{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2025,12,31]],"date-time":"2025-12-31T18:23:51Z","timestamp":1767205431148,"version":"build-2238731810"},"reference-count":89,"publisher":"MDPI AG","issue":"6","license":[{"start":{"date-parts":[[2021,6,5]],"date-time":"2021-06-05T00:00:00Z","timestamp":1622851200000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":["www.mdpi.com"],"crossmark-restriction":true},"short-container-title":["Universe"],"abstract":"<jats:p>The growing interest in axion-like particles (ALPs) stems from the fact that they provide successful theoretical explanations of physics phenomena, from the anomaly of the CP-symmetry conservation in strong interactions to the observation of an unexpectedly large TeV photon flux from astrophysical sources, at distances where the strong absorption by the intergalactic medium should make the signal very dim. In this latter condition, which is the focus of this review, a possible explanation is that TeV photons convert to ALPs in the presence of strong and\/or extended magnetic fields, such as those in the core of galaxy clusters or around compact objects, or even those in the intergalactic space. This mixing affects the observed \u03b3-ray spectrum of distant sources, either by signal recovery or the production of irregularities in the spectrum, called \u2018wiggles\u2019, according to the specific microscopic realization of the ALP and the ambient magnetic field at the source, and in the Milky Way, where ALPs may be converted back to \u03b3 rays. ALPs are also proposed as candidate particles for the Dark Matter. Imaging Atmospheric Cherenkov telescopes (IACTs) have the potential to detect the imprint of ALPs in the TeV spectrum from several classes of sources. In this contribution, we present the ALP case and review the past decade of searches for ALPs with this class of instruments.<\/jats:p>","DOI":"10.3390\/universe7060185","type":"journal-article","created":{"date-parts":[[2021,6,7]],"date-time":"2021-06-07T01:56:40Z","timestamp":1623031000000},"page":"185","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":5,"title":["Axion-like Particle Searches with IACTs"],"prefix":"10.3390","volume":"7","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-1209-2542","authenticated-orcid":false,"given":"Ivana","family":"Batkovi\u0107","sequence":"first","affiliation":[{"name":"Dipartimento di Fisica e Astronomia (DFA) G. Galilei, Universit\u00e0 di Padova (UniPD), I-35131 Padova, Italy"},{"name":"Istituto di Fisica Nucleare (INFN), sez. Padova, I-35131 Padova, Italy"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3288-2517","authenticated-orcid":false,"given":"Alessandro","family":"De Angelis","sequence":"additional","affiliation":[{"name":"Dipartimento di Fisica e Astronomia (DFA) G. Galilei, Universit\u00e0 di Padova (UniPD), I-35131 Padova, Italy"},{"name":"Istituto di Fisica Nucleare (INFN), sez. Padova, I-35131 Padova, Italy"},{"name":"Instituto Superior T\u00e9cnico, Universidade de Lisboa and LIP, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-9104-3214","authenticated-orcid":false,"given":"Michele","family":"Doro","sequence":"additional","affiliation":[{"name":"Dipartimento di Fisica e Astronomia (DFA) G. Galilei, Universit\u00e0 di Padova (UniPD), I-35131 Padova, Italy"},{"name":"Istituto di Fisica Nucleare (INFN), sez. Padova, I-35131 Padova, Italy"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-1530-3031","authenticated-orcid":false,"given":"Marina","family":"Manganaro","sequence":"additional","affiliation":[{"name":"Department of Physics, University of Rijeka, 51000 Rijeka, Croatia"}]}],"member":"1968","published-online":{"date-parts":[[2021,6,5]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"1440","DOI":"10.1103\/PhysRevLett.38.1440","article-title":"CP conservation in the presence of pseudoparticles","volume":"38","author":"Peccei","year":"1977","journal-title":"Phys. Rev. Lett."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"223","DOI":"10.1103\/PhysRevLett.40.223","article-title":"A New Light Boson?","volume":"40","author":"Weinberg","year":"1978","journal-title":"Phys. Rev. Lett."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"279","DOI":"10.1103\/PhysRevLett.40.279","article-title":"Problem of Strong P and T Invariance in the Presence of Instantons","volume":"40","author":"Wilczek","year":"1978","journal-title":"Phys. Rev. Lett."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"081803","DOI":"10.1103\/PhysRevLett.124.081803","article-title":"Measurement of the Permanent Electric Dipole Moment of the Neutron","volume":"124","author":"Abel","year":"2020","journal-title":"Phys. Rev. Lett."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"103","DOI":"10.1103\/PhysRevLett.43.103","article-title":"Weak-Interaction Singlet and Strong CP Invariance","volume":"43","author":"Kim","year":"1979","journal-title":"Phys. Rev. Lett."},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"493","DOI":"10.1016\/0550-3213(80)90209-6","article-title":"Can confinement ensure natural CP invariance of strong interactions?","volume":"166","author":"Shifman","year":"1980","journal-title":"Nucl. Phys. B"},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"199","DOI":"10.1016\/0370-2693(81)90590-6","article-title":"A simple solution to the strong CP problem with a harmless axion","volume":"104","author":"Dine","year":"1981","journal-title":"Phys. Lett. B"},{"key":"ref_8","first-page":"260","article-title":"On Possible Suppression of the Axion Hadron Interactions","volume":"31","author":"Zhitnitsky","year":"1980","journal-title":"Sov. J. Nucl. Phys."},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"39","DOI":"10.1016\/0370-2693(82)90011-9","article-title":"A second massless axion?","volume":"114","author":"Anselm","year":"1982","journal-title":"Phys. Lett. B"},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"2001","DOI":"10.1103\/PhysRevD.37.2001","article-title":"Experimental test for arion \u21c6 photon oscillations in a homogeneous constant magnetic field","volume":"37","author":"Anselm","year":"1988","journal-title":"Phys. Rev. D"},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"013","DOI":"10.1088\/1475-7516\/2012\/06\/013","article-title":"WISPy cold dark matter","volume":"2012","author":"Arias","year":"2012","journal-title":"J. Cosmol. Astropart. Phys."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"584","DOI":"10.1038\/nphys4109","article-title":"New CAST limit on the axion-photon interaction","volume":"13","author":"Anastassopoulos","year":"2017","journal-title":"Nat. Phys."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"1415","DOI":"10.1103\/PhysRevLett.51.1415","article-title":"Experimental Tests of the \u201cInvisible\u201d Axion","volume":"51","author":"Sikivie","year":"1983","journal-title":"Phys. Rev. Lett."},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"193","DOI":"10.1016\/j.phpro.2014.12.031","article-title":"The Next Generation of Axion Helioscopes: The International Axion Observatory (IAXO)","volume":"61","author":"Vogel","year":"2015","journal-title":"Phys. Procedia"},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"115016","DOI":"10.1103\/PhysRevD.64.115016","article-title":"Search for solar axions using 7Li","volume":"64","author":"Bradley","year":"2001","journal-title":"Phys. Rev. D"},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"261301","DOI":"10.1103\/PhysRevLett.118.261301","article-title":"First Searches for Axions and Axionlike Particles with the LUX Experiment","volume":"118","author":"Akerib","year":"2017","journal-title":"Phys. Rev. Lett."},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"092002","DOI":"10.1103\/PhysRevD.92.092002","article-title":"New exclusion limits on scalar and pseudoscalar axionlike particles from light shining through a wall","volume":"92","author":"Ballou","year":"2015","journal-title":"Phys. Rev. D"},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"149","DOI":"10.1016\/j.physletb.2010.04.066","article-title":"New ALPS results on hidden-sector lightweights","volume":"689","author":"Ehret","year":"2010","journal-title":"Phys. Lett. B"},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"151301","DOI":"10.1103\/PhysRevLett.120.151301","article-title":"Search for Invisible Axion Dark Matter with the Axion Dark Matter Experiment","volume":"120","author":"Du","year":"2018","journal-title":"Phys. Rev. Lett."},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"135","DOI":"10.1016\/j.dark.2017.01.003","article-title":"Searching for galactic axions through magnetized media: The QUAX proposal","volume":"15","author":"Barbieri","year":"2017","journal-title":"Phys. Dark Universe"},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"102004","DOI":"10.1103\/PhysRevD.103.102004","article-title":"Search for invisible axion dark matter of mass ma = 43 \u03bceV with the QUAX-a\u03b3 experiment","volume":"103","author":"Alesini","year":"2021","journal-title":"Phys. Rev. D"},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"171801","DOI":"10.1103\/PhysRevLett.124.171801","article-title":"Axion Search with a Quantum-Limited Ferromagnetic Haloscope","volume":"124","author":"Crescini","year":"2020","journal-title":"Phys. Rev. Lett."},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"311","DOI":"10.1007\/s11214-008-9319-2","article-title":"Non-Thermal Processes in Cosmological Simulations","volume":"134","author":"Dolag","year":"2008","journal-title":"Space Sci. Rev."},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"225","DOI":"10.1086\/374923","article-title":"XMM-Newton Observations of the Perseus Cluster. I. The Temperature and Surface Brightness Structure","volume":"590","author":"Churazov","year":"2003","journal-title":"Astrophys. J."},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"1500","DOI":"10.1111\/j.1365-2966.2006.10244.x","article-title":"Magnetic fields in the centre of the Perseus cluster","volume":"368","author":"Taylor","year":"2006","journal-title":"MNRAS"},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"A99","DOI":"10.1051\/0004-6361\/201118502","article-title":"Constraining cosmic rays and magnetic fields in the Perseus galaxy cluster with TeV observations by the MAGIC telescopes","volume":"541","author":"Alvarez","year":"2012","journal-title":"A&A"},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"036","DOI":"10.1088\/1475-7516\/2017\/12\/036","article-title":"A new bound on axion-like particles","volume":"2017","author":"Marsh","year":"2017","journal-title":"J. Cosmol. Astropart. Phys."},{"key":"ref_28","doi-asserted-by":"crossref","first-page":"1237","DOI":"10.1103\/PhysRevD.37.1237","article-title":"Mixing of the photon with low-mass particles","volume":"37","author":"Raffelt","year":"1988","journal-title":"Phys. Rev. D"},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"035027","DOI":"10.1103\/PhysRevD.87.035027","article-title":"First lower limits on the photon-axion-like particle coupling from very high energy gamma-ray observations","volume":"87","author":"Meyer","year":"2013","journal-title":"Phys. Rev. D"},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"075024","DOI":"10.1103\/PhysRevD.86.075024","article-title":"Hardening of TeV gamma spectrum of active galactic nuclei in galaxy clusters by conversions of photons into axionlike particles","volume":"86","author":"Horns","year":"2012","journal-title":"Phys. Rev. D"},{"key":"ref_31","doi-asserted-by":"crossref","first-page":"004","DOI":"10.1088\/1475-7516\/2009\/12\/004","article-title":"Stochastic conversions of TeV photons into axion-like particles in extragalactic magnetic fields","volume":"2009","author":"Mirizzi","year":"2009","journal-title":"J. Cosmol. Astropart. Phys."},{"key":"ref_32","doi-asserted-by":"crossref","unstructured":"Long, G., Chen, S., Xu, S., and Zhang, H.H. (2021). Probing \u03bceV ALPs with future LHAASO observation of AGN \u03b3-ray spectra. arXiv.","DOI":"10.1103\/PhysRevD.104.083014"},{"key":"ref_33","doi-asserted-by":"crossref","first-page":"121301","DOI":"10.1103\/PhysRevD.76.121301","article-title":"Evidence for a new light spin-zero boson from cosmological gamma-ray propagation?","volume":"76","author":"Roncadelli","year":"2007","journal-title":"Phys. Rev. D"},{"key":"ref_34","doi-asserted-by":"crossref","first-page":"063001","DOI":"10.1103\/PhysRevD.77.063001","article-title":"Milky Way as a kiloparsec-scale axionscope","volume":"77","author":"Simet","year":"2008","journal-title":"Phys. Rev. D"},{"key":"ref_35","doi-asserted-by":"crossref","first-page":"123511","DOI":"10.1103\/PhysRevD.79.123511","article-title":"Hints of the existence of axionlike particles from the gamma-ray spectra of cosmological sources","volume":"79","author":"Paneque","year":"2009","journal-title":"Phys. Rev. D"},{"key":"ref_36","doi-asserted-by":"crossref","first-page":"105030","DOI":"10.1103\/PhysRevD.84.105030","article-title":"Relevance of axionlike particles for very-high-energy astrophysics","volume":"84","author":"Galanti","year":"2011","journal-title":"Phys. Rev. D"},{"key":"ref_37","doi-asserted-by":"crossref","first-page":"231102","DOI":"10.1103\/PhysRevLett.99.231102","article-title":"Detecting Axionlike Particles with Gamma Ray Telescopes","volume":"99","author":"Hooper","year":"2007","journal-title":"Phys. Rev. Lett."},{"key":"ref_38","doi-asserted-by":"crossref","first-page":"L21","DOI":"10.1111\/j.1745-3933.2008.00602.x","article-title":"Photon propagation and the very high energy \u03b3-ray spectra of blazars: How transparent is the Universe?","volume":"394","author":"Mansutti","year":"2009","journal-title":"MNRAS"},{"key":"ref_39","doi-asserted-by":"crossref","first-page":"102003","DOI":"10.1103\/PhysRevD.88.102003","article-title":"Constraints on axionlike particles with H.E.S.S. from the irregularity of the PKS 2155-304 energy spectrum","volume":"88","author":"Abramowski","year":"2013","journal-title":"Phys. Rev. D"},{"key":"ref_40","doi-asserted-by":"crossref","first-page":"14","DOI":"10.1088\/0004-637X\/757\/1\/14","article-title":"A New Model of the Galactic Magnetic Field","volume":"757","author":"Jansson","year":"2012","journal-title":"Astrophys. J."},{"key":"ref_41","doi-asserted-by":"crossref","first-page":"573","DOI":"10.1051\/0004-6361:20078671","article-title":"Radio observational constraints on Galactic 3D-emission models","volume":"477","author":"Sun","year":"2008","journal-title":"A&A"},{"key":"ref_42","doi-asserted-by":"crossref","first-page":"192","DOI":"10.1088\/0004-637X\/738\/2\/192","article-title":"Deriving the Global Structure of the Galactic Magnetic Field from Faraday Rotation Measures of Extragalactic Sources","volume":"738","author":"Pshirkov","year":"2011","journal-title":"Astrophys. J."},{"key":"ref_43","doi-asserted-by":"crossref","unstructured":"Davies, J., Meyer, M., and Cotter, G. (2020). Relevance of Jet Magnetic Field Structure for Blazar ALP Searches. arXiv.","DOI":"10.1103\/PhysRevD.103.023008"},{"key":"ref_44","doi-asserted-by":"crossref","first-page":"161101","DOI":"10.1103\/PhysRevLett.116.161101","article-title":"Search for Spectral Irregularities due to Photon-Axionlike-Particle Oscillations with the Fermi Large Area Telescope","volume":"116","author":"Ajello","year":"2016","journal-title":"Phys. Rev. Lett."},{"key":"ref_45","doi-asserted-by":"crossref","first-page":"48","DOI":"10.1088\/1475-7516\/2021\/02\/048","article-title":"Sensitivity of the Cherenkov Telescope Array for probing cosmology and fundamental physics with gamma-ray propagation","volume":"2021","author":"Abdalla","year":"2021","journal-title":"J. Cosmol. Astropart. Phys."},{"key":"ref_46","doi-asserted-by":"crossref","first-page":"19","DOI":"10.1016\/j.astropartphys.2012.06.002","article-title":"Evolution of ground-based gamma-ray astronomy from the early days to the Cherenkov Telescope Arrays. Seeing the High-Energy Universe with the Cherenkov Telescope Array\u2014The Science Explored with the CTA","volume":"43","author":"Hillas","year":"2013","journal-title":"Astropart. Phys."},{"key":"ref_47","doi-asserted-by":"crossref","first-page":"379","DOI":"10.1086\/167599","article-title":"Observation of TeV Gamma Rays from the Crab Nebula Using the Atmospheric Cerenkov Imaging Technique","volume":"342","author":"Weekes","year":"1989","journal-title":"Astrophys. J."},{"key":"ref_48","doi-asserted-by":"crossref","first-page":"331","DOI":"10.1016\/j.newar.2003.12.004","article-title":"The status of the HESS project","volume":"48","author":"Hinton","year":"2004","journal-title":"New Astron. Rev."},{"key":"ref_49","unstructured":"Lorenz, E., and MAGIC Collaboration (2001, January 8\u201315). Status of the 17 m diameter Magic telescope. Proceedings of the International Cosmic Ray Conference, Hamburg, Germany."},{"key":"ref_50","doi-asserted-by":"crossref","first-page":"221","DOI":"10.1016\/S0927-6505(01)00152-9","article-title":"VERITAS: The Very Energetic Radiation Imaging Telescope Array System","volume":"17","author":"Weekes","year":"2002","journal-title":"Astropart. Phys."},{"key":"ref_51","unstructured":"Hillas, A.M. (1985, January 11\u201323). Cerenkov Light Images of EAS Produced by Primary Gamma Rays and by Nuclei. Proceedings of the 19th International Cosmic Ray Conference (ICRC19), San Diego, CA, USA."},{"key":"ref_52","doi-asserted-by":"crossref","first-page":"137","DOI":"10.1016\/0927-6505(94)90036-1","article-title":"New methods of atmospheric Cherenkov imaging for gamma-ray astronomy. I. The false source method","volume":"2","author":"Fomin","year":"1994","journal-title":"Astropart. Phys."},{"key":"ref_53","doi-asserted-by":"crossref","first-page":"096901","DOI":"10.1088\/0034-4885\/71\/9\/096901","article-title":"High energy astrophysics with ground-based gamma ray detectors","volume":"71","author":"Aharonian","year":"2008","journal-title":"Rep. Prog. Phys."},{"key":"ref_54","first-page":"30","article-title":"Measurement of the Crab Nebula spectrum over three decades in energy with the MAGIC telescopes","volume":"5","author":"Ansoldi","year":"2015","journal-title":"J. High Energy Astrophys."},{"key":"ref_55","doi-asserted-by":"crossref","first-page":"023001","DOI":"10.1103\/PhysRevD.76.023001","article-title":"Signatures of axionlike particles in the spectra of TeV gamma-ray sources","volume":"76","author":"Mirizzi","year":"2007","journal-title":"Phys. Rev. D"},{"key":"ref_56","doi-asserted-by":"crossref","first-page":"123011","DOI":"10.1103\/PhysRevD.76.123011","article-title":"Effects of axion-photon mixing on gamma-ray spectra from magnetized astrophysical sources","volume":"76","author":"Hochmuth","year":"2007","journal-title":"Phys. Rev. D"},{"key":"ref_57","doi-asserted-by":"crossref","first-page":"439","DOI":"10.1016\/0370-2693(96)00778-2","article-title":"SN 1987A gamma-ray limits on the conversion of pseudoscalars","volume":"383","author":"Brockway","year":"1996","journal-title":"Phys. Lett. B"},{"key":"ref_58","doi-asserted-by":"crossref","first-page":"2372","DOI":"10.1103\/PhysRevLett.77.2372","article-title":"Gamma Rays from SN 1987A due to Pseudoscalar Conversion","volume":"77","author":"Grifols","year":"1996","journal-title":"Phys. Rev. Lett."},{"key":"ref_59","doi-asserted-by":"crossref","first-page":"006","DOI":"10.1088\/1475-7516\/2015\/02\/006","article-title":"Revisiting the SN1987A gamma-ray limit on ultralight axion-like particles","volume":"2015","author":"Payez","year":"2015","journal-title":"J. Cosmol. Astropart. Phys."},{"key":"ref_60","unstructured":"Raffelt, G.G. (1996). Stars as Laboratories for Fundamental Physics: The Astrophysics of Neutrinos, Axions, and Other Weakly Interacting Particles, University of Chicago Press."},{"key":"ref_61","doi-asserted-by":"crossref","first-page":"51","DOI":"10.1007\/978-3-540-73518-2_3","article-title":"Astrophysical Axion Bounds","volume":"Volume 741","author":"Kuster","year":"2008","journal-title":"Axions"},{"key":"ref_62","doi-asserted-by":"crossref","first-page":"045019","DOI":"10.1103\/PhysRevD.93.045019","article-title":"Constraints on axions and axionlike particles from Fermi Large Area Telescope observations of neutron stars","volume":"93","author":"Berenji","year":"2016","journal-title":"Phys. Rev. D"},{"key":"ref_63","doi-asserted-by":"crossref","first-page":"843","DOI":"10.1103\/PhysRevD.34.843","article-title":"Axion mass limits may be improved by pulsar x-ray measurements","volume":"34","author":"Morris","year":"1986","journal-title":"Phys. Rev. D"},{"key":"ref_64","doi-asserted-by":"crossref","first-page":"015","DOI":"10.1088\/1475-7516\/2011\/01\/015","article-title":"New constraints for heavy axion-like particles from supernovae","volume":"2011","author":"Giannotti","year":"2011","journal-title":"J. Cosmol. Astropart. Phys."},{"key":"ref_65","doi-asserted-by":"crossref","first-page":"110406","DOI":"10.1103\/PhysRevLett.96.110406","article-title":"Experimental Observation of Optical Rotation Generated in Vacuum by a Magnetic Field","volume":"96","author":"Zavattini","year":"2006","journal-title":"Phys. Rev. Lett."},{"key":"ref_66","doi-asserted-by":"crossref","first-page":"847","DOI":"10.1016\/j.physletb.2007.12.012","article-title":"Axion-like particles, cosmic magnetic fields and gamma-ray astrophysics","volume":"659","author":"Mansutti","year":"2008","journal-title":"Phys. Lett. B"},{"key":"ref_67","doi-asserted-by":"crossref","first-page":"425","DOI":"10.1146\/annurev.aa.22.090184.002233","article-title":"The Origin of Ultra-High-Energy Cosmic Rays","volume":"22","author":"Hillas","year":"1984","journal-title":"Annu. Rev. Astron. Astrophys."},{"key":"ref_68","doi-asserted-by":"crossref","first-page":"1","DOI":"10.1016\/S0370-2693(00)01113-8","article-title":"An infrared background-TeV gamma-ray crisis?","volume":"493","author":"Protheroe","year":"2000","journal-title":"Phys. Lett. B"},{"key":"ref_69","unstructured":"MAGIC Collaboration, Albert, J., Aliu, E., Anderhub, H., Antonelli, L.A., Antoranz, P., Backes, M., Baixeras, C., Barrio, J.A., and Bartko, H. (2008). Very-High-Energy gamma rays from a Distant Quasar: How Transparent Is the Universe?. Science, 320, 1752."},{"key":"ref_70","doi-asserted-by":"crossref","first-page":"L8","DOI":"10.1088\/2041-8205\/730\/1\/L8","article-title":"MAGIC Discovery of Very High Energy Emission from the FSRQ PKS 1222+21","volume":"730","author":"Antonelli","year":"2011","journal-title":"Astrophys. J. Lett."},{"key":"ref_71","doi-asserted-by":"crossref","first-page":"085036","DOI":"10.1103\/PhysRevD.86.085036","article-title":"Evidence for an axion-like particle from PKS 1222+216?","volume":"86","author":"Tavecchio","year":"2012","journal-title":"Phys. Rev. D"},{"key":"ref_72","doi-asserted-by":"crossref","first-page":"809","DOI":"10.1051\/0004-6361:20000538","article-title":"The multifrequency emission of Mrk 501. From radio to TeV gamma-rays","volume":"367","author":"Sol","year":"2001","journal-title":"Astron. Astrophys."},{"key":"ref_73","doi-asserted-by":"crossref","first-page":"043005","DOI":"10.1103\/PhysRevD.86.043005","article-title":"Irregularity in gamma ray source spectra as a signature of axionlike particles","volume":"86","author":"Wouters","year":"2012","journal-title":"Phys. Rev. D"},{"key":"ref_74","doi-asserted-by":"crossref","unstructured":"Cheng, J.G., He, Y.J., Liang, Y.F., Lu, R.J., and Liang, E.W. (2020). Revisiting the analysis of axion-like particles with the Fermi-LAT gamma-ray observation of NGC1275. arXiv.","DOI":"10.1016\/j.physletb.2021.136611"},{"key":"ref_75","doi-asserted-by":"crossref","first-page":"24","DOI":"10.1140\/epjc\/s10052-015-3869-8","article-title":"The PVLAS experiment: Measuring vacuum magnetic birefringence and dichroism with a birefringent Fabry-Perot cavity","volume":"76","author":"Ejlli","year":"2016","journal-title":"Eur. Phys. J. C"},{"key":"ref_76","doi-asserted-by":"crossref","first-page":"063009","DOI":"10.1103\/PhysRevD.97.063009","article-title":"New bounds on axionlike particles from the Fermi Large Area Telescope observation of PKS 2155-304","volume":"97","author":"Zhang","year":"2018","journal-title":"Phys. Rev. D"},{"key":"ref_77","doi-asserted-by":"crossref","first-page":"042","DOI":"10.1088\/1475-7516\/2019\/06\/042","article-title":"Constraints on axion-like particle properties with TeV gamma-ray observations of Galactic sources","volume":"2019","author":"Liang","year":"2019","journal-title":"J. Cosmol. Astropart. Phys."},{"key":"ref_78","unstructured":"Malyshev, D., Neronov, A., Semikoz, D., Santangelo, A., and Jochum, J. (2018). Improved limit on axion-like particles from gamma-ray data on Perseus cluster. arXiv."},{"key":"ref_79","doi-asserted-by":"crossref","unstructured":"Guo, J., Li, H.J., Bi, X.J., Lin, S.J., and Yin, P.F. (2020). The implications of the axion like particle from the Fermi-LAT and H.E.S.S. observations of PG 1553+113 and PKS 2155-304. arXiv.","DOI":"10.1088\/1674-1137\/abcd2e"},{"key":"ref_80","doi-asserted-by":"crossref","first-page":"A75","DOI":"10.1051\/0004-6361\/201220631","article-title":"Evidence for a cosmological effect in \u03b3-ray spectra of BL Lacertae","volume":"554","author":"Sanchez","year":"2013","journal-title":"A&A"},{"key":"ref_81","doi-asserted-by":"crossref","first-page":"023","DOI":"10.1088\/1475-7516\/2013\/11\/023","article-title":"fparticle mixing in the GeV gamma-ray blazar data?","volume":"2013","author":"Mena","year":"2013","journal-title":"J. Cosmol. Astropart. Phys."},{"key":"ref_82","doi-asserted-by":"crossref","first-page":"123","DOI":"10.1093\/mnras\/stz1144","article-title":"Blazar VHE spectral alterations induced by photon-ALP oscillations","volume":"487","author":"Galanti","year":"2019","journal-title":"MNRAS"},{"key":"ref_83","doi-asserted-by":"crossref","first-page":"063003","DOI":"10.1103\/PhysRevD.97.063003","article-title":"Searching for spectral oscillations due to photon-axionlike particle conversion using the Fermi-LAT observations of bright supernova remnants","volume":"97","author":"Xia","year":"2018","journal-title":"Phys. Rev. D"},{"key":"ref_84","doi-asserted-by":"crossref","first-page":"123004","DOI":"10.1103\/PhysRevD.100.123004","article-title":"Searching for the possible signal of the photon-axionlike particle oscillation in the combined GeV and TeV spectra of supernova remnants","volume":"100","author":"Xia","year":"2019","journal-title":"Phys. Rev. D"},{"key":"ref_85","doi-asserted-by":"crossref","first-page":"003","DOI":"10.1088\/1475-7516\/2014\/09\/003","article-title":"On detecting oscillations of gamma rays into axion-like particles in turbulent and coherent magnetic fields","volume":"2014","author":"Meyer","year":"2014","journal-title":"J. Cosmol. Astropart. Phys."},{"key":"ref_86","doi-asserted-by":"crossref","first-page":"1553","DOI":"10.1093\/mnras\/stz3410","article-title":"Hint at an axion-like particle from the redshift dependence of blazar spectra","volume":"493","author":"Galanti","year":"2020","journal-title":"Mon. Not. R. Astron. Soc."},{"key":"ref_87","unstructured":"Bai, X., Bi, B.Y., Bi, X.J., Cao, Z., Chen, S.Z., Chen, Y., Chiavassa, A., Cui, X.H., Dai, Z.G., and della Volpe, D. (2019). The Large High Altitude Air Shower Observatory (LHAASO) Science White Paper. arXiv."},{"key":"ref_88","doi-asserted-by":"crossref","first-page":"431","DOI":"10.1002\/asna.202113946","article-title":"The Southern Wide-Field Gamma-ray Observatory","volume":"342","year":"2021","journal-title":"Astron. Nachrichten"},{"key":"ref_89","unstructured":"Spector, A. (2016). ALPS II technical overview and status report. arXiv."}],"updated-by":[{"DOI":"10.3390\/universe8020074","type":"correction","label":"Correction","source":"publisher","updated":{"date-parts":[[2021,6,5]],"date-time":"2021-06-05T00:00:00Z","timestamp":1622851200000}}],"container-title":["Universe"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/2218-1997\/7\/6\/185\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,8,3]],"date-time":"2025-08-03T23:04:24Z","timestamp":1754262264000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/2218-1997\/7\/6\/185"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2021,6,5]]},"references-count":89,"journal-issue":{"issue":"6","published-online":{"date-parts":[[2021,6]]}},"alternative-id":["universe7060185"],"URL":"https:\/\/doi.org\/10.3390\/universe7060185","relation":{},"ISSN":["2218-1997"],"issn-type":[{"value":"2218-1997","type":"electronic"}],"subject":[],"published":{"date-parts":[[2021,6,5]]}}}