# Axion-like Particle Searches with IACTs

^{1}

^{2}

^{3}

^{4}

^{*}

## Abstract

**:**

## 1. Axion and Axion-Like-Particles

#### Experimental Searches for ALPs

## 2. Phenomenology of the Mixing between Gamma-Rays and ALP and Propagation in the Astrophysical Environment

#### 2.1. ALP Propagation

#### 2.2. Probability of ALP-Gamma Conversion

#### 2.3. Gamma-Ray Survival Probability

#### 2.4. Astrophysical Magnetic Field and Photon Survival

#### 2.5. A Concrete Example of the Photon Survival Probability

## 3. A Decade of Results with IACTs

#### 3.1. VHE $\gamma $-ray Detection and Analysis Techniques

#### 3.2. Astrophysical Targets for ALPs Searches with IACTs

#### 3.3. Critical Energy and Parameter Space for $\gamma $-ray Studies

#### 3.4. H.E.S.S. Results with PKS 2155-304

#### 3.5. Studies on Spectral Irregularities of NGC 1275

`Pass 8`event analysis, and produced ALP predictions by including the photon–ALP conversion in the intracluster magnetic field and in the galactic magnetic field of the Milky Way. A fit of the time-averaged spectrum of NGC 1275 and ALPs models was made, and a likelihood analysis was performed. In Figure 6, one can see the likelihood of one of the event types, together with the best spectral fit with and without ALPs. To evaluate the ALPs hypothesis, Ajello et al. [44] exploited a likelihood ratio test statistics ($TS$). In the procedure, a time-averaged spectrum is modelled by a smooth function, and likelihood is extracted for each reconstructed energy bin ${k}^{\prime}$, $\mathcal{L}({\mu}_{{k}^{\prime}},\theta |{D}_{{k}^{\prime}})$, where ${\mu}_{{k}^{\prime}}$ is the expected number of photons in the photon–ALP conversion scenario, $\theta $ are the nuisance parameters of the fit, and ${D}_{{k}^{\prime}}$ is the observed photon count. For each set of ALPs parameters and magnetic field, the joint likelihood of all reconstructed energy bins ${k}^{\prime}$ is maximized and the best-fit parameters are determined. Among the different turbulent magnetic field realizations, simulated by accounting for its randomness, the one corresponding to the 0.95 quantile of the likelihood distribution is chosen. The likelihood ratio test is performed as

#### 3.6. Combined Fermi-LAT and H.E.S.S. Observations of PKS 2155-304

#### 3.7. H.E.S.S. Study with Galactic Sources

#### 3.8. Supernova Remnants

#### 3.9. Studies Obtained Comparing Data from Different Blazars

#### 3.10. ALP-Photon Back Conversion in the Galactic Magnetic Field

## 4. Outlook: The Cherenkov Telescope Array

## 5. Summary and Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

## Abbreviations

ADMX | Axion Dark Matter eXperiment |

AGN | Active Galactic Nucleus |

ALPs | Axion-like particles |

ALPS | Any Light Particle Search |

ATNF | The Australia Telescope National Facility |

CAST | The CERN Axion Solar Telescope |

CERN | European Council for Nuclear Research |

CDM | Cold Dark Matter |

CMB | Cosmic Microwave Background |

CP | Charge-Parity |

DAMA | Dark Matter experiment |

DARMA | De Angelis, Roncadelli and Mansutti |

DFSZ | Dine–Fischler–Srednicki–Zhitnitsky |

DM | Dark Matter |

EBL | Extragalactic Background Light |

FSRQ | Flat Spectrum Radio Quasar |

GMF | Galactic Magnetic Field |

HE | high-energy (E > 100 MeV) |

H.E.S.S. | The High-Energy Stereoscopic System |

IACT | Imaging Atmospheric Cherenkov Telescope |

IAXO | The International Axion Observatory |

ICMF | Intracluster Magnetic Field |

IGMF | Intergalactic Magnetic Field |

KSVZ | Kim–Shifman–Vainshtein–Zakharov |

LHAASO | The Large High-Altitude Air Shower Observatory |

LHC | Large Hadron Collider |

LIV | Lorentz Invariance Violation |

MAGIC | Major Atmospheric Gamma-ray Cherenkov |

OSQAR | The Optical Search for QED Vacuum Bifringence |

PQ | Peccei-Quinn |

PVLAS | The Polarization of the Vacuum with Laser |

QCD | Quantum Chromo-Dynamics |

QUAX | QUest for AXions |

SM | Standard Model |

SWGO | The Southern Wide-field Gamma-ray Observatory |

VERITAS | Very Energetic Radiation Imaging Telescope Array System |

VHE | very-high-energy (E > 100 GeV) |

WISPs | Weakly Interacting Slim Particles |

## Notes

1 | By external magnetic field, we mean that the field is present outside the photon-ALP system itself and not generated during or by the interaction. |

2 | http://cta.irap.omp.eu/ctools/ (accessed on 3 June 2020). |

3 | https://gammapy.org/ (accessed on 3 June 2020). |

4 | https://github.com/me-manu/gammaALPs (accessed on 3 June 2020). |

## References

- Peccei, R.D.; Quinn, H.R. CP conservation in the presence of pseudoparticles. Phys. Rev. Lett.
**1977**, 38, 1440–1443. [Google Scholar] [CrossRef] [Green Version] - Weinberg, S. A New Light Boson? Phys. Rev. Lett.
**1978**, 40, 223–226. [Google Scholar] [CrossRef] - Wilczek, F. Problem of Strong P and T Invariance in the Presence of Instantons. Phys. Rev. Lett.
**1978**, 40, 279–282. [Google Scholar] [CrossRef] - Abel, C.; Afach, S.; Ayres, N.J.; Baker, C.A.; Ban, G.; Bison, G.; Bodek, K.; Bondar, V.; Burghoff, M.; Chanel, E.; et al. Measurement of the Permanent Electric Dipole Moment of the Neutron. Phys. Rev. Lett.
**2020**, 124, 081803. [Google Scholar] [CrossRef] [Green Version] - Kim, J.E. Weak-Interaction Singlet and Strong CP Invariance. Phys. Rev. Lett.
**1979**, 43, 103–107. [Google Scholar] [CrossRef] - Shifman, M.A.; Vainshtein, A.I.; Zakharov, V.I. Can confinement ensure natural CP invariance of strong interactions? Nucl. Phys. B
**1980**, 166, 493–506. [Google Scholar] [CrossRef] - Dine, M.; Fischler, W.; Srednicki, M. A simple solution to the strong CP problem with a harmless axion. Phys. Lett. B
**1981**, 104, 199–202. [Google Scholar] [CrossRef] - Zhitnitsky, A. On Possible Suppression of the Axion Hadron Interactions. Sov. J. Nucl. Phys.
**1980**, 31, 260. (In Russian) [Google Scholar] - Anselm, A.; Uraltsev, N. A second massless axion? Phys. Lett. B
**1982**, 114, 39–41. [Google Scholar] [CrossRef] - Anselm, A.A. Experimental test for arion ⇆ photon oscillations in a homogeneous constant magnetic field. Phys. Rev. D
**1988**, 37, 2001–2004. [Google Scholar] [CrossRef] - Arias, P.; Cadamuro, D.; Goodsell, M.; Jaeckel, J.; Redondo, J.; Ringwald, A. WISPy cold dark matter. J. Cosmol. Astropart. Phys.
**2012**, 2012, 013. [Google Scholar] [CrossRef] - Anastassopoulos, V.; Aune, S.; Barth, K.; Belov, A.; Bräuninger, H.; Cantatore, G.; Carmona, J.M.; Castel, J.F.; Cetin, S.A.; Christensen, F.; et al. New CAST limit on the axion-photon interaction. Nat. Phys.
**2017**, 13, 584–590. [Google Scholar] - Sikivie, P. Experimental Tests of the “Invisible” Axion. Phys. Rev. Lett.
**1983**, 51, 1415–1417. [Google Scholar] [CrossRef] - Vogel, J.K.; Armengaud, E.; Avignone, F.T.; Betz, M.; Brax, P.; Brun, P.; Cantatore, G.; Carmona, J.M.; Carosi, G.P.; Caspers, F.; et al. The Next Generation of Axion Helioscopes: The International Axion Observatory (IAXO). Phys. Procedia
**2015**, 61, 193–200. [Google Scholar] [CrossRef] - Krčmar, M.; Krečak, Z.; LjubičiĆ, A.; Stipčević, M.; Bradley, D.A. Search for solar axions using
^{7}Li. Phys. Rev. D**2001**, 64, 115016. [Google Scholar] [CrossRef] [Green Version] - Akerib, D.S.; Alsum, S.; Aquino, C.; Araújo, H.M.; Bai, X.; Bailey, A.J.; Balajthy, J.; Beltrame, P.; Bernard, E.P.; Bernstein, A.; et al. First Searches for Axions and Axionlike Particles with the LUX Experiment. Phys. Rev. Lett.
**2017**, 118, 261301. [Google Scholar] [CrossRef] [PubMed] - Ballou, R.; Deferne, G.; Finger, M.; Finger, M.; Flekova, L.; Hosek, J.; Kunc, S.; Macuchova, K.; Meissner, K.A.; Pugnat, P.; et al. New exclusion limits on scalar and pseudoscalar axionlike particles from light shining through a wall. Phys. Rev. D
**2015**, 92, 092002. [Google Scholar] [CrossRef] [Green Version] - Ehret, K.; Frede, M.; Ghazaryan, S.; Hildebrandt, M.; Knabbe, E.A.; Kracht, D.; Lindner, A.; List, J.; Meier, T.; Meyer, N.; et al. New ALPS results on hidden-sector lightweights. Phys. Lett. B
**2010**, 689, 149–155. [Google Scholar] [CrossRef] - Du, N.; Force, N.; Khatiwada, R.; Lentz, E.; Ottens, R.; Rosenberg, L.J.; Rybka, G.; Carosi, G.; Woollett, N.; Bowring, D.; et al. Search for Invisible Axion Dark Matter with the Axion Dark Matter Experiment. Phys. Rev. Lett.
**2018**, 120, 151301. [Google Scholar] [CrossRef] [PubMed] [Green Version] - Barbieri, R.; Braggio, C.; Carugno, G.; Gallo, C.S.; Lombardi, A.; Ortolan, A.; Pengo, R.; Ruoso, G.; Speake, C.C. Searching for galactic axions through magnetized media: The QUAX proposal. Phys. Dark Universe
**2017**, 15, 135–141. [Google Scholar] [CrossRef] [Green Version] - Alesini, D.; Braggio, C.; Carugno, G.; Crescini, N.; D’Agostino, D.; Di Gioacchino, D.; Di Vora, R.; Falferi, P.; Gambardella, U.; Gatti, C.; et al. Search for invisible axion dark matter of mass m
_{a}= 43 μeV with the QUAX-aγ experiment. Phys. Rev. D**2021**, 103, 102004. [Google Scholar] [CrossRef] - Crescini, N.; Alesini, D.; Braggio, C.; Carugno, G.; D’Agostino, D.; Di Gioacchino, D.; Falferi, P.; Gambardella, U.; Gatti, C.; Iannone, G.; et al. Axion Search with a Quantum-Limited Ferromagnetic Haloscope. Phys. Rev. Lett.
**2020**, 124, 171801. [Google Scholar] [CrossRef] [PubMed] - Dolag, K.; Bykov, A.M.; Diaferio, A. Non-Thermal Processes in Cosmological Simulations. Space Sci. Rev.
**2008**, 134, 311–335. [Google Scholar] [CrossRef] [Green Version] - Churazov, E.; Forman, W.; Jones, C.; Böhringer, H. XMM-Newton Observations of the Perseus Cluster. I. The Temperature and Surface Brightness Structure. Astrophys. J.
**2003**, 590, 225–237. [Google Scholar] [CrossRef] [Green Version] - Taylor, G.B.; Gugliucci, N.E.; Fabian, A.C.; Sanders, J.S.; Gentile, G.; Allen, S.W. Magnetic fields in the centre of the Perseus cluster. MNRAS
**2006**, 368, 1500–1506. [Google Scholar] [CrossRef] [Green Version] - Aleksić, J.; Alvarez, E.A.; Antonelli, L.A.; Antoranz, P.; Asensio, M.; Backes, M.; Barres de Almeida, U.; Barrio, J.A.; Bastieri, D.; Becerra González, J.; et al. Constraining cosmic rays and magnetic fields in the Perseus galaxy cluster with TeV observations by the MAGIC telescopes. A&A
**2012**, 541, A99. [Google Scholar] - Marsh, M.C.D.; Russell, H.R.; Fabian, A.C.; McNamara, B.R.; Nulsen, P.; Reynolds, C.S. A new bound on axion-like particles. J. Cosmol. Astropart. Phys.
**2017**, 2017, 036. [Google Scholar] [CrossRef] [Green Version] - Raffelt, G.; Stodolsky, L. Mixing of the photon with low-mass particles. Phys. Rev. D
**1988**, 37, 1237–1249. [Google Scholar] [CrossRef] [Green Version] - Meyer, M.; Horns, D.; Raue, M. First lower limits on the photon-axion-like particle coupling from very high energy gamma-ray observations. Phys. Rev. D
**2013**, 87, 035027. [Google Scholar] [CrossRef] [Green Version] - Horns, D.; Maccione, L.; Meyer, M.; Mirizzi, A.; Montanino, D.; Roncadelli, M. Hardening of TeV gamma spectrum of active galactic nuclei in galaxy clusters by conversions of photons into axionlike particles. Phys. Rev. D
**2012**, 86, 075024. [Google Scholar] [CrossRef] [Green Version] - Mirizzi, A.; Montanino, D. Stochastic conversions of TeV photons into axion-like particles in extragalactic magnetic fields. J. Cosmol. Astropart. Phys.
**2009**, 2009, 004. [Google Scholar] [CrossRef] [Green Version] - Long, G.; Chen, S.; Xu, S.; Zhang, H.H. Probing μeV ALPs with future LHAASO observation of AGN γ-ray spectra. arXiv
**2021**, arXiv:2101.10270. [Google Scholar] - de Angelis, A.; Roncadelli, M.; Mansutti, O. Evidence for a new light spin-zero boson from cosmological gamma-ray propagation? Phys. Rev. D
**2007**, 76, 121301. [Google Scholar] [CrossRef] [Green Version] - Simet, M.; Hooper, D.; Serpico, P.D. Milky Way as a kiloparsec-scale axionscope. Phys. Rev. D
**2008**, 77, 063001. [Google Scholar] [CrossRef] [Green Version] - Sánchez-Conde, M.A.; Paneque, D.; Bloom, E.; Prada, F.; Domínguez, A. Hints of the existence of axionlike particles from the gamma-ray spectra of cosmological sources. Phys. Rev. D
**2009**, 79, 123511. [Google Scholar] [CrossRef] - de Angelis, A.; Galanti, G.; Roncadelli, M. Relevance of axionlike particles for very-high-energy astrophysics. Phys. Rev. D
**2011**, 84, 105030. [Google Scholar] [CrossRef] [Green Version] - Hooper, D.; Serpico, P.D. Detecting Axionlike Particles with Gamma Ray Telescopes. Phys. Rev. Lett.
**2007**, 99, 231102. [Google Scholar] [CrossRef] [PubMed] [Green Version] - de Angelis, A.; Mansutti, O.; Persic, M.; Roncadelli, M. Photon propagation and the very high energy γ-ray spectra of blazars: How transparent is the Universe? MNRAS
**2009**, 394, L21–L25. [Google Scholar] [CrossRef] [Green Version] - Abramowski, A.; Acero, F.; Aharonian, F.; Ait Benkhali, F.; Akhperjanian, A.G.; Angüner, E.; Anton, G.; Balenderan, S.; Balzer, A.; Barnacka, A.; et al. Constraints on axionlike particles with H.E.S.S. from the irregularity of the PKS 2155-304 energy spectrum. Phys. Rev. D
**2013**, 88, 102003. [Google Scholar] [CrossRef] [Green Version] - Jansson, R.; Farrar, G.R. A New Model of the Galactic Magnetic Field. Astrophys. J.
**2012**, 757, 14. [Google Scholar] [CrossRef] - Sun, X.H.; Reich, W.; Waelkens, A.; Enßlin, T.A. Radio observational constraints on Galactic 3D-emission models. A&A
**2008**, 477, 573–592. [Google Scholar] - Pshirkov, M.S.; Tinyakov, P.G.; Kronberg, P.P.; Newton-McGee, K.J. Deriving the Global Structure of the Galactic Magnetic Field from Faraday Rotation Measures of Extragalactic Sources. Astrophys. J.
**2011**, 738, 192. [Google Scholar] [CrossRef] [Green Version] - Davies, J.; Meyer, M.; Cotter, G. Relevance of Jet Magnetic Field Structure for Blazar ALP Searches. arXiv
**2020**, arXiv:2011.08123. [Google Scholar] - Ajello, M.; Albert, A.; Anderson, B.; Baldini, L.; Barbiellini, G.; Bastieri, D.; Bellazzini, R.; Bissaldi, E.; Blandford, R.D.; Bloom, E.D.; et al. Search for Spectral Irregularities due to Photon-Axionlike-Particle Oscillations with the Fermi Large Area Telescope. Phys. Rev. Lett.
**2016**, 116, 161101. [Google Scholar] [CrossRef] [Green Version] - Abdalla, H.; Abe, H.; Acero, F.; Acharyya, A.; Adam, R.; Agudo, I.; Aguirre-Santaella, A.; Alfaro, R.; Alfaro, J.; Alispach, C.; et al. Sensitivity of the Cherenkov Telescope Array for probing cosmology and fundamental physics with gamma-ray propagation. J. Cosmol. Astropart. Phys.
**2021**, 2021, 48. [Google Scholar] [CrossRef] - Hillas, A. 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—The Science Explored with the CTA. Astropart. Phys.
**2013**, 43, 19–43. [Google Scholar] [CrossRef] - Weekes, T.C.; Cawley, M.F.; Fegan, D.J.; Gibbs, K.G.; Hillas, A.M.; Kowk, P.W.; Lamb, R.C.; Lewis, D.A.; Macomb, D.; Porter, N.A.; et al. Observation of TeV Gamma Rays from the Crab Nebula Using the Atmospheric Cerenkov Imaging Technique. Astrophys. J.
**1989**, 342, 379. [Google Scholar] [CrossRef] - Hinton, J.A.; HESS Collaboration. The status of the HESS project. New Astron. Rev.
**2004**, 48, 331–337. [Google Scholar] [CrossRef] [Green Version] - Lorenz, E.; MAGIC Collaboration. Status of the 17 m diameter Magic telescope. In Proceedings of the International Cosmic Ray Conference, Hamburg, Germany, 8–15 August 2001; Volume 7, p. 2789. [Google Scholar]
- Weekes, T.C.; Badran, H.; Biller, S.D.; Bond, I.; Bradbury, S.; Buckley, J.; Carter-Lewis, D.; Catanese, M.; Criswell, S.; Cui, W.; et al. VERITAS: The Very Energetic Radiation Imaging Telescope Array System. Astropart. Phys.
**2002**, 17, 221–243. [Google Scholar] [CrossRef] - Hillas, A.M. Cerenkov Light Images of EAS Produced by Primary Gamma Rays and by Nuclei. In Proceedings of the 19th International Cosmic Ray Conference (ICRC19), San Diego, CA, USA, 11–23 August 1985; Volume 3, p. 445. [Google Scholar]
- Fomin, V.P.; Stepanian, A.A.; Lamb, R.C.; Lewis, D.A.; Punch, M.; Weekes, T.C. New methods of atmospheric Cherenkov imaging for gamma-ray astronomy. I. The false source method. Astropart. Phys.
**1994**, 2, 137–150. [Google Scholar] [CrossRef] - Aharonian, F.; Buckley, J.; Kifune, T.; Sinnis, G. High energy astrophysics with ground-based gamma ray detectors. Rep. Prog. Phys.
**2008**, 71, 096901. [Google Scholar] [CrossRef] - Aleksić, J.; Ansoldi, S.; Antonelli, L.A.; Antoranz, P.; Babic, A.; Bangale, P.; Barrio, J.A.; Becerra González, J.; Bednarek, W.; Bernardini, E.; et al. Measurement of the Crab Nebula spectrum over three decades in energy with the MAGIC telescopes. J. High Energy Astrophys.
**2015**, 5, 30–38. [Google Scholar] [CrossRef] - Mirizzi, A.; Raffelt, G.G.; Serpico, P.D. Signatures of axionlike particles in the spectra of TeV gamma-ray sources. Phys. Rev. D
**2007**, 76, 023001. [Google Scholar] [CrossRef] [Green Version] - Hochmuth, K.A.; Sigl, G. Effects of axion-photon mixing on gamma-ray spectra from magnetized astrophysical sources. Phys. Rev. D
**2007**, 76, 123011. [Google Scholar] [CrossRef] [Green Version] - Brockway, J.W.; Carlson, E.D.; Raffelt, G.G. SN 1987A gamma-ray limits on the conversion of pseudoscalars. Phys. Lett. B
**1996**, 383, 439–443. [Google Scholar] [CrossRef] [Green Version] - Grifols, J.A.; Massó, E.; Toldrà, R. Gamma Rays from SN 1987A due to Pseudoscalar Conversion. Phys. Rev. Lett.
**1996**, 77, 2372–2375. [Google Scholar] [CrossRef] [Green Version] - Payez, A.; Evoli, C.; Fischer, T.; Giannotti, M.; Mirizzi, A.; Ringwald, A. Revisiting the SN1987A gamma-ray limit on ultralight axion-like particles. J. Cosmol. Astropart. Phys.
**2015**, 2015, 006. [Google Scholar] [CrossRef] [Green Version] - Raffelt, G.G. Stars as Laboratories for Fundamental Physics: The Astrophysics of Neutrinos, Axions, and Other Weakly Interacting Particles; University of Chicago Press: Chicago, IL, USA, 1996. [Google Scholar]
- Raffelt, G.G. Astrophysical Axion Bounds. In Axions; Kuster, M., Raffelt, G., Beltrán, B., Eds.; Springer: Berlin/Heidelberg, Germany, 2008; Volume 741, p. 51. [Google Scholar]
- Berenji, B.; Gaskins, J.; Meyer, M. Constraints on axions and axionlike particles from Fermi Large Area Telescope observations of neutron stars. Phys. Rev. D
**2016**, 93, 045019. [Google Scholar] [CrossRef] [Green Version] - Morris, D.E. Axion mass limits may be improved by pulsar x-ray measurements. Phys. Rev. D
**1986**, 34, 843–848. [Google Scholar] [CrossRef] [PubMed] - Giannotti, M.; Duffy, L.D.; Nita, R. New constraints for heavy axion-like particles from supernovae. J. Cosmol. Astropart. Phys.
**2011**, 2011, 015. [Google Scholar] [CrossRef] [Green Version] - Zavattini, E.; Zavattini, G.; Ruoso, G.; Polacco, E.; Milotti, E.; Karuza, M.; Gastaldi, U.; di Domenico, G.; Della Valle, F.; Cimino, R.; et al. Experimental Observation of Optical Rotation Generated in Vacuum by a Magnetic Field. Phys. Rev. Lett.
**2006**, 96, 110406. [Google Scholar] [CrossRef] [PubMed] [Green Version] - de Angelis, A.; Mansutti, O.; Roncadelli, M. Axion-like particles, cosmic magnetic fields and gamma-ray astrophysics. Phys. Lett. B
**2008**, 659, 847–855. [Google Scholar] [CrossRef] [Green Version] - Hillas, A.M. The Origin of Ultra-High-Energy Cosmic Rays. Annu. Rev. Astron. Astrophys.
**1984**, 22, 425–444. [Google Scholar] [CrossRef] - Protheroe, R.J.; Meyer, H. An infrared background-TeV gamma-ray crisis? Phys. Lett. B
**2000**, 493, 1–6. [Google Scholar] [CrossRef] [Green Version] - MAGIC Collaboration; Albert, J.; Aliu, E.; Anderhub, H.; Antonelli, L.A.; Antoranz, P.; Backes, M.; Baixeras, C.; Barrio, J.A.; Bartko, H.; et al. Very-High-Energy gamma rays from a Distant Quasar: How Transparent Is the Universe? Science
**2008**, 320, 1752. [Google Scholar] - Aleksić, J.; Antonelli, L.A.; Antoranz, P.; Backes, M.; Barrio, J.A.; Bastieri, D.; Becerra González, J.; Bednarek, W.; Berdyugin, A.; Berger, K.; et al. MAGIC Discovery of Very High Energy Emission from the FSRQ PKS 1222+21. Astrophys. J. Lett.
**2011**, 730, L8. [Google Scholar] [CrossRef] - Tavecchio, F.; Roncadelli, M.; Galanti, G.; Bonnoli, G. Evidence for an axion-like particle from PKS 1222+216? Phys. Rev. D
**2012**, 86, 085036. [Google Scholar] [CrossRef] [Green Version] - Katarzyński, K.; Sol, H.; Kus, A. The multifrequency emission of Mrk 501. From radio to TeV gamma-rays. Astron. Astrophys.
**2001**, 367, 809–825. [Google Scholar] [CrossRef] - Wouters, D.; Brun, P. Irregularity in gamma ray source spectra as a signature of axionlike particles. Phys. Rev. D
**2012**, 86, 043005. [Google Scholar] [CrossRef] [Green Version] - Cheng, J.G.; He, Y.J.; Liang, Y.F.; Lu, R.J.; Liang, E.W. Revisiting the analysis of axion-like particles with the Fermi-LAT gamma-ray observation of NGC1275. arXiv
**2020**, arXiv:2010.12396. [Google Scholar] - Della Valle, F.; Ejlli, A.; Gastaldi, U.; Messineo, G.; Milotti, E.; Pengo, R.; Ruoso, G.; Zavattini, G. The PVLAS experiment: Measuring vacuum magnetic birefringence and dichroism with a birefringent Fabry-Perot cavity. Eur. Phys. J. C
**2016**, 76, 24. [Google Scholar] [CrossRef] [Green Version] - Zhang, C.; Liang, Y.F.; Li, S.; Liao, N.H.; Feng, L.; Yuan, Q.; Fan, Y.Z.; Ren, Z.Z. New bounds on axionlike particles from the Fermi Large Area Telescope observation of PKS 2155-304. Phys. Rev. D
**2018**, 97, 063009. [Google Scholar] [CrossRef] [Green Version] - Liang, Y.F.; Zhang, C.; Xia, Z.Q.; Feng, L.; Yuan, Q.; Fan, Y.Z. Constraints on axion-like particle properties with TeV gamma-ray observations of Galactic sources. J. Cosmol. Astropart. Phys.
**2019**, 2019, 042. [Google Scholar] [CrossRef] - Malyshev, D.; Neronov, A.; Semikoz, D.; Santangelo, A.; Jochum, J. Improved limit on axion-like particles from gamma-ray data on Perseus cluster. arXiv
**2018**, arXiv:1805.04388. [Google Scholar] - Guo, J.; Li, H.J.; Bi, X.J.; Lin, S.J.; Yin, P.F. 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
**2020**, arXiv:2002.07571. [Google Scholar] - Sanchez, D.A.; Fegan, S.; Giebels, B. Evidence for a cosmological effect in γ-ray spectra of BL Lacertae. A&A
**2013**, 554, A75. [Google Scholar] - Mena, O.; Razzaque, S. fparticle mixing in the GeV gamma-ray blazar data? J. Cosmol. Astropart. Phys.
**2013**, 2013, 023. [Google Scholar] [CrossRef] [Green Version] - Galanti, G.; Tavecchio, F.; Roncadelli, M.; Evoli, C. Blazar VHE spectral alterations induced by photon-ALP oscillations. MNRAS
**2019**, 487, 123–132. [Google Scholar] [CrossRef] [Green Version] - Xia, Z.Q.; Zhang, C.; Liang, Y.F.; Feng, L.; Yuan, Q.; Fan, Y.Z.; Wu, J. Searching for spectral oscillations due to photon-axionlike particle conversion using the Fermi-LAT observations of bright supernova remnants. Phys. Rev. D
**2018**, 97, 063003. [Google Scholar] [CrossRef] [Green Version] - Xia, Z.Q.; Liang, Y.F.; Feng, L.; Yuan, Q.; Fan, Y.Z.; Wu, J. Searching for the possible signal of the photon-axionlike particle oscillation in the combined GeV and TeV spectra of supernova remnants. Phys. Rev. D
**2019**, 100, 123004. [Google Scholar] [CrossRef] [Green Version] - Meyer, M.; Montanino, D.; Conrad, J. On detecting oscillations of gamma rays into axion-like particles in turbulent and coherent magnetic fields. J. Cosmol. Astropart. Phys.
**2014**, 2014, 003. [Google Scholar] [CrossRef] [Green Version] - Galanti, G.; Roncadelli, M.; De Angelis, A.; Bignami, G.F. Hint at an axion-like particle from the redshift dependence of blazar spectra. Mon. Not. R. Astron. Soc.
**2020**, 493, 1553–1564. [Google Scholar] [CrossRef] - Bai, X.; Bi, B.Y.; Bi, X.J.; Cao, Z.; Chen, S.Z.; Chen, Y.; Chiavassa, A.; Cui, X.H.; Dai, Z.G.; della Volpe, D.; et al. The Large High Altitude Air Shower Observatory (LHAASO) Science White Paper. arXiv
**2019**, arXiv:1905.02773. [Google Scholar] - Barres de Almeida, U.; the SWGO Collaboration. The Southern Wide-Field Gamma-ray Observatory. Astron. Nachrichten
**2021**, 342, 431–437. [Google Scholar] [CrossRef] - Spector, A. ALPS II technical overview and status report. arXiv
**2016**, arXiv:1611.05863. [Google Scholar]

**Figure 1.**ALPs parameter space with current constraints (last update: July 2020). The collected limits, references and plots are available in the git-hub repository: https://cajohare.github.io/AxionLimits/ (accessed on 3 June 2020).

**Figure 3.**Photon survival probability for ${m}_{a}=100\phantom{\rule{3.33333pt}{0ex}}\mathrm{neV}$ and ${g}_{a\gamma \gamma}=1\times {10}^{-11}$ GeV${}^{-1}$. Obtained using the GAMMAALPs code: https://github.com/me-manu/gammaALPs (accessed on 3 June 2020).

**Figure 4.**ALPs parameter space available for gamma-ray observations. Reprinted from Hooper and Serpico [37].

**Figure 5.**(

**Left**) Schematic view of spectral irregularity quantification. Reprinted from Abramowski et al. [39]. (

**Right**) Constraints on ALPs parameter space set by CAST, compared with results from the previous helioscope Sumico and DAMA experiment, as well as with PVLAS [75] and OSQAR [17] experiments, constraints set by H.E.S.S collaboration, observations of SN1987A, Solar astrophysics and Dark Matter (DM) searches. Reprinted from Anastassopoulos et al. [12].

**Figure 6.**(

**Left**) Likelihood curves for one event type and best spectral fits with and without ALPs. Reprinted from Ajello et al. [44]. (

**Right**) Projected limits on the ALPs parameter space obtained with the Fermi-LAT study of the NGC 1275 data, compared with the results from other experiments at the time. Reprinted from Ajello et al. [44].

**Figure 7.**(

**Left**) Likelihood curves for the observed spectrum of PKS 2155-304. Solid lines represent, best fits including the photon–ALP oscillations and best spectral fit without oscillations included. Reprinted from Zhang et al. [76]. (

**Right**) Comparison of exclusion regions derived in [76], compared with exclusion regions from H.E.S.S. observations of PKS-2155-304 [39] and Fermi-LAT observations of NGC 1275 [44]. Reprinted from Zhang et al. [76].

**Figure 8.**(

**Top**) Projected CTA exclusions on the ALPs parameter space for different assumptions on the intracluster magnetic field parameters. Reprinted from Abdalla et al. [45]. (

**Bottom**) Projected limits from the CTA simulations, compared to constraints on the ALPs parameter space with Fermi LAT and H.E.S.S. Reprinted from Abdalla et al. [45].

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. |

© 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Batković, I.; De Angelis, A.; Doro, M.; Manganaro, M.
Axion-like Particle Searches with IACTs. *Universe* **2021**, *7*, 185.
https://doi.org/10.3390/universe7060185

**AMA Style**

Batković I, De Angelis A, Doro M, Manganaro M.
Axion-like Particle Searches with IACTs. *Universe*. 2021; 7(6):185.
https://doi.org/10.3390/universe7060185

**Chicago/Turabian Style**

Batković, Ivana, Alessandro De Angelis, Michele Doro, and Marina Manganaro.
2021. "Axion-like Particle Searches with IACTs" *Universe* 7, no. 6: 185.
https://doi.org/10.3390/universe7060185