Searching for gamma-ray blazar candidates among the unidentified INTEGRAL sources
The identification of low-energy counterparts for gamma-ray sources is one of the biggest challenge in modern gamma-ray astronomy. Recently, we developed and successfully applied a new association method to recognize gamma-ray blazar candidates that could be possible counterparts for the unidentified gamma-ray sources above 100 MeV in the second Fermi Large Area Telescope (LAT) catalog (2FGL). This method is based on the Infrared (IR) colors of the recent Wide-Field Infrared Survey Explorer (WISE) all-sky survey. In this letter we applied our new association method to the case of unidentified INTEGRAL sources (UISs) listed in the fourth soft gamma-ray source catalog (4IC). Only 86 UISs out of the 113 can be analyzed, due to the sky coverage of the WISE Preliminary data release. Among these 86 UISs, we found that 18 appear to have a gamma-ray blazar candidate within their positional error region. Finally, we analyzed the SWIFT archival data available for 10 out these 18 gamma-ray blazar candidates, and we found that 7 out of 10 are clearly detected in soft X-rays and/or in the optical-ultraviolet band. We cannot confirm the associations between the UISs and the selected gamma-ray blazar candidates due to the discrepancies between the INTEGRAL and the soft X-ray spectra. However, the discovery of the soft X-ray counterparts for the selected gamma-ray blazar candidates adds an important clue to help understand their origin and to confirm their blazar nature.
💡 Research Summary
The paper tackles the long‑standing problem of identifying low‑energy counterparts for high‑energy γ‑ray sources, focusing on the unidentified sources listed in the fourth INTEGRAL catalog (4IC). Building on a method previously developed for the Fermi‑LAT 2FGL catalog, the authors use the infrared (IR) colors measured by the Wide‑Field Infrared Survey Explorer (WISE) to single out blazar‑like candidates. Blazars occupy a distinct region in the WISE color–color diagram (primarily W1‑W2 versus W2‑W3) because their non‑thermal synchrotron emission dominates the mid‑IR band.
The study begins by cross‑matching the 113 unidentified INTEGRAL sources (UISs) with the WISE Preliminary Release. Due to incomplete sky coverage, only 86 UISs could be examined. Applying the WISE color criteria, the authors find that 18 of these 86 sources contain at least one WISE object whose colors are consistent with known γ‑ray blazars and that lies within the INTEGRAL positional error ellipse. This demonstrates that the IR‑color technique, originally validated for Fermi‑LAT sources, can also be employed for the harder X‑ray/γ‑ray regime probed by INTEGRAL.
To test the plausibility of these associations, the authors retrieve archival Swift observations for ten of the eighteen candidates. Swift’s X‑Ray Telescope (XRT) provides coverage in the 0.3–10 keV band, while the UV/Optical Telescope (UVOT) supplies simultaneous UV‑optical photometry (UVW1, U, B filters). Seven of the ten candidates are clearly detected either in soft X‑rays, in the UV/optical bands, or in both. The detection of a soft X‑ray counterpart is a strong indicator of blazar activity, as blazars typically exhibit a bright, variable synchrotron component that extends from radio up to X‑ray energies.
However, a direct association between the INTEGRAL hard X‑ray detections (20–100 keV) and the Swift soft X‑ray measurements proves problematic. The spectral shapes derived from INTEGRAL differ markedly from those obtained with Swift/XRT; the photon indices and flux normalizations are inconsistent for most of the examined sources. The authors discuss several possible explanations: (1) intrinsic variability of blazars, which can cause large flux changes on timescales shorter than the interval between the INTEGRAL and Swift observations; (2) the presence of additional components (e.g., a separate X‑ray binary or a non‑blazar active galactic nucleus) within the INTEGRAL error region; and (3) limitations of the WISE color selection, which, while efficient, cannot completely exclude non‑blazar AGN that share similar mid‑IR colors.
Given these discrepancies, the paper refrains from claiming a definitive identification of the UISs with the proposed blazar candidates. Nonetheless, the discovery of soft X‑ray and UV/optical counterparts for the majority of the examined candidates provides valuable ancillary evidence supporting their blazar nature.
The authors conclude by outlining a roadmap for future work. They advocate for simultaneous observations with instruments that bridge the soft and hard X‑ray regimes, such as NuSTAR, to obtain a continuous spectral coverage and to test for spectral continuity. Optical spectroscopy is recommended to measure redshifts and to confirm the presence of broad emission lines characteristic of blazars. Long‑term multi‑wavelength monitoring would help quantify variability patterns, a hallmark of blazar activity, and could be used to refine the IR‑color selection criteria. Finally, they suggest expanding the cross‑matching to include radio surveys (e.g., NVSS, SUMSS) and γ‑ray catalogs to further reduce false positives.
In summary, this work demonstrates that WISE infrared colors remain a powerful tool for flagging blazar candidates even among hard X‑ray/γ‑ray sources detected by INTEGRAL. While the current data set does not allow a firm association for any individual UIS, the multi‑wavelength detections reported here lay the groundwork for more definitive identifications in forthcoming studies.