The Palermo Swift-BAT hard X-ray catalogue III. Results after 54 months of sky survey

We present the Second Palermo Swift-BAT hard X-ray catalogue obtained by analysing data acquired in the first 54 months of the Swift mission. Using our software dedicated to the analysis of data from coded mask telescopes, we analysed the BAT survey data in three energy bands (15-30 keV, 15-70 keV, 15-150 keV), obtaining a list of 1256 detections above a significance threshold of 4.8 standard deviations. The identification of the source counterparts is pursued using two strategies: the analysis of field observations of soft X-ray instruments and cross-correlation of our catalogue with source databases.The survey covers 50% of the sky to a 15–150 keV flux limit of 1.0 x 10^-11 erg s^-1 cm^-2 and 9.2 x 10^-12 erg s^-1 cm^-2 for |b|< 10 degrees and |b|> 10 degrees, respectively. The Second Palermo Swift-BAT hard X-ray catalogue includes 1079 (86%) hard X-ray sources with an associated counterpart (26 with a double association and 2 with a triple association) and 177 BAT excesses (14%) that still lack a counterpart. The distribution of the BAT sources among the different object classes consists of 19% Galactic sources, 57% extragalactic sources, and 10% sources with a counterpart at softer energies whose nature has not yet been determined. About half of the BAT associated sources lack a counterpart in the ROSAT catalogues. This suggests that either moderate or strong absorption may be preventing their detection in the ROSAT energy band. The comparison of our BAT catalogue with the Fermi Large Area Telescope First Source Catalogue identifies 59 BAT/Fermi correspondences: 48 blazars, 3 Seyfert galaxies, 1 interacting galaxy, 3 high mass X-ray binaries, and 4 pulsars/supernova remnants. This small number of correspondences indicates that different populations make the sky shine in these two different energy bands.

💡 Research Summary

The paper presents the Second Palermo Swift‑BAT Hard X‑ray Catalogue, derived from a systematic re‑analysis of the first 54 months of Swift mission data. Using a dedicated software suite optimized for coded‑mask telescopes, the authors processed the BAT survey in three independent energy bands (15‑30 keV, 15‑70 keV, and 15‑150 keV). By applying a detection threshold of 4.8 σ, they identified 1 256 statistically significant excesses across the sky.

Source identification was carried out through a two‑pronged approach. First, the team examined pointed observations from soft‑X‑ray instruments (Swift‑XRT, XMM‑Newton, Chandra) to locate counterparts within the BAT positional uncertainties. Second, they performed automated cross‑correlations with major astronomical databases (SIMBAD, NED, VizieR) to match BAT detections with previously catalogued objects. This effort resulted in firm associations for 1 079 sources (86 % of the sample), including 26 double and 2 triple associations, while 177 excesses (14 %) remain without a counterpart.

The survey reaches a flux limit of 1.0 × 10⁻¹¹ erg s⁻¹ cm⁻² (15‑150 keV) for low Galactic latitudes (|b| < 10°) and 9.2 × 10⁻¹² erg s⁻¹ cm⁻² for higher latitudes, covering roughly half of the sky at these sensitivities. The identified sources are distributed as follows: 19 % Galactic objects (primarily high‑mass X‑ray binaries, pulsars, and supernova‑remnant remnants), 57 % extragalactic objects (dominated by active galactic nuclei, especially blazars and Seyfert galaxies), and about 10 % with softer‑X‑ray counterparts whose nature is still ambiguous. Notably, more than half of the BAT‑detected sources lack a counterpart in the ROSAT (0.1‑2.4 keV) catalogues, implying that moderate to heavy absorption suppresses their soft‑X‑ray emission. This highlights the BAT catalogue’s capability to uncover a population of heavily absorbed AGN that are invisible to softer X‑ray surveys.

A cross‑comparison with the Fermi Large Area Telescope First Source Catalogue revealed 59 coincidences: 48 blazars, 3 Seyfert galaxies, 1 interacting galaxy, 3 high‑mass X‑ray binaries, and 4 pulsars/supernova‑remnant systems. The relatively small overlap underscores the distinct astrophysical populations that dominate the hard X‑ray (∼15‑150 keV) and high‑energy gamma‑ray (≥100 MeV) regimes.

In summary, the Second Palermo Swift‑BAT catalogue represents a significant improvement over the first release in terms of sensitivity, sky coverage, and identification completeness. It provides a valuable sample of absorbed AGN, Galactic compact objects, and previously unrecognized hard X‑ray emitters. The authors stress that follow‑up observations across the electromagnetic spectrum—particularly in the optical/infrared for classification and in the soft X‑ray band for absorption studies—will be essential to fully characterize the 177 unidentified BAT excesses. Moreover, the catalogue serves as a crucial reference for upcoming missions such as eROSITA, Athena, and the Cherenkov Telescope Array, facilitating multi‑wavelength studies of the high‑energy universe.