J/A+A/710/A243 Symbiotic star candidates in inner Galactic disk (Bao+, 2026)
X-ray luminous late-type giants:
An overlooked population contributing to the Galactic ridge iron line emission.
Bao T., Ponti G., Xu X.-J., Morris M.R., Levin B., Mori K., Mandel S.,
Locatelli N., Munoz-Darias T., Casares J., Torres M.A.P.
<Astron. Astrophys. 710, A243 (2026)>
=2026A&A...710A.243B 2026A&A...710A.243B (SIMBAD/NED BibCode)
ADC_Keywords: Galactic plane ; X-ray sources ; Binaries, X-ray
Keywords: binaries: symbiotic - Galaxy: disk - Galaxy: stellar content -
X-ray: stars
Abstract:
The origin of the highly ionized iron emission (FeXXV at 6.7keV)
characterizing the Galactic ridge X-ray emission (GRXE) remains a
fundamental puzzle in high-energy astrophysics. Although the GRXE
continuum is largely resolved into discrete populations of cataclysmic
variables and coronally active stars, these sources exhibit Fe XXV
equivalent widths significantly lower than that of the total GRXE,
leaving the intense iron line emission unexplained. In this work, we
cross-correlated the XMM-Newton survey of the inner Galactic disk with
Gaia DR3 astrometry to identify and characterize hard X-ray sources
(>2keV) with reliable stellar counterparts. We selected 107 X-ray
sources located within the red giant branch of the color-magnitude
diagram, many of which are verified long-period variables. These
sources exhibit high X-ray luminosities (Lx∼1031-1033erg/s),
significantly exceeding the typical coronal saturation levels of
single giants. Their X-ray spectra are notably harder than those of
quiescent stellar coronae, with plasma temperatures reaching up to kT
∼6keV and a prominent emission feature at ∼6.7keV. The combination
of high Lx, hard spectra, and intense FeXXV emission identifies this
population as accretion-powered binaries associated with late-type
giants. Our analysis demonstrates that this population contributes
∼20% of the total GRXE continuum and ∼40% of its iron line emission,
providing a key component to resolving the Galactic X-ray background
puzzle.
Description:
This table provides X-ray and optical properties for 107 symbiotic
star candidates detected in the XMM-Newton heritage survey toward the
inner Galactic disk. For each source, the data includes the reliable
Gaia counterpart identification, associated photometric information,
and X-ray luminosity derived from XMM-Newton data. Counterparts in
VPHAS+ survey are also included, providing the r-Halpha and u-i color.
Sources with Halpha excess (r-Halpha>0.8) are labeled by *.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 86 107 Source properties for 107 SySt candidates
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See also:
J/A+A/682/A34 : SRG/eROSITA all-sky survey catalogs (eRASS1) (Merloni+, 2024)
Byte-by-byte Description of file: tablea1.dat
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Bytes Format Units Label Explanations
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1- 3 I3 --- ID [1/107] Source index
4- 13 F10.4 deg RAdeg Right Ascension (J2000) from X-ray
14- 22 F9.4 deg DEdeg Declination (J2000) from X-ray
23- 27 F5.2 arcsec Sep Separation between X-ray and Gaia position
28- 33 F6.2 mag Gmag Gaia DR3 G-band magnitude (1)
34- 38 F5.2 mag BP-RP Gaia DR3 BP-RP color (1)
39- 44 F6.2 kpc Dist Geometric distance from Gaia DR3
45- 51 F7.2 kpc dmaxX Max detectable distance (X-ray sensitivity)
52- 57 F6.2 kpc dmaxG Max detectable distance (Gaia sensitivity)
58- 64 F7.2 [10-7W] logLX Log of X-ray luminosity (0.2-12keV)
65- 69 F5.2 --- pany Probability that any association is correct
70- 74 F5.2 --- pi Relative probability of the match
76 A1 --- f_(r-Ha)v [*] Note '*' if
(r-Hα)v>0.8(Hα excess)
77- 81 F5.2 mag (r-Ha)v VPHAS+ (r-Halpha) color
82- 86 F5.2 mag (u-i)v ? VPHAS+ (u-i) color
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Note (1): All the magnitudes and color are extinction corrected.
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Acknowledgements:
Tong Bao, xzbaotong(at)gmail.com
License: CC-BY-4.0
(End) Patricia Vannier [CDS] 12-May-2026