# Searches for Ultra-High-Energy Photons at the Pierre Auger Observatory

^{†}

## Abstract

**:**

## 1. Introduction

## 2. Photon-Induced Air Showers

## 3. The Pierre Auger Observatory

## 4. Searches for a Diffuse Flux of UHE Photons

#### 4.1. A Search for Photons with Energies above $2\phantom{\rule{0.166667em}{0ex}}\phantom{\rule{0.166667em}{0ex}}\times \phantom{\rule{0.166667em}{0ex}}\phantom{\rule{0.166667em}{0ex}}{10}^{17}\phantom{\rule{0.166667em}{0ex}}\mathrm{eV}$ Using Hybrid Data from the Low-Energy Extensions of the Pierre Auger Observatory

#### 4.2. A Search for Ultra-High-Energy Photons at the Pierre Auger Observatory Exploiting Air-Shower Universality

#### 4.3. Search for Photons above ${10}^{19}\phantom{\rule{0.166667em}{0ex}}\mathrm{eV}$ with the Surface Detector of the Pierre Auger Observatory

#### 4.4. Summary of the Searches for a Diffuse Flux of UHE Photons

## 5. Searches for UHE Photons from Point Sources and Transient Events

#### 5.1. A Search for Point Sources of EeV Photons

#### 5.2. A Targeted Search for Point Sources of EeV Photons with the Pierre Auger Observatory

#### 5.3. Follow-Up Search for UHE Photons from Gravitational Wave Sources with the Pierre Auger Observatory

## 6. Outlook

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## Appendix A

**The Pierre Auger Collaboration:**

**P. Abreu ${}^{\mathbf{71}}$, M. Aglietta ${}^{\mathbf{53},\mathbf{51}}$, I. Allekotte ${}^{\mathbf{1}}$, K. Almeida Cheminant ${}^{\mathbf{69}}$, A. Almela ${}^{\mathbf{8},\mathbf{12}}$, J. Alvarez-Muñiz ${}^{\mathbf{78}}$, J. Ammerman Yebra ${}^{\mathbf{78}}$, G.A. Anastasi ${}^{\mathbf{53},\mathbf{51}}$, L. Anchordoqui ${}^{\mathbf{85}}$, B. Andrada ${}^{\mathbf{8}}$, S. Andringa ${}^{\mathbf{71}}$, C. Aramo ${}^{\mathbf{49}}$, P.R. Araújo Ferreira ${}^{\mathbf{41}}$, E. Arnone ${}^{\mathbf{62},\mathbf{51}}$, J. C. Arteaga Velázquez ${}^{\mathbf{66}}$, H. Asorey ${}^{\mathbf{8}}$, P. Assis ${}^{\mathbf{71}}$, G. Avila ${}^{\mathbf{11}}$, E. Avocone ${}^{\mathbf{56},\mathbf{45}}$, A.M. Badescu ${}^{\mathbf{74}}$, A. Bakalova ${}^{\mathbf{31}}$, A. Balaceanu ${}^{\mathbf{72}}$, F. Barbato ${}^{\mathbf{44},\mathbf{45}}$, J.A. Bellido ${}^{\mathbf{13},\mathbf{68}}$, C. Berat ${}^{\mathbf{35}}$, M.E. Bertaina ${}^{\mathbf{62},\mathbf{51}}$, G. Bhatta ${}^{\mathbf{69}}$, P.L. Biermann ${}^{\mathbf{f}}$, V. Binet ${}^{\mathbf{6}}$, K. Bismark ${}^{\mathbf{38},\mathbf{8}}$, T. Bister ${}^{\mathbf{41}}$, J. Biteau ${}^{\mathbf{36}}$, J. Blazek ${}^{\mathbf{31}}$, C. Bleve ${}^{\mathbf{35}}$, J. Blümer ${}^{\mathbf{40}}$, M. Boháčová ${}^{\mathbf{31}}$, D. Boncioli ${}^{\mathbf{56},\mathbf{45}}$, C. Bonifazi ${}^{\mathbf{9},\mathbf{25}}$, L. Bonneau Arbeletche ${}^{\mathbf{21}}$, N. Borodai ${}^{\mathbf{69}}$, J. Brack ${}^{\mathbf{g}}$, T. Bretz ${}^{\mathbf{41}}$, P.G. Brichetto Orchera ${}^{\mathbf{8}}$, F.L. Briechle ${}^{\mathbf{41}}$, P. Buchholz ${}^{\mathbf{43}}$, A. Bueno ${}^{\mathbf{77}}$, S. Buitink ${}^{\mathbf{15}}$, M. Buscemi ${}^{\mathbf{46},\mathbf{60}}$, M. Büsken ${}^{\mathbf{38},\mathbf{8}}$, A. Bwembya ${}^{\mathbf{79},\mathbf{80}}$, K.S. Caballero-Mora ${}^{\mathbf{65}}$, L. Caccianiga ${}^{\mathbf{58},\mathbf{48}}$, I. Caracas ${}^{\mathbf{37}}$, R. Caruso ${}^{\mathbf{57},\mathbf{46}}$, A. Castellina ${}^{\mathbf{53},\mathbf{51}}$, F. Catalani ${}^{\mathbf{18}}$, G. Cataldi ${}^{\mathbf{47}}$, L. Cazon ${}^{\mathbf{78}}$, M. Cerda ${}^{\mathbf{10}}$, J.A. Chinellato ${}^{\mathbf{21}}$, J. Chudoba ${}^{\mathbf{31}}$, L. Chytka ${}^{\mathbf{32}}$, R.W. Clay ${}^{\mathbf{13}}$, A.C. Cobos Cerutti ${}^{\mathbf{7}}$, R. Colalillo ${}^{\mathbf{59},\mathbf{49}}$, A. Coleman ${}^{\mathbf{89}}$, M.R. Coluccia ${}^{\mathbf{47}}$, R. Conceição ${}^{\mathbf{71}}$, A. Condorelli ${}^{\mathbf{44},\mathbf{45}}$, G. Consolati ${}^{\mathbf{48},\mathbf{54}}$, F. Contreras ${}^{\mathbf{11}}$, F. Convenga ${}^{\mathbf{40}}$, D. Correia dos Santos ${}^{\mathbf{27}}$, C.E. Covault ${}^{\mathbf{83}}$, M. Cristinziani ${}^{\mathbf{43}}$, S. Dasso ${}^{\mathbf{5},\mathbf{3}}$, K. Daumiller ${}^{\mathbf{40}}$, B.R. Dawson ${}^{\mathbf{13}}$, R.M. de Almeida ${}^{\mathbf{27}}$, J. de Jesús ${}^{\mathbf{8},\mathbf{40}}$, S.J. de Jong ${}^{\mathbf{79},\mathbf{80}}$, J.R.T. de Mello Neto ${}^{\mathbf{25},\mathbf{26}}$, I. De Mitri ${}^{\mathbf{44},\mathbf{45}}$, J. de Oliveira ${}^{\mathbf{17}}$, D. de Oliveira Franco ${}^{\mathbf{21}}$, F. de Palma ${}^{\mathbf{55},\mathbf{47}}$, V. de Souza ${}^{\mathbf{19}}$, E. De Vito ${}^{\mathbf{55},\mathbf{47}}$, A. Del Popolo ${}^{\mathbf{57},\mathbf{46}}$, O. Deligny ${}^{\mathbf{33}}$, L. Deval ${}^{\mathbf{40},\mathbf{8}}$, A. di Matteo ${}^{\mathbf{51}}$, M. Dobre ${}^{\mathbf{72}}$, C. Dobrigkeit ${}^{\mathbf{21}}$, J.C. D’Olivo ${}^{\mathbf{67}}$, L.M. Domingues Mendes ${}^{\mathbf{71}}$, R.C. dos Anjos ${}^{\mathbf{24}}$, J. Ebr ${}^{\mathbf{31}}$, M. Eman ${}^{\mathbf{79},\mathbf{80}}$, R. Engel ${}^{\mathbf{38},\mathbf{40}}$, I. Epicoco ${}^{\mathbf{55},\mathbf{47}}$, M. Erdmann ${}^{\mathbf{41}}$, A. Etchegoyen ${}^{\mathbf{8},\mathbf{12}}$, H. Falcke ${}^{\mathbf{79},\mathbf{81},\mathbf{80}}$, J. Farmer ${}^{\mathbf{88}}$, G. Farrar ${}^{\mathbf{87}}$, A.C. Fauth ${}^{\mathbf{21}}$, N. Fazzini ${}^{\mathbf{d}}$, F. Feldbusch ${}^{\mathbf{39}}$, F. Fenu ${}^{\mathbf{62},\mathbf{51}}$, B. Fick ${}^{\mathbf{86}}$, J.M. Figueira ${}^{\mathbf{8}}$, A. Filipčič ${}^{\mathbf{76},\mathbf{75}}$, T. Fitoussi ${}^{\mathbf{40}}$, T. Fodran ${}^{\mathbf{79}}$, T. Fujii ${}^{\mathbf{88},\mathbf{e}}$, A. Fuster ${}^{\mathbf{8},\mathbf{12}}$, C. Galea ${}^{\mathbf{79}}$, C. Galelli ${}^{\mathbf{58},\mathbf{48}}$, B. García ${}^{\mathbf{7}}$, H. Gemmeke ${}^{\mathbf{39}}$, F. Gesualdi ${}^{\mathbf{8},\mathbf{40}}$, A. Gherghel-Lascu ${}^{\mathbf{72}}$, P.L. Ghia ${}^{\mathbf{33}}$, U. Giaccari ${}^{\mathbf{79}}$, M. Giammarchi ${}^{\mathbf{48}}$, J. Glombitza ${}^{\mathbf{41}}$, F. Gobbi ${}^{\mathbf{10}}$, F. Gollan ${}^{\mathbf{8}}$, G. Golup ${}^{\mathbf{1}}$, M. Gómez Berisso ${}^{\mathbf{1}}$, P.F. Gómez Vitale ${}^{\mathbf{11}}$, J.P. Gongora ${}^{\mathbf{11}}$, J.M. González ${}^{\mathbf{1}}$, N. González ${}^{\mathbf{14}}$, I. Goos ${}^{\mathbf{1}}$, D. Góra ${}^{\mathbf{69}}$, A. Gorgi ${}^{\mathbf{53},\mathbf{51}}$, M. Gottowik ${}^{\mathbf{78}}$, T.D. Grubb ${}^{\mathbf{13}}$, F. Guarino ${}^{\mathbf{59},\mathbf{49}}$, G.P. Guedes ${}^{\mathbf{22}}$, E. Guido ${}^{\mathbf{43}}$, S. Hahn ${}^{\mathbf{40},\mathbf{8}}$, P. Hamal ${}^{\mathbf{31}}$, M.R. Hampel ${}^{\mathbf{8}}$, P. Hansen ${}^{\mathbf{4}}$, D. Harari ${}^{\mathbf{1}}$, V.M. Harvey ${}^{\mathbf{13}}$, A. Haungs ${}^{\mathbf{40}}$, T. Hebbeker ${}^{\mathbf{41}}$, D. Heck ${}^{\mathbf{40}}$, C. Hojvat ${}^{\mathbf{d}}$, J.R. Hörandel ${}^{\mathbf{79},\mathbf{80}}$, P. Horvath ${}^{\mathbf{32}}$, M. Hrabovský ${}^{\mathbf{32}}$, T. Huege ${}^{\mathbf{40},\mathbf{15}}$, A. Insolia ${}^{\mathbf{57},\mathbf{46}}$, P.G. Isar ${}^{\mathbf{73}}$, P. Janecek ${}^{\mathbf{31}}$, J.A. Johnsen ${}^{\mathbf{84}}$, J. Jurysek ${}^{\mathbf{31}}$, A. Kääpä ${}^{\mathbf{37}}$, K.H. Kampert ${}^{\mathbf{37}}$, B. Keilhauer ${}^{\mathbf{40}}$, A. Khakurdikar ${}^{\mathbf{79}}$, V.V. Kizakke Covilakam ${}^{\mathbf{8},\mathbf{40}}$, H.O. Klages ${}^{\mathbf{40}}$, M. Kleifges ${}^{\mathbf{39}}$, J. Kleinfeller ${}^{\mathbf{10}}$, F. Knapp ${}^{\mathbf{38}}$, N. Kunka ${}^{\mathbf{39}}$, B.L. Lago ${}^{\mathbf{16}}$, N. Langner ${}^{\mathbf{41}}$, M.A. Leigui de Oliveira ${}^{\mathbf{23}}$, V. Lenok ${}^{\mathbf{38}}$, A. Letessier-Selvon ${}^{\mathbf{34}}$, I. Lhenry-Yvon ${}^{\mathbf{33}}$, D. Lo Presti ${}^{\mathbf{57},\mathbf{46}}$, L. Lopes ${}^{\mathbf{71}}$, R. López ${}^{\mathbf{63}}$, L. Lu ${}^{\mathbf{90}}$, Q. Luce ${}^{\mathbf{38}}$, J.P. Lundquist ${}^{\mathbf{75}}$, A. Machado Payeras ${}^{\mathbf{21}}$, G. Mancarella ${}^{\mathbf{55},\mathbf{47}}$, D. Mandat ${}^{\mathbf{31}}$, B.C. Manning ${}^{\mathbf{13}}$, J. Manshanden ${}^{\mathbf{42}}$, P. Mantsch ${}^{\mathbf{d}}$, S. Marafico ${}^{\mathbf{33}}$, F.M. Mariani ${}^{\mathbf{58},\mathbf{48}}$, A.G. Mariazzi ${}^{\mathbf{4}}$, I.C. Mariş ${}^{\mathbf{14}}$, G. Marsella ${}^{\mathbf{60},\mathbf{46}}$, D. Martello ${}^{\mathbf{55},\mathbf{47}}$, S. Martinelli ${}^{\mathbf{40},\mathbf{8}}$, O. Martínez Bravo ${}^{\mathbf{63}}$, M.A. Martins ${}^{\mathbf{78}}$, M. Mastrodicasa ${}^{\mathbf{56},\mathbf{45}}$, H.J. Mathes ${}^{\mathbf{40}}$, J. Matthews ${}^{\mathbf{a}}$, G. Matthiae ${}^{\mathbf{61},\mathbf{50}}$, E. Mayotte ${}^{\mathbf{84},\mathbf{37}}$, S. Mayotte ${}^{\mathbf{84}}$, P.O. Mazur ${}^{\mathbf{d}}$, G. Medina-Tanco ${}^{\mathbf{67}}$, D. Melo ${}^{\mathbf{8}}$, A. Menshikov ${}^{\mathbf{39}}$, S. Michal ${}^{\mathbf{32}}$, M.I. Micheletti ${}^{\mathbf{6}}$, L. Miramonti ${}^{\mathbf{58},\mathbf{48}}$, S. Mollerach ${}^{\mathbf{1}}$, F. Montanet ${}^{\mathbf{35}}$, L. Morejon ${}^{\mathbf{37}}$, C. Morello ${}^{\mathbf{53},\mathbf{51}}$, A.L. Müller ${}^{\mathbf{31}}$, K. Mulrey ${}^{\mathbf{79},\mathbf{80}}$, R. Mussa ${}^{\mathbf{51}}$, M. Muzio ${}^{\mathbf{87}}$, W.M. Namasaka ${}^{\mathbf{37}}$, A. Nasr-Esfahani ${}^{\mathbf{37}}$, L. Nellen ${}^{\mathbf{67}}$, G. Nicora ${}^{\mathbf{2}}$, M. Niculescu-Oglinzanu ${}^{\mathbf{72}}$, M. Niechciol ${}^{\mathbf{43}}$, D. Nitz ${}^{\mathbf{86}}$, I. Norwood ${}^{\mathbf{86}}$, D. Nosek ${}^{\mathbf{30}}$, V. Novotny ${}^{\mathbf{30}}$, L. Nožka ${}^{\mathbf{32}}$, A Nucita ${}^{\mathbf{55},\mathbf{47}}$, L.A. Núñez ${}^{\mathbf{29}}$, C. Oliveira ${}^{\mathbf{19}}$, M. Palatka ${}^{\mathbf{31}}$, J. Pallotta ${}^{\mathbf{2}}$, G. Parente ${}^{\mathbf{78}}$, A. Parra ${}^{\mathbf{63}}$, J. Pawlowsky ${}^{\mathbf{37}}$, M. Pech ${}^{\mathbf{31}}$, J. Pȩkala ${}^{\mathbf{69}}$, R. Pelayo ${}^{\mathbf{64}}$, E.E. Pereira Martins ${}^{\mathbf{38},\mathbf{8}}$, J. Perez Armand ${}^{\mathbf{20}}$, C. Pérez Bertolli ${}^{\mathbf{8},\mathbf{40}}$, L. Perrone ${}^{\mathbf{55},\mathbf{47}}$, S. Petrera ${}^{\mathbf{44},\mathbf{45}}$, C. Petrucci ${}^{\mathbf{56},\mathbf{45}}$, T. Pierog ${}^{\mathbf{40}}$, M. Pimenta ${}^{\mathbf{71}}$, M. Platino ${}^{\mathbf{8}}$, B. Pont ${}^{\mathbf{79}}$, M. Pothast ${}^{\mathbf{80},\mathbf{79}}$, M. Pourmohammad Shavar ${}^{\mathbf{60},\mathbf{46}}$, P. Privitera ${}^{\mathbf{88}}$, M. Prouza ${}^{\mathbf{31}}$, A. Puyleart ${}^{\mathbf{86}}$, S. Querchfeld ${}^{\mathbf{37}}$, J. Rautenberg ${}^{\mathbf{37}}$, D. Ravignani ${}^{\mathbf{8}}$, M. Reininghaus ${}^{\mathbf{38}}$, J. Ridky ${}^{\mathbf{31}}$, F. Riehn ${}^{\mathbf{71}}$, M. Risse ${}^{\mathbf{43}}$, V. Rizi ${}^{\mathbf{56},\mathbf{45}}$, W. Rodrigues de Carvalho ${}^{\mathbf{79}}$, J. Rodriguez Rojo ${}^{\mathbf{11}}$, M.J. Roncoroni ${}^{\mathbf{8}}$, S. Rossoni ${}^{\mathbf{42}}$, M. Roth ${}^{\mathbf{40}}$, E. Roulet ${}^{\mathbf{1}}$, A.C. Rovero ${}^{\mathbf{5}}$, P. Ruehl ${}^{\mathbf{43}}$, A. Saftoiu ${}^{\mathbf{72}}$, M. Saharan ${}^{\mathbf{79}}$, F. Salamida ${}^{\mathbf{56},\mathbf{45}}$, H. Salazar ${}^{\mathbf{63}}$, G. Salina ${}^{\mathbf{50}}$, J.D. Sanabria Gomez ${}^{\mathbf{29}}$, F. Sánchez ${}^{\mathbf{8}}$, E.M. Santos ${}^{\mathbf{20}}$, E. Santos ${}^{\mathbf{31}}$, F. Sarazin ${}^{\mathbf{84}}$, R. Sarmento ${}^{\mathbf{71}}$, R. Sato ${}^{\mathbf{11}}$, P. Savina ${}^{\mathbf{90}}$, C.M. Schäfer ${}^{\mathbf{40}}$, V. Scherini ${}^{\mathbf{55},\mathbf{47}}$, H. Schieler ${}^{\mathbf{40}}$, M. Schimassek ${}^{\mathbf{40}}$, M. Schimp ${}^{\mathbf{37}}$, F. Schlüter ${}^{\mathbf{40},\mathbf{8}}$, D. Schmidt ${}^{\mathbf{38}}$, O. Scholten ${}^{\mathbf{15}}$, H. Schoorlemmer ${}^{\mathbf{79},\mathbf{80}}$, P. Schovánek ${}^{\mathbf{31}}$, F.G. Schröder ${}^{\mathbf{89},\mathbf{40}}$, J. Schulte ${}^{\mathbf{41}}$, T. Schulz ${}^{\mathbf{40}}$, S.J. Sciutto ${}^{\mathbf{4}}$, M. Scornavacche ${}^{\mathbf{8},\mathbf{40}}$, A. Segreto ${}^{\mathbf{52},\mathbf{46}}$, S. Sehgal ${}^{\mathbf{37}}$, S.U. Shivashankara ${}^{\mathbf{75}}$, G. Sigl ${}^{\mathbf{42}}$, G. Silli ${}^{\mathbf{8}}$, O. Sima ${}^{\mathbf{72},\mathbf{b}}$, R. Smau ${}^{\mathbf{72}}$, R. Šmída ${}^{\mathbf{88}}$, P. Sommers ${}^{\mathbf{h}}$, J.F. Soriano ${}^{\mathbf{85}}$, R. Squartini ${}^{\mathbf{10}}$, M. Stadelmaier ${}^{\mathbf{31}}$, D. Stanca ${}^{\mathbf{72}}$, S. Stanič ${}^{\mathbf{75}}$, J. Stasielak ${}^{\mathbf{69}}$, P. Stassi ${}^{\mathbf{35}}$, M. Straub ${}^{\mathbf{41}}$, A. Streich ${}^{\mathbf{38},\mathbf{8}}$, M. Suárez-Durán ${}^{\mathbf{14}}$, T. Sudholz ${}^{\mathbf{13}}$, T. Suomijärvi ${}^{\mathbf{36}}$, A.D. Supanitsky ${}^{\mathbf{8}}$, Z. Szadkowski ${}^{\mathbf{70}}$, A. Tapia ${}^{\mathbf{28}}$, C. Taricco ${}^{\mathbf{62},\mathbf{51}}$, C. Timmermans ${}^{\mathbf{80},\mathbf{79}}$, O. Tkachenko ${}^{\mathbf{40}}$, P. Tobiska ${}^{\mathbf{31}}$, C.J. Todero Peixoto ${}^{\mathbf{18}}$, B. Tomé ${}^{\mathbf{71}}$, Z. Torrès ${}^{\mathbf{35}}$, A. Travaini ${}^{\mathbf{10}}$, P. Travnicek ${}^{\mathbf{31}}$, C. Trimarelli ${}^{\mathbf{56},\mathbf{45}}$, M. Tueros ${}^{\mathbf{4}}$, R. Ulrich ${}^{\mathbf{40}}$, M. Unger ${}^{\mathbf{40}}$, L. Vaclavek ${}^{\mathbf{32}}$, M. Vacula ${}^{\mathbf{32}}$, J.F. Valdés Galicia ${}^{\mathbf{67}}$, L. Valore ${}^{\mathbf{59},\mathbf{49}}$, E. Varela ${}^{\mathbf{63}}$, A. Vásquez-Ramírez ${}^{\mathbf{29}}$, D. Veberič ${}^{\mathbf{40}}$, C. Ventura ${}^{\mathbf{26}}$, I.D. Vergara Quispe ${}^{\mathbf{4}}$, V. Verzi ${}^{\mathbf{50}}$, J. Vicha ${}^{\mathbf{31}}$, J. Vink ${}^{\mathbf{82}}$, S. Vorobiov ${}^{\mathbf{75}}$, C. Watanabe ${}^{\mathbf{25}}$, A.A. Watson ${}^{\mathbf{c}}$, A. Weindl ${}^{\mathbf{40}}$, L. Wiencke ${}^{\mathbf{84}}$, H. Wilczyński ${}^{\mathbf{69}}$, D. Wittkowski ${}^{\mathbf{37}}$, B. Wundheiler ${}^{\mathbf{8}}$, A. Yushkov ${}^{\mathbf{31}}$, O. Zapparrata ${}^{\mathbf{14}}$, E. Zas ${}^{\mathbf{78}}$, D. Zavrtanik ${}^{\mathbf{75},\mathbf{76}}$, M. Zavrtanik ${}^{\mathbf{76},\mathbf{75}}$, L. Zehrer ${}^{\mathbf{75}}$**

- Centro Atómico Bariloche and Instituto Balseiro (CNEA-UNCuyo-CONICET), San Carlos de Bariloche, Argentina
- Centro de Investigaciones en Láseres y Aplicaciones, CITEDEF and CONICET, Villa Martelli, Argentina
- Departamento de Física and Departamento de Ciencias de la Atmósfera y los Océanos, FCEyN, Universidad de Buenos Aires and CONICET, Buenos Aires, Argentina
- IFLP, Universidad Nacional de La Plata and CONICET, La Plata, Argentina
- Instituto de Astronomía y Física del Espacio (IAFE, CONICET-UBA), Buenos Aires, Argentina
- Instituto de Física de Rosario (IFIR)—CONICET/U.N.R. and Facultad de Ciencias Bioquímicas y Farmacéuticas U.N.R., Rosario, Argentina
- Instituto de Tecnologías en Detección y Astropartículas (CNEA, CONICET, UNSAM), and Universidad Tecnológica Nacional—Facultad Regional Mendoza (CONICET/CNEA), Mendoza, Argentina
- Instituto de Tecnologías en Detección y Astropartículas (CNEA, CONICET, UNSAM), Buenos Aires, Argentina
- International Center of Advanced Studies and Instituto de Ciencias Físicas, ECyT-UNSAM and CONICET, Campus Miguelete—San Martín, Buenos Aires, Argentina
- Observatorio Pierre Auger, Malargüe, Argentina
- Observatorio Pierre Auger and Comisión Nacional de Energía Atómica, Malargüe, Argentina
- Facultad Regional Buenos Aires, Universidad Tecnológica Nacional, Buenos Aires, Argentina
- University of Adelaide, Adelaide, S.A., Australia
- Université Libre de Bruxelles (ULB), Brussels, Belgium
- Vrije Universiteit Brussels, Brussels, Belgium
- Centro Federal de Educação Tecnológica Celso Suckow da Fonseca, Nova Friburgo, Brazil
- Instituto Federal de Educação, Ciência e Tecnologia do Rio de Janeiro (IFRJ), Brazil
- Escola de Engenharia de Lorena, Universidade de São Paulo, Lorena, SP, Brazil
- Instituto de Física de São Carlos, Universidade de São Paulo, São Carlos, SP, Brazil
- Instituto de Física, Universidade de São Paulo, São Paulo, SP, Brazil
- Universidade Estadual de Campinas, IFGW, Campinas, SP, Brazil
- Universidade Estadual de Feira de Santana, Feira de Santana, Brazil
- Universidade Federal do ABC, Santo André, SP, Brazil
- Universidade Federal do Paraná, Setor Palotina, Palotina, Brazil
- Instituto de Física, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ, Brazil
- Universidade Federal do Rio de Janeiro (UFRJ), Observatório do Valongo, Rio de Janeiro, RJ, Brazil
- Universidade Federal Fluminense, EEIMVR, Volta Redonda, RJ, Brazil
- Universidad de Medellín, Medellín, Colombia
- Universidad Industrial de Santander, Bucaramanga, Colombia
- Faculty of Mathematics and Physics, Institute of Particle and Nuclear Physics, Charles University, Prague, Czech Republic
- Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic
- Palacky University, RCPTM, Olomouc, Czech Republic
- CNRS/IN2P3, IJCLab, Université Paris-Saclay, Orsay, France
- Laboratoire de Physique Nucléaire et de Hautes Energies (LPNHE), Sorbonne Université, Université de Paris, CNRS-IN2P3, Paris, France
- Université Grenoble Alpes, CNRS, Grenoble Institute of Engineering Université Grenoble Alpes, LPSC-IN2P3, 38000 Grenoble, France
- Université Paris-Saclay, CNRS/IN2P3, IJCLab, Orsay, France
- Department of Physics, Bergische Universität Wuppertal, Wuppertal, Germany
- Karlsruhe Institute of Technology (KIT), Institute for Experimental Particle Physics, Karlsruhe, Germany
- Karlsruhe Institute of Technology (KIT), Institut für Prozessdatenverarbeitung und Elektronik, Karlsruhe, Germany
- Karlsruhe Institute of Technology (KIT), Institute for Astroparticle Physics, Karlsruhe, Germany
- RWTH Aachen University, III. Physikalisches Institut A, Aachen, Germany
- II. Institut für Theoretische Physik, Universität Hamburg, Hamburg, Germany
- Department Physik—Experimentelle Teilchenphysik, Universität Siegen, Siegen, Germany
- Gran Sasso Science Institute, L’Aquila, Italy
- INFN Laboratori Nazionali del Gran Sasso, Assergi (L’Aquila), Italy
- INFN, Sezione di Catania, Catania, Italy
- INFN, Sezione di Lecce, Lecce, Italy
- INFN, Sezione di Milano, Milano, Italy
- INFN, Sezione di Napoli, Napoli, Italy
- INFN, Sezione di Roma “Tor Vergata”, Roma, Italy
- INFN, Sezione di Torino, Torino, Italy
- Istituto di Astrofisica Spaziale e Fisica Cosmica di Palermo (INAF), Palermo, Italy
- Osservatorio Astrofisico di Torino (INAF), Torino, Italy
- Politecnico di Milano, Dipartimento di Scienze e Tecnologie Aerospaziali, Milano, Italy
- Dipartimento di Matematica e Fisica “E. De Giorgi”, Università del Salento, Lecce, Italy
- Dipartimento di Scienze Fisiche e Chimiche, Università dell’Aquila, L’Aquila, Italy
- Dipartimento di Fisica e Astronomia “Ettore Majorana”, Università di Catania, Catania, Italy
- Dipartimento di Fisica, Università di Milano, Milano, Italy
- Dipartimento di Fisica “Ettore Pancini”, Università di Napoli “Federico II”, Napoli, Italy
- Dipartimento di Fisica e Chimica ”E. Segrè”, Università di Palermo, Palermo, Italy
- Dipartimento di Fisica, Università di Roma “Tor Vergata”, Roma, Italy
- Dipartimento di Fisica, Università Torino, Torino, Italy
- Benemérita Universidad Autónoma de Puebla, Puebla, México
- Unidad Profesional Interdisciplinaria en Ingeniería y Tecnologías Avanzadas del Instituto Politécnico Nacional (UPIITA-IPN), México, D.F., México
- Universidad Autónoma de Chiapas, Tuxtla Gutiérrez, Chiapas, México
- Universidad Michoacana de San Nicolás de Hidalgo, Morelia, Michoacán, México
- Universidad Nacional Autónoma de México, México, D.F., México
- Facultad de Ciencias Naturales y Formales, Universidad Nacional de San Agustin de Arequipa, Arequipa, Peru
- Institute of Nuclear Physics PAN, Krakow, Poland
- Faculty of High-Energy Astrophysics, University of Łódź, Łódź, Poland
- Laboratório de Instrumentação e Física Experimental de Partículas—LIP and Instituto Superior Técnico—IST, Universidade de Lisboa—UL, Lisboa, Portugal
- “Horia Hulubei” National Institute for Physics and Nuclear Engineering, Bucharest-Magurele, Romania
- Institute of Space Science, Bucharest-Magurele, Romania
- University Politehnica of Bucharest, Bucharest, Romania
- Center for Astrophysics and Cosmology (CAC), University of Nova Gorica, Nova Gorica, Slovenia
- Experimental Particle Physics Department, J. Stefan Institute, Ljubljana, Slovenia
- Universidad de Granada and C.A.F.P.E., Granada, Spain
- Instituto Galego de Física de Altas Enerxías (IGFAE), Universidade de Santiago de Compostela, Santiago de Compostela, Spain
- IMAPP, Radboud University Nijmegen, Nijmegen, The Netherlands
- Nationaal Instituut voor Kernfysica en Hoge Energie Fysica (NIKHEF), Science Park, Amsterdam, The Netherlands
- Stichting Astronomisch Onderzoek in Nederland (ASTRON), Dwingeloo, The Netherlands
- Faculty of Science, Universiteit van Amsterdam, Amsterdam, The Netherlands
- Case Western Reserve University, Cleveland, OH, USA
- Colorado School of Mines, Golden, CO, USA
- Department of Physics and Astronomy, Lehman College, City University of New York, Bronx, NY, USA
- Michigan Technological University, Houghton, MI, USA
- New York University, New York, NY, USA
- Enrico Fermi Institute, University of Chicago, Chicago, IL, USA
- Department of Physics and Astronomy, Bartol Research Institute, University of Delaware, Newark, DE, USA
- Department of Physics and WIPAC, University of Wisconsin-Madison, Madison, WI, USA

- a
- Louisiana State University, Baton Rouge, LA, USA
- b
- also at University of Bucharest, Physics Department, Bucharest, Romania
- c
- School of Physics and Astronomy, University of Leeds, Leeds, United Kingdom
- d
- Fermi National Accelerator Laboratory, Fermilab, Batavia, IL, USA
- e
- now at Graduate School of Science, Osaka Metropolitan University, Osaka, Japan
- f
- Max-Planck-Institut für Radioastronomie, Bonn, Germany
- g
- Colorado State University, Fort Collins, CO, USA
- h
- Pennsylvania State University, University Park, PA, USA

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**Figure 1.**Schematic depiction of the main differences between photon-induced air showers and those initiated by primary nuclei (protons or heavier nuclei).

**Figure 2.**

**Left**: map of the Pierre Auger Observatory [17]; each dot represents one SD station; the four FD sites at the border of the SD array are also shown.

**Top right**: the fluorescence telescopes at the FD site Los Leones; even though the picture was taken during the daytime, the shutters were had been opened for maintenance.

**Bottom right**: a single SD station in the Pampa Amarilla.

**Figure 3.**Normalized distributions of the three discriminating observables ${X}_{\mathrm{max}}$, ${S}_{b}$ and ${N}_{\mathrm{stations}}$ used in the photon search based on HeCo data. The (simulated) photon sample is shown in blue, the (simulated) proton sample in red, and the data sample in black. In addition, normalized distributions of the final discriminator $\beta $—which is based on a MVA combining ${X}_{\mathrm{max}}$, ${S}_{b}$ and ${N}_{\mathrm{stations}}$ as well as the photon energy and the zenith angle—are displayed. The dashed line denotes the median of the photon test sample, which is used as the photon candidate cut. In all plots, only events with ${E}_{\gamma}\phantom{\rule{0.166667em}{0ex}}>\phantom{\rule{0.166667em}{0ex}}2\phantom{\rule{0.166667em}{0ex}}\times \phantom{\rule{0.166667em}{0ex}}{10}^{17}\phantom{\rule{0.166667em}{0ex}}\mathrm{eV}$ are shown. For more details, see [23].

**Figure 4.**(

**Top left**) scatter plot of ${X}_{\mathrm{max}}$ and ${F}_{\mu}$, i.e., the observables used in the hybrid search for photons using air-shower universality, for simulated primary photons (blue) and protons (red); The contour lines enclose 90%, 50% and 10%, respectively, of the events. (

**Top right**) distributions of the Fisher discriminant f for simulated primary photons (signal, blue) and protons (background, red), and for the burnt sample (black); the dashed red line marks the tail of the proton distribution; the dashed blue line indicates the median of the photon distribution. (

**Bottom**) the tail of the distribution of f for the hybrid data sample (black dots); the dashed line represents the photon-candidate cut; the shaded blue regions show the $1\sigma $, $2\sigma $ and $3\sigma $ uncertainty bands for background expectation. For more details, see [24].

**Figure 5.**(

**Top**) distributions of $\Delta $ (

**left**) and ${L}_{\mathrm{LDF}}$ (

**right**), i.e., the observables used in the search for photons based on SD-only data, as a function of the photon energy ${E}_{\gamma}$ for simulated primary photons (in blue) and a fraction of the data sample (in red) that is used as a burnt sample; the bands represent one standard deviation of the photon distributions. (

**Bottom**) distributions of the Fisher discriminant for the burnt sample (grey), the search sample (red) and simulated primary photons (non-preshowering in blue and preshowering in light blue), weighted with an ${E}^{-2}$ spectrum; the search sample and the photon distributions are scaled to have the same integral as the burn sample one; the vertical line indicates the value of the photon-candidate cut; the dashed line shows the result of the fit of an exponential to the 5% of events in the burnt sample with the largest values of the Fisher discriminant. For more details, see [25].

**Figure 6.**Current upper limits on the integral photon flux determined from data collected by the Pierre Auger Observatory (red, blue and gray circles). We also show the upper limits published by other experiments: KASCADE-Grande (orange crosses) [34], EAS-MSU (magenta triangles) [35]) and Telescope Array (green squares from [36] and turquoise squares from [37]). The ranges of expected GZK photon fluxes under the assumption of two different pure-proton scenarios are shown as the red and gray bands (following [2,5], respectively). The green band shows the expected GZK photon flux, assuming a mixed composition that would fit the Auger data [3], while the blue band denotes the range of photon fluxes that would be expected from cosmic-ray interactions with matter in the Milky Way [4]. In addition, the expected photon fluxes from the decay of super-heavy dark matter particles are included (decay into hadrons, $X\to q\overline{q}$, based on [38]: dashed violet line for a mass of the SHDM particles ${M}_{X}={10}^{10}\phantom{\rule{0.166667em}{0ex}}\mathrm{GeV}$ and a lifetime ${\tau}_{X}=3\phantom{\rule{0.166667em}{0ex}}\times \phantom{\rule{0.166667em}{0ex}}{10}^{21}\phantom{\rule{0.166667em}{0ex}}\mathrm{yr}$ [SHDM Ia]; brown dot-dashed line for ${M}_{X}={10}^{12}\phantom{\rule{0.166667em}{0ex}}\mathrm{GeV}$ and ${\tau}_{X}={10}^{23}\phantom{\rule{0.166667em}{0ex}}\mathrm{yr}$ [SHDM Ib]; decay into leptons, $X\to \nu \overline{\nu}$, based on [39]: dashed gray line for ${M}_{X}={10}^{10}\phantom{\rule{0.166667em}{0ex}}\mathrm{GeV}$ and ${\tau}_{X}=3\phantom{\rule{0.166667em}{0ex}}\times \phantom{\rule{0.166667em}{0ex}}{10}^{21}\phantom{\rule{0.166667em}{0ex}}\mathrm{yr}$ [SHDM II]; the exact lines have been obtained through personal communication with one of the authors).

**Figure 7.**Celestial map, in Galactic coordinates, of upper limits on the incoming photon flux [42]. The white regions indicate regions of the sky that are either not in the field of view of the Pierre Auger Observatory (northern hemisphere) or omitted in this analysis (southern celestial pole). For more details, see [42].

**Figure 8.**(

**Left**) The three classes of selected gravitational wave sources in the follow-up search for photons in association with gravitational wave events, as defined by their 50% localization region (${\Omega}_{50\%}$) and luminosity distance (${D}_{L}$); the circled markers in the acceptance region mark the events which had at least some overlap with the field of view of the SD at any time. (

**Right**) Preliminary upper limits on the spectral fluence of UHE photons at Earth for each of the selected gravitational wave sources; the uncertainty bars include both the directional uncertainty of the gravitational wave event (blue) and the uncertainty due to the choice of the spectral index used to calculate the spectral fluence (red); for the second event, the uncertainty bars extend beyond the plotted range, since this source is located right at the edge of the field of view of the Pierre Auger Observatory. For more details, see [44].

**Table 1.**Compilation of the upper limits on the integral photon flux determined through the three analyses discussed in the previous sections.

Detector | ${\mathit{E}}_{0}\phantom{\rule{0.166667em}{0ex}}\left[\mathbf{eV}\right]$ | ${\mathbf{\Phi}}_{\mathit{\gamma},\phantom{\rule{0.166667em}{0ex}}\mathbf{U}.\mathbf{L}.}^{95\%}({\mathit{E}}_{\mathit{\gamma}}>{\mathit{E}}_{0})\phantom{\rule{0.166667em}{0ex}}\left[{\mathbf{km}}^{-2}\phantom{\rule{0.166667em}{0ex}}{\mathbf{yr}}^{-1}\phantom{\rule{0.166667em}{0ex}}{\mathbf{sr}}^{-1}\right]$ | Reference |
---|---|---|---|

HeCo + SD $750\phantom{\rule{0.166667em}{0ex}}\mathrm{m}$ | $2\phantom{\rule{0.166667em}{0ex}}\times \phantom{\rule{0.166667em}{0ex}}{10}^{17}$ | $2.72$ | [23] |

$3\phantom{\rule{0.166667em}{0ex}}\times \phantom{\rule{0.166667em}{0ex}}{10}^{17}$ | $2.50$ | ||

$5\phantom{\rule{0.166667em}{0ex}}\times \phantom{\rule{0.166667em}{0ex}}{10}^{17}$ | $2.74$ | ||

${10}^{18}$ | $3.55$ | ||

FD + SD $1500\phantom{\rule{0.166667em}{0ex}}\mathrm{m}$ | ${10}^{18}$ | $4\phantom{\rule{0.166667em}{0ex}}\times \phantom{\rule{0.166667em}{0ex}}{10}^{-2}$ | [24] |

$2\phantom{\rule{0.166667em}{0ex}}\times \phantom{\rule{0.166667em}{0ex}}{10}^{18}$ | $1.1\phantom{\rule{0.166667em}{0ex}}\times \phantom{\rule{0.166667em}{0ex}}{10}^{-2}$ | ||

$3\phantom{\rule{0.166667em}{0ex}}\times \phantom{\rule{0.166667em}{0ex}}{10}^{18}$ | $0.35\phantom{\rule{0.166667em}{0ex}}\times \phantom{\rule{0.166667em}{0ex}}{10}^{-2}$ | ||

$5\phantom{\rule{0.166667em}{0ex}}\times \phantom{\rule{0.166667em}{0ex}}{10}^{18}$ | $0.23\phantom{\rule{0.166667em}{0ex}}\times \phantom{\rule{0.166667em}{0ex}}{10}^{-2}$ | ||

${10}^{19}$ | $0.21\phantom{\rule{0.166667em}{0ex}}\times \phantom{\rule{0.166667em}{0ex}}{10}^{-2}$ | ||

SD$1500\phantom{\rule{0.166667em}{0ex}}\mathrm{m}$ | ${10}^{19}$ | $2.11\phantom{\rule{0.166667em}{0ex}}\times \phantom{\rule{0.166667em}{0ex}}{10}^{-3}$ | [25] |

$2\phantom{\rule{0.166667em}{0ex}}\times \phantom{\rule{0.166667em}{0ex}}{10}^{19}$ | $0.312\phantom{\rule{0.166667em}{0ex}}\times \phantom{\rule{0.166667em}{0ex}}{10}^{-3}$ | ||

$4\phantom{\rule{0.166667em}{0ex}}\times \phantom{\rule{0.166667em}{0ex}}{10}^{19}$ | $0.172\phantom{\rule{0.166667em}{0ex}}\times \phantom{\rule{0.166667em}{0ex}}{10}^{-3}$ |

**Table 2.**Combined unweighted probabilities $\mathcal{P}$ and weighted probabilities ${\mathcal{P}}_{w}$ for the 12 target sets analyzed in [43]. In addition, selected information on the most significant target from each target set is given: the unpenalized (p) and penalized (${p}^{*}$) p-values and the derived upper limit on the photon flux at 95% C.L.. More details on the most significant targets, e.g., the galactic coordinates and upper limits on the energy flux, can be found in [43].

Class | N | $\mathcal{P}$ | ${\mathcal{P}}_{\mathit{w}}$ | p | ${\mathit{p}}^{*}$ | ${\mathit{f}}_{\mathbf{UL}}^{0.95}$ [km${}^{-2}$ yr${}^{-1}$] |
---|---|---|---|---|---|---|

msec pulsars | 67 | 0.14 | 0.57 | 0.010 | 0.476 | 0.043 |

$\gamma $-ray pulsars | 75 | 0.98 | 0.97 | 0.007 | 0.431 | 0.045 |

Low-mass X-ray binaries | 87 | 0.74 | 0.13 | 0.014 | 0.718 | 0.046 |

High-mass X-ray binaries | 48 | 0.84 | 0.33 | 0.040 | 0.856 | 0.036 |

H.E.S.S. pulsar wind nebulae | 17 | 0.90 | 0.92 | 0.104 | 0.845 | 0.038 |

H.E.S.S. other | 16 | 0.52 | 0.12 | 0.042 | 0.493 | 0.040 |

H.E.S.S. unidentified | 20 | 0.45 | 0.79 | 0.014 | 0.251 | 0.045 |

Microquasars | 13 | 0.48 | 0.29 | 0.037 | 0.391 | 0.045 |

Magnetars | 16 | 0.89 | 0.30 | 0.115 | 0.858 | 0.031 |

Galactic Center | 1 | 0.59 | 0.59 | 0.471 | 0.471 | 0.024 |

Large Magellanic Cloud | 3 | 0.62 | 0.52 | 0.463 | 0.845 | 0.030 |

Centaurus A | 1 | 0.31 | 0.31 | 0.221 | 0.221 | 0.031 |

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