Exploring the Nature of Galactic Unassociated Sources Detected by the Fermi-LAT
F. Acero, A. Acharyya, A. Adelfio, M. Ajello, E. Aviano, L. Baldini, J. Ballet, C. Bartolini, D. Bastieri, J. Becerra González, R. Bellazzini, Aakash Bhat, E. Bissaldi, R. Bonino, P. Bruel, R. A. Cameron, P. A. Caraveo, Fausto Casaburo, F. Casini, E. Cavazzuti, Nicolò Cibrario, S. Ciprini, G. Cozzolongo, P. Cristarella Orestano, F. Cuna, S. Cutini, F. D’Ammando, Pedro De la Torre Luque, Davide Depalo, Niccolò Di Lalla, A. Dinesh, L. Di Venere, A. Domínguez, Jordan Eagle, Cristina Fernández-Suárez, A. Fiori, Y. Fukazawa, S. Funk, P. Fusco, F. Gargano, C. Gasbarra, D. Gasparrini, S. Germani, Federica Giacchino, N. Giglietto, M. Giliberti, F. Giordano, M. Giroletti, I. A. Grenier, M.-H. Grondin, S. Guiriec, Rahul Gupta, M. Hashizume, E. Hays, J. W. Hewitt, A H Airasca, D. Horan, X. T. Hou, T. Kayanoki, M. Kerr, M. Kuss, Dimitrios A. Langis, Adrien Laviron, M. Lemoine‐Goumard, A. Liguori, Jian Li, Ioannis Liodakis, Pierpaolo Loizzo, F. Longo, F. Loparco, S. López Pérez, L. Lorusso, B. Lott, M. N. Lovellette, P. Lubrano, S. Maldera, D. Malyshev, G. Martí-Devesa, R Martinelli, M. N. Mazziotta, I. Mereu, Miltiadis Michailidis, P. F. Michelson, Tsunefumi Mizuno, P. Monti-Guarnieri, M. E. Monzani, A. Morselli, Michela Negro, N. Omodei, M. Orienti, E. Orlando, D. Paneque, G. Panzarini, M. Persic, M. Pesce-Rollins, R. Pillera, T. A. Porter, G. Principe, S. Rainò, R. Rando
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 12 CO tracer is explored using the 13 CO 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.