro@gmail.com

Received: 2 January 2025
Accepted: 25 April 2025

Abstract

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.