Exploring the Nature of Galactic Unassociated Sources Detected by the Fermi-LAT

Acero F, Acharyya A, Adelfio A, Ajello M, Aviano E, Baldini L, Ballet J, Bartolini C, Bastieri D, Becerra Gonzalez J, Bellazzini R, Bhat A, Bissaldi E, Bonino R, Bruel P, Cameron RA, Caraveo PA, Casaburo F, Casini F, Cavazzuti E, Cibrario N, Ciprini S, Cozzolongo G, Cristarella Orestano P, Cuna F, Cutini S, D’Ammando F, de la Torre Luque P, Depalo D, Di Lalla N, Dinesh A, Di Venere L, Domínguez A, Eagle J, Fernández-Suárez C, Fiori A, Fukazawa Y, Funk S, Fusco P, Gargano F, Gasbarra C, Gasparrini D, Germani S, Giacchino F, Giglietto N, Giliberti M, Giordano F, Giroletti M, Grenier IA, Grondin MH, Guiriec S, Gupta R, Hashizume M, Hays E, Hewitt JW, Holzmann Airasca A, Horan D, Hou X, Kayanoki T, Kerr M, Kuss M, Langis DA, Laviron A, Lemoine-Goumard M, Liguori A, Li J, Liodakis I, Loizzo P, Longo F, Loparco F, Pérez SL, Lorusso L, Lott B, Lovellette MN, Lubrano P, Maldera S, Malyshev D, Martí-Devesa G, Martinelli R, Mazziotta MN, Mereu I, Michailidis M, Michelson PF, Mizuno T, Monti-Guarnieri P, Monzani ME, Morselli A, Negro M, Omodei N, Orienti M, Orlando E, Paneque D, Panzarini G, Persic M, Pesce-Rollins M, Pillera R, Porter TA, Principe G, Rainò S, Rando R, Reimer A, Reimer O, Sánchez-Conde M, Saz Parkinson PM, Serini D, Sgrò C, Siskind EJ, Smith DA, Spandre G, Spinelli P, Strong AW, Suson DJ, Tajima H, Thayer JB, Thompson DJ, Tibaldo L, Torres DF, Valverde J, Wood K, Yu Q, Zaharijas G, Zhang W (2026)

Publication Type: Journal article

Publication year: 2026

Journal

DOI: 10.3847/1538-4357/ae74d3

Abstract

We investigate the nature of the unassociated sources detected by the Fermi-LAT close (∣b∣ < 10°) to the Galactic plane, representing 16% of all sources in the 4FGL-DR4 catalog. The bulk of these sources (referred to as soft Galactic unassociated sources, SGUs) exhibit properties not found in known classes of gamma-ray emitters, as confirmed by a machine-learning classification approach. In particular, these properties include a steep, curved spectrum peaking below 1 GeV and a specific Galactic-latitude distribution with both a narrow and a broad component (dubbed the spike and the shoulder, respectively). Some source clusters are highlighted. New plausible source classes are explored, but only star-forming regions are found to account for a significant fraction (at most 10%) of the unassociated population. A thorough search for counterparts to the 175 brightest sources brings out a number of plausible counterparts but does not reveal clues about the nature of the whole population. We investigate the possibility that SGUs originate from mismodeled clumps of diffuse emission. Using Monte Carlo simulations, the SGU spectra can be reproduced in this scenario under an ad hoc condition concerning the clump spatial extension. The possible connection between the SGUs and gas not accounted for by the ¹²CO tracer is explored using the ¹³CO MOPRA data but leads to inconclusive results. The origin of SGUs being related to diffuse emission remains plausible. However, a scenario whereby SGUs represent a new class of gamma-ray emitters cannot be fully excluded.

Authors with CRIS profile

Involved external institutions

National Institute for Nuclear Physics / Istituto Nazionale di Fisica Nucleare (INFN) Italy (IT) Laboratoire Leprince-Ringuet (LLR) France (FR) University of Southern Denmark / Syddansk Universitet Denmark (DK) Clemson University United States (USA) (US) Università degli Studi di Trieste Italy (IT) Yunnan Observatories China (CN) Université Paris-Saclay France (FR) Universidad Complutense de Madrid (UCM) Spain (ES) University of Pisa / Università di Pisa (UniPi) Italy (IT) Instituto de Astrofísica de Canarias (IAC) Spain (ES) Universität Potsdam Germany (DE) Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) United States (USA) (US) National Institute for Astrophysics / Istituto Nazionale Astrofisica (INAF) Italy (IT) Italian Space Agency / Agenzia Spaziale Italiana (ASI) Italy (IT) Universidad Autónoma de Madrid (UAM) Spain (ES) National Aeronautics and Space Administration (NASA) United States (USA) (US) Hiroshima University / 広島大学 Japan (JP) Université de Bordeaux France (FR) University of North Florida (UNF) United States (USA) (US) U.S. Naval Research Laboratory United States (USA) (US) Foundation for Research and Technology-Hellas (FORTH) / Ίδρυμα Τεχνολογίας και Έρευνας Greece (GR) University of Science and Technology of China (USTC) China (CN) Institute of Space Sciences (IEEC-CSIC) Spain (ES) Louisiana State University United States (USA) (US) Leopold-Franzens-Universität Innsbruck / University of Innsbruck Austria (AT) NYCB Real-Time Computing, Inc. United States (USA) (US) Max-Planck-Institut für extraterrestrische Physik (MPE) / Max Planck Institute for Extraterrestrial Physics Germany (DE) Purdue University Northwest / Purdue University Calumet United States (USA) (US) Nagoya University / 名古屋大学 Japan (JP) The Aerospace Corporation United States (USA) (US) Max-Planck-Institut für Physik (Werner-Heisenberg-Institut, MPP) / Max Planck Institute for Physics Germany (DE) Santa Cruz Institute for Particle Physics United States (USA) (US) Marquette University United States (USA) (US) Praxis, Inc. United States (USA) (US) Institut de Recherche en Astrophysique et Planétologie (IRAP) France (FR) University of Nova Gorica / Univerza v Novi Gorici Slovenia (SI) Institute of High Energy Physics (IHEP) / 中国科学院高能物理研究所 China (CN)

How to cite

APA:

Acero, F., Acharyya, A., Adelfio, A., Ajello, M., Aviano, E., Baldini, L.,... Zhang, W. (2026). Exploring the Nature of Galactic Unassociated Sources Detected by the Fermi-LAT. Astrophysical Journal. https://doi.org/10.3847/1538-4357/ae74d3

MLA:

Acero, F., et al. "Exploring the Nature of Galactic Unassociated Sources Detected by the Fermi-LAT." Astrophysical Journal (2026).

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