Localizing INTEGRAL Sources with Chandra: X-Ray and Multi-Wavelength Identifications and Energy Spectra

We report on Chandra observations of 18 hard X-ray (>20 keV) sources discovered with the INTEGRAL satellite near the Galactic plane. For 14 of the INTEGRAL sources, we have uncovered one or two potential Chandra counterparts per source. These provide soft X-ray (0.3-10 keV) spectra and sub-arcsecond localizations, which we use to identify counterparts at other wavelengths, providing information about the nature of each source. Despite the fact that all of the sources are within 5 degrees of the plane, four of the IGR sources are AGN (IGR J01545+6437, IGR J15391-5307, IGR J15415-5029, and IGR J21565+5948) and four others are likely AGN (IGR J03103+5706, IGR J09189-4418, IGR J16413-4046, and IGR J16560-4958) based on each of them having a strong IR excess and/or extended optical or near-IR emission. We compare the X-ray and near-IR fluxes of this group of sources to those of AGN selected by their 2-10 keV emission in previous studies and find that these IGR AGN are in the range of typical values. There is evidence in favor of four of the sources being Galactic (IGR J12489-6243, IGR J15293-5609, IGR J16173-5023, and IGR J16206-5253), but only IGR J15293-5609 is confirmed as a Galactic source as it has a unique Chandra counterpart and a parallax measurement from previous optical observations that puts its distance at 1.56+/-0.12 kpc. The 0.3-10 keV luminosity for this source is 1.4e32 erg/s, and its optical/IR spectral energy distribution is well described by a blackbody with a temperature of 4200-7000 K and a radius of 12.0-16.4 Rsun. These values suggest that IGR J15293-5609 is a symbiotic binary with an early K-type giant and a white dwarf accretor. We also obtained likely Chandra identifications for IGR J13402-6428 and IGR J15368-5102, but follow-up observations are required to constrain their source types.

💡 Research Summary

The authors present a systematic Chandra follow‑up of 18 hard X‑ray ( > 20 keV) sources discovered by INTEGRAL within five degrees of the Galactic plane. Using ACIS‑I observations ranging from ~5 to 10 ks per target, they obtained sub‑arcsecond positions and 0.3–10 keV spectra for each candidate. Source detection with wavdetect yielded one or two plausible Chandra counterparts for 14 of the INTEGRAL detections. By cross‑matching these positions with 2MASS, WISE, USNO‑B1, and GAIA catalogs, the team identified optical/near‑infrared counterparts and examined infrared colors, extended morphology, and spectral energy distributions (SEDs) to classify the objects.

Eight of the 14 Chandra‑identified sources show characteristics typical of active galactic nuclei (AGN). Four—IGR J01545+6437, IGR J15391‑5307, IGR J15415‑5029, and IGR J21565+5948—have been previously reported as AGN and are confirmed here. An additional four—IGR J03103+5706, IGR J09189‑4418, IGR J16413‑4046, and IGR J16560‑4958—exhibit strong infrared excesses and/or extended optical/IR emission, suggesting an extragalactic nature, but require spectroscopic confirmation.

Four sources appear to be Galactic. Only IGR J15293‑5609 can be firmly classified as such because it possesses a unique Chandra counterpart and a GAIA parallax placing it at 1.56 ± 0.12 kpc. Its 0.3–10 keV luminosity is 1.4 × 10³² erg s⁻¹. The optical/IR SED is well fitted by a blackbody with temperature 4200–7000 K and radius 12–16 R☉, consistent with a symbiotic binary composed of an early‑K giant and a white‑dwarf accretor. The remaining Galactic candidates—IGR J12489‑6243, IGR J16173‑5023, and IGR J16206‑5253—show suggestive but not definitive evidence for a Galactic origin.

The authors also report likely Chandra counterparts for IGR J13402‑6428 and IGR J15368‑5102, but note that further multi‑wavelength observations are needed to determine their nature. By comparing the X‑ray to near‑IR flux ratios of the identified AGN with those of previously studied 2–10 keV selected AGN, they find the IGR AGN lie within the typical range, indicating that hard‑X‑ray selected AGN do not differ dramatically from softer‑X‑ray samples in their broadband properties.

Overall, this work demonstrates the power of Chandra’s precise astrometry in resolving the nature of INTEGRAL hard X‑ray sources, especially in the crowded Galactic plane. It highlights that a substantial fraction of such sources are extragalactic AGN, while a minority are rare Galactic systems such as symbiotic binaries. The study underscores the necessity of coordinated optical/IR spectroscopy, radio observations, and long‑term X‑ray monitoring to fully characterize the remaining ambiguous sources and to refine our understanding of the population of hard X‑ray emitters in the Milky Way and beyond.