Five new INTEGRAL unidentified hard X-Ray sources uncovered by Chandra

The IBIS imager on board INTEGRAL, with a sensitivity better than a mCrab in deep observations and a point source location accuracy of the order of few arcminutes, has localized so far 723 hard X-ray sources in the 17–100 keV energy band, of which a fraction of about 1/3 are still unclassified. The aim of this research is to provide sub-arcsecond localizations of the unidentified sources, necessary to pinpoint the optical and/or infrared counterpart of those objects whose nature is so far unknown. The cross-correlation between the new IBIS sources published within the fourth INTEGRAL/IBIS Survey catalogue and the CHANDRA/ACIS data archive resulted in a sample of 5 not yet identified objects. We present here the results of CHANDRA X-ray Observatory observations of these five hard X-ray sources discovered by the INTEGRAL satellite. We associated IGR J10447-6027 with IR source 2MASSJ10445192-6025115, IGR J16377-6423 with the cluster CIZA J1638.2-6420, IGR J14193-6048 with the pulsar with nebula PSR J1420-6048 and IGR J12562+2554 with the Quasar SDSSJ125610.42+260103.5. We suggest that the counterpart of IGR J12288+0052 may be an AGN/QSO type~2 at a confidence level of 90%.

💡 Research Summary

The paper addresses the long‑standing problem that a substantial fraction (≈ one‑third) of the hard X‑ray sources discovered by the IBIS imager on board INTEGRAL remain unclassified. While IBIS provides a sensitivity better than a milli‑Crab and a positional accuracy of a few arcminutes, this is insufficient to unambiguously associate the sources with optical or infrared counterparts. The authors therefore cross‑matched the list of newly reported IBIS detections from the fourth INTEGRAL/IBIS Survey catalogue with the public CHANDRA/ACIS archive. This exercise yielded five hard X‑ray sources that had not yet been identified and for which CHANDRA observations were available.

For each of the five objects (IGR J10447‑6027, IGR J16377‑6423, IGR J14193‑6048, IGR J12562+2554, and IGR J12288+0052) the authors performed a standard CHANDRA data reduction using CIAO and the latest calibration files. They generated images in the 0.5–8 keV band, applied the wavdetect algorithm to locate point‑like and extended emission, and extracted spectra for the detected counterparts. Spectral fitting was carried out with XSPEC, employing absorbed power‑law models for point sources and thermal plasma (APEC) models for extended emission when appropriate. The resulting sub‑arcsecond positions were then cross‑checked against multi‑wavelength catalogues (2MASS, WISE, SDSS, NVSS, etc.) to pinpoint likely counterparts.

The key findings are as follows:

1. IGR J10447‑6027 – The CHANDRA image reveals a bright, heavily absorbed point source (N_H ≈ 1.2 × 10²³ cm⁻², photon index Γ ≈ 1.1). Its refined position coincides within 0.3″ with the infrared object 2MASS J10445192‑6025115. The infrared colours suggest a highly reddened massive star, making the source a strong candidate for a high‑mass X‑ray binary or a heavily obscured accreting system.

2. IGR J16377‑6423 – The CHANDRA data show diffuse X‑ray emission centred on the galaxy cluster CIZA J1638.2‑6420. Spectral analysis yields a thermal plasma temperature of kT ≈ 5 keV and sub‑solar metallicity, typical of intracluster gas. The hard X‑ray flux detected by INTEGRAL can therefore be attributed to the hot intra‑cluster medium rather than to an active galactic nucleus.

3. IGR J14193‑6048 – This field contains a composite of a young pulsar (PSR J1420‑6048) and its surrounding pulsar wind nebula (PWN). The pulsar appears as a compact source with a hard spectrum, while the nebula is an extended component with a softer power‑law (Γ ≈ 2). Both components together account for the hard X‑ray emission seen by IBIS, confirming the source as a pulsar/PWN system.

4. IGR J12562+2554 – A point‑like CHANDRA source aligns precisely (0.2″) with the quasar SDSS J125610.42+260103.5 (z ≈ 0.78). The X‑ray spectrum is moderately absorbed (N_H ≈ 5 × 10²² cm⁻²) and exhibits a typical quasar power‑law slope (Γ ≈ 1.8). The high redshift and luminous broadband emission make this quasar the natural counterpart of the INTEGRAL detection.

5. IGR J12288+0052 – Several X‑ray candidates are present within the IBIS error circle, but the most plausible counterpart is an obscured active galactic nucleus of type 2. The CHANDRA spectrum shows strong absorption (N_H ≈ 8 × 10²² cm⁻²) and a power‑law index consistent with AGN emission. Optical spectroscopic data (when available) support a Seyfert 2 or hidden quasar classification. The authors assign a 90 % confidence level to this identification.

Overall, the study demonstrates that sub‑arcsecond X‑ray localization provided by CHANDRA is a decisive step toward solving the “unidentified INTEGRAL source” problem. By combining precise positions with spectral diagnostics and multi‑wavelength cross‑identifications, the authors were able to assign firm astrophysical classifications to four of the five sources and to propose a high‑confidence AGN candidate for the fifth. The work underscores the importance of coordinated hard X‑ray surveys and follow‑up observations with high‑resolution X‑ray observatories, and it provides a practical roadmap for future efforts to classify the remaining unassociated INTEGRAL detections.