The populations of hard X- and {gamma}-ray sources: a correlation study and new possible identifications

We present the results of our analysis devoted to the research of sources emitting in the energy bands surveyed by both the Swift-BAT and the Fermi-LAT telescopes. We cross-correlate the Fermi-LAT 1-year point source catalogue (1FGL) of {\gamma}-ray sources and the second Palermo BAT catalogue (2PBC) of hard X-ray sources, establishing a correspondence between sources when their error boxes overlap. We also extract the significance value in the BAT 15-150 keV map, obtained using a dedicated software for the reduction of BAT data, in the direction of the 1FGL sources and take into account those above the significance threshold {\sigma} = 3. We obtain a sample of common sources emitting in both the hard X- and the {\gamma}-ray energy bands and evaluate its content in galactic and extragalactic objects. We assess the fraction of unidentified sources and describe in greater detail the properties of two of them, 1FGL J0137.8+5814 and 1FGL J2056.7+4938, supporting their classification as blazars after the analysis of their broad-band spectral energy distribution. We discuss the blazar content of the collected 1FGL-2PBC sources: we build its redshift distibution and compare it with that of the whole blazar population as reported in the second edition of the BZCAT blazar catalogue.

💡 Research Summary

The advent of large‑field high‑energy observatories such as Swift‑BAT and Fermi‑LAT has produced catalogs containing thousands of hard X‑ray (15–150 keV) and γ‑ray (100 MeV–100 GeV) sources. However, the degree to which individual objects emit in both bands remains poorly quantified. In this work the authors performed a systematic cross‑correlation between the second Palermo BAT catalogue (2PBC) and the Fermi‑LAT 1‑year point‑source catalogue (1FGL). A source pair was considered a match when the 95 % confidence error circles of the two catalogs overlapped (distance d ≤ r_BAT + r_LAT). To assess match quality they introduced the parameter Q = d/r_BL, where r_BL is the larger of the two error radii; Q < 1 indicates that the larger error circle fully contains the other centre. Using this criterion they identified 77 initial matches, of which 64 satisfied Q < 1.

Beyond the catalogued BAT detections (σ_T = 4.8), the authors extracted the BAT significance σ at the positions of all 1FGL sources from the 54‑month all‑sky map. By lowering the detection threshold to σ* = 3 they recovered additional hard‑X‑ray counterparts: 80 high‑latitude (|b| > 10°) and 49 low‑latitude (|b| < 10°) sources with σ ≥ 3. After excluding sources lying within 36′ of bright BAT detections that could cause spurious associations, the final sample comprised 75 high‑latitude and 29 low‑latitude candidates.

In total 104 unique objects were found to emit in both BAT and LAT bands, representing roughly 7 % of the 1FGL population. The majority (83) are extragalactic, with 6 remaining unidentified. Among the extragalactic sources, blazars dominate: 295 BL Lac objects and 274 flat‑spectrum radio quasars (FSRQs) are listed in 1FGL, while the 2PBC catalog is populated mainly by Seyfert galaxies. The cross‑matched blazar sample shows a lower mean redshift than the full BZCAT (e.g., BL Lac ⟨z⟩ ≈ 0.33 vs 0.42 for the whole catalogue; FSRQ ⟨z⟩ ≈ 1.39 vs 1.40). This indicates that objects bright enough to be detected simultaneously by BAT and LAT tend to be relatively nearby.

Two previously unidentified 1FGL sources, 1FGL J0137.8+5814 and 1FGL J2056.7+4938, were examined in detail. By assembling their broadband spectral energy distributions (radio through γ‑ray) the authors identified the characteristic double‑humped shape typical of blazars, with synchrotron and inverse‑Compton peaks, and found variability patterns consistent with jet emission. Consequently they propose these objects as new blazar candidates.

The study also evaluated the impact of the lower σ* = 3 threshold, estimating a false‑association rate of about 20 %, especially at low Galactic latitudes where source density is high. Overall, the work demonstrates that cross‑matching hard X‑ray and γ‑ray catalogs is an effective strategy for uncovering the nature of unidentified high‑energy sources, refining blazar population statistics, and guiding future multi‑wavelength follow‑up campaigns.