Hard X-ray Sources in Fermi UFOs

Alessandro Paggi; Ioannis Liodakis; John Antoniadis; Chien-Ting Chen; Steven R. Ehlert; Daniel Gruen; Philip Kaaret; Ignacio De La Calle Perez; Elena Jimenez Bailon; Mykhailo Ilin

arXiv · 2606.00331

Hard X-ray Sources in Fermi UFOs

Alessandro Paggi·Ioannis Liodakis·John Antoniadis·Chien-Ting Chen·Steven R. Ehlert·Daniel Gruen·Philip Kaaret·Ignacio De La Calle Perez·Elena Jimenez Bailon·Mykhailo Ilin

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Abstract

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/blazars candidates (2 sources), Seyfert galaxies (5 sources), X-ray binaries (2 sources), generic X-ray sources (1 source), cataclysmic variables (2 sources), and variable stars (3 sources). Blazars and Seyfert galaxies are likely lower-energy counterparts to the UFOs, since their mid-IR colors and broad-band spectral energy distributions suggest significant jetted, non-thermal 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. Cataclysmic variables have been suggested as potential $γ$-ray emitters so, if confirmed, the 2 sources classified as cataclysmic variables would represent the first $γ$-ray emitting sources of this kind. Finally, we consider the association of the 3 variable stars with the UFOs unlikely.

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Alessandro Paggi, Ioannis Liodakis, John Antoniadis, Chien-Ting Chen, Steven R. Ehlert, Daniel Gruen, Philip Kaaret, Ignacio De La Calle Perez, Elena Jimenez Bailon, Mykhailo Ilin. 2026-05-29. Hard X-ray Sources in Fermi UFOs. https://arxiv.org/abs/2606.00331

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