2025A&A...694A.176U

Aims. We searched for the multi-wavelength (X-ray, optical, and radio) counterparts to the unassociated gamma-ray sources (UGS) of the Fermi 4FGL-DR4 catalogue. The main goal was to identify new blazars and/or new active galactic nuclei that emit at GeV energies [such as (narrow-line) Seyfert-1 and radio galaxies].
Methods. We focused on sky regions that were observed by the Swift satellite and that overlap with the reported positions of the UGSs. Our primary interest are extragalactic sources, and we therefore focused on UGSs that are located outside the Galactic plane (|b|> 10°). Because of the large number of sources (about 1800 UGS), we developed a pipeline to automatise the search for counterparts and significantly reduce the computational time for the analysis. Our association process began by identifying potential X-ray counterparts for each UGS. When one was found, we further searched for corresponding radio and optical counterparts in the X-ray counterpart error box to minimise ambiguities.
Results. Out of the 1284 UGSs in the 4FGL-DR4 catalogue, 714 were observed at least once by Swift/X-ray Telescope (Swift/XRT). We detected at least one X-ray source within the Fermi error box with a significance of ≥3σ for 274 of these γ-ray emitters. Of these, 193 UGSs have a single potential X-ray counterpart (referred to as UGS1), while 81 have multiple potential X-ray counterparts within the Fermi error box (referred to as UGS2). Of the UGS2, 54 have two X-ray counterparts, 11 have three, and the remaining 16 have more than three counterparts. Each UGS1 has an optical counterpart, and 113 might also be associated with a radio counterpart. We compared the properties of the possible counterpart with those of the γ-ray emitters identified by Fermi with the aim to assess the goodness of our associations.