Hard X-Ray Sources in Fermi UFOs

Identification and/or association of unidentified γ -ray sources with lower-energy counterparts represents a key challenge in modern astronomy, due to the relatively large positional uncertainty provided by γ -ray detectors. We selected unidentified Fermi objects (UFOs) positionally compatible with hard X-ray sources in the latest Palermo Swift-BAT hard X-ray Catalog and in the SRG/ART-XC all-sky X-ray survey Catalog, to identify lower-energy sources and possibly associate them to the UFOs. We found 17 UFOs with overlapping hard X-ray sources. We then collected soft X-ray data from Swift-XRT, Chandra-ACIS, XMM-Newton-EPIC, and eROSITA, identified 16 soft X-ray counterparts to the hard X-ray sources, and associate 15 with known astronomical objects, classified as: blazars/blazar candidates (two sources), Seyfert galaxies (five sources), X-ray binaries (two sources), generic X-ray sources (one source), cataclysmic variables (CVs; two sources), and variable stars (three sources). Blazars and Seyfert galaxies are likely lower-energy counterparts to the UFOs, since their mid-IR colors and broadband spectral energy distributions suggest significant jetted, nonthermal emission. X-ray binaries can be potential lower-energy counterparts to the UFOs, since this class of sources has been already observed to emit γ -rays. The generic X-ray source has been proposed as a pulsar candidate, and we therefore suggest that it can be the lower-energy counterpart to the UFO. CVs have been suggested as potential γ -ray emitters, so, if confirmed, the two sources classified as CVs would represent the first γ -ray emitting sources of this kind. Finally, we consider the association of the three variable stars with the UFOs unlikely.