Mirabal, N.et al.2020-09-012020-09-012016-04-20M. Ajello et al., Search for Spectral Irregularities due to Photon–Axionlike-Particle Oscillations with the Fermi Large Area Telescope, Phys. Rev. Lett. 116, 161101 (2016), DOI:https://doi.org/10.1103/PhysRevLett.116.161101https://doi.org/10.1103/PhysRevLett.116.161101http://hdl.handle.net/11603/19554M. Ajello, A. Albert, B. Anderson, L. Baldini, G. Barbiellini, D. Bastieri, R. Bellazzini, E. Bissaldi, R. D. Blandford, E. D. Bloom, R. Bonino, E. Bottacini, J. Bregeon, P. Bruel, R. Buehler, G. A. Caliandro, R. A. Cameron, M. Caragiulo, P. A. Caraveo, C. Cecchi, A. Chekhtman, S. Ciprini, J. Cohen-Tanugi, J. Conrad, F. Costanza, F. D’Ammando, A. de Angelis, F. de Palma, R. Desiante, M. Di Mauro, L. Di Venere, A. Domínguez, P. S. Drell, C. Favuzzi, W. B. Focke, A. Franckowiak, Y. Fukazawa, S. Funk, P. Fusco, F. Gargano, D. Gasparrini, N. Giglietto, T. Glanzman, G. Godfrey, S. Guiriec, D. Horan, G. Jóhannesson, M. Katsuragawa, S. Kensei, M. Kuss, S. Larsson, L. Latronico, J. Li, L. Li, F. Longo, F. Loparco, P. Lubrano, G. M. Madejski, S. Maldera, A. Manfreda, M. Mayer, M. N. Mazziotta, M. Meyer, P. F. Michelson, N. Mirabal, T. Mizuno, M. E. Monzani, A. Morselli, I. V. Moskalenko, S. Murgia, M. Negro, E. Nuss, C. Okada, E. Orlando, J. F. Ormes, D. Paneque, J. S. Perkins, M. Pesce-Rollins, F. Piron, G. Pivato, T. A. Porter, S. Rainò, R. Rando, M. Razzano, A. Reimer, M. Sánchez-Conde, C. Sgrò,D. Simone, E. J. Siskind, F. Spada, G. Spandre, P. Spinelli, H. Takahashi, J. B. Thayer, D. F. Torres, G. Tosti, E. Troja, Y. Uchiyama, K. S. Wood, M. Wood, G. Zaharijas, and S. Zimmer.We report on the search for spectral irregularities induced by oscillations between photons and axionlike-particles (ALPs) in the γ-ray spectrum of NGC 1275, the central galaxy of the Perseus cluster. Using 6 years of Fermi Large Area Telescope data, we find no evidence for ALPs and exclude couplings above 5×10⁻¹² GeV⁻¹ for ALP masses 0.5≲ma≲5 neV at 95% confidence. The limits are competitive with the sensitivity of planned laboratory experiments, and, together with other bounds, strongly constrain the possibility that ALPs can reduce the γ-ray opacity of the Universe.7 pagesen-USThis item is likely protected under Title 17 of the U.S. Copyright Law. Unless on a Creative Commons license, for uses protected by Copyright Law, contact the copyright holder or the author.Public Domain Mark 1.0This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. LawText