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

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Searches for Ultra-High-Energy Photons

at the Pierre Auger Observatory

The Pierre Auger Collaboration

P. Abreu71, M. Aglietta53,51, I. Allekotte1, K. Almeida Cheminant69, A. Almela8,12, J. Alvarez-Mu˜niz78, J. Am-

merman Yebra78, G.A. Anastasi53,51 , L. Anchordoqui85 , B. Andrada8, S. Andringa71, C. Aramo49, P.R. Ara´ujo

Ferreira41, E. Arnone62,51, J. C. Arteaga Vel´azquez66, H. Asorey8, P. Assis71, G. Avila11, E. Avocone56,45,

A.M. Badescu74, A. Bakalova31 , A. Balaceanu72, F. Barbato44,45, J.A. Bellido13,68, C. Berat35, M.E. Bertaina62,51,

G. Bhatta69, P.L. Biermannf, V. Binet6, K. Bismark38,8, T. Bister41, J. Biteau36, J. Blazek31, C. Bleve35,

J. Bl¨umer40, M. Boh´aˇcov´a31 , D. Boncioli56,45, C. Bonifazi9,25, L. Bonneau Arbeletche21, N. Borodai69, J. Brackg,

T. Bretz41, P.G. Brichetto Orchera8, F.L. Briechle41, P. Buchholz43, A. Bueno77, S. Buitink15, M. Buscemi46,60,

M. B¨usken38,8, A. Bwembya79,80, K.S. Caballero-Mora65, L. Caccianiga58,48, I. Caracas37, R. Caruso57,46,

A. Castellina53,51, F. Catalani18, G. Cataldi47, L. Cazon78, M. Cerda10, J.A. Chinellato21, J. Chudoba31,

L. Chytka32, R.W. Clay13, A.C. Cobos Cerutti7, R. Colalillo59,49, A. Coleman89, M.R. Coluccia47, R. Con-

cei¸c˜ao71, A. Condorelli44,45, G. Consolati48,54 , F. Contreras11, F. Convenga40, D. Correia dos Santos27,

C.E. Covault83, M. Cristinziani43, S. Dasso5,3, K. Daumiller40, B.R. Dawson13, R.M. de Almeida27, J. de

Jes´us8,40, S.J. de Jong79,80, J.R.T. de Mello Neto25,26, I. De Mitri44,45 , J. de Oliveira17, D. de Oliveira

Franco21, F. de Palma55,47, V. de Souza19, E. De Vito55,47, A. Del Popolo57,46, O. Deligny33, L. Deval40,8, A. di

Matteo51, M. Dobre72, C. Dobrigkeit21, J.C. D’Olivo67, L.M. Domingues Mendes71, R.C. dos Anjos24, J. Ebr31,

M. Eman79,80, R. Engel38,40, I. Epicoco55,47, M. Erdmann41 , A. Etchegoyen8,12, H. Falcke79,81,80, J. Farmer88,

G. Farrar87, A.C. Fauth21, N. Fazzinid, F. Feldbusch39, F. Fenu62,51, B. Fick86, J.M. Figueira8, A. Filipˇciˇc76,75,

T. Fitoussi40, T. Fodran79, T. Fujii88,e, A. Fuster8,12, C. Galea79, C. Galelli58,48, B. Garc´ıa7, H. Gemmeke39,

F. Gesualdi8,40, A. Gherghel-Lascu72, P.L. Ghia33 , U. Giaccari79, M. Giammarchi48, J. Glombitza41, F. Gobbi10 ,

F. Gollan8, G. Golup1, M. G´omez Berisso1, P.F. G´omez Vitale11, J.P. Gongora11, J.M. Gonz´alez1, N. Gonz´alez14,

I. Goos1, D. G´ora69 , A. Gorgi53,51, M. Gottowik78, T.D. Grubb13, F. Guarino59,49 , G.P. Guedes22, E. Guido43,

S. Hahn40,8, P. Hamal31, M.R. Hampel8, P. Hansen4, D. Harari1, V.M. Harvey13, A. Haungs40, T. Hebbeker41,

D. Heck40, C. Hojvatd, J.R. H¨orandel79,80, P. Horvath32, M. Hrabovsk´y32 , T. Huege40,15, A. Insolia57,46,

P.G. Isar73, P. Janecek31, J.A. Johnsen84, J. Jurysek31, A. K¨a¨ap¨a37 , K.H. Kampert37 , B. Keilhauer40,

A. Khakurdikar79, V.V. Kizakke Covilakam8,40, H.O. Klages40, M. Kleifges39, J. Kleinfeller10, F. Knapp38,

N. Kunka39, B.L. Lago16, N. Langner41, M.A. Leigui de Oliveira23, V. Lenok38, A. Letessier-Selvon34, I. Lhenry-

Yvon33, D. Lo Presti57,46, L. Lopes71, R. L´opez63, L. Lu90 , Q. Luce38 , J.P. Lundquist75 , A. Machado Payeras21,

G. Mancarella55,47, D. Mandat31, B.C. Manning13, J. Manshanden42, P. Mantschd, S. Maraﬁco33,

F.M. Mariani58,48, A.G. Mariazzi4, I.C. Mari¸s14, G. Marsella60,46, D. Martello55,47 , S. Martinelli40,8, O. Mart´ınez

Bravo63, M.A. Martins78, M. Mastrodicasa56,45, H.J. Mathes40, J. Matthewsa, G. Matthiae61,50 , E. Mayotte84,37,

S. Mayotte84, P.O. Mazurd, G. Medina-Tanco67 , D. Melo8, A. Menshikov39, S. Michal32, M.I. Micheletti6,

L. Miramonti58,48, S. Mollerach1, F. Montanet35, L. Morejon37, C. Morello53,51 , A.L. M¨uller31, K. Mulrey79,80 ,

R. Mussa51, M. Muzio87, W.M. Namasaka37 , A. Nasr-Esfahani37, L. Nellen67, G. Nicora2, M. Niculescu-

Oglinzanu72, M. Niechciol43, D. Nitz86, I. Norwood86 , D. Nosek30, V. Novotny30 , L. Noˇzka32 , A Nucita55,47,

L.A. N´u˜nez29, C. Oliveira19, M. Palatka31, J. Pallotta2, G. Parente78, A. Parra63, J. Pawlowsky37, M. Pech31,

J. P¸ekala69, R. Pelayo64 , E.E. Pereira Martins38,8, J. Perez Armand20, C. P´erez Bertolli8,40, L. Perrone55,47 ,

S. Petrera44,45, C. Petrucci56,45, T. Pierog40, M. Pimenta71, M. Platino8, B. Pont79, M. Pothast80,79, M. Pourmo-

hammad Shavar60,46, P. Privitera88 , M. Prouza31, A. Puyleart86, S. Querchfeld37 , J. Rautenberg37, D. Ravignani8,

M. Reininghaus38, J. Ridky31, F. Riehn71, M. Risse43, V. Rizi56,45, W. Rodrigues de Carvalho79 , J. Ro-

driguez Rojo11 , M.J. Roncoroni8, S. Rossoni42, M. Roth40, E. Roulet1, A.C. Rovero5, P. Ruehl43, A. Saftoiu72 ,

M. Saharan79, F. Salamida56,45 , H. Salazar63 , G. Salina50, J.D. Sanabria Gomez29, F. S´anchez8, E.M. Santos20,

E. Santos31, F. Sarazin84, R. Sarmento71, R. Sato11 , P. Savina90, C.M. Sch¨afer40, V. Scherini55,47, H. Schieler40,

M. Schimassek40, M. Schimp37, F. Schl¨uter40,8, D. Schmidt38, O. Scholten15, H. Schoorlemmer79,80,

P. Schov´anek31 , F.G. Schr¨oder89,40, J. Schulte41, T. Schulz40 , S.J. Sciutto4, M. Scornavacche8,40, A. Segreto52,46 ,

S. Sehgal37, S.U. Shivashankara75, G. Sigl42, G. Silli8, O. Sima72,b, R. Smau72, R. ˇ

Sm´ıda88, P. Sommersh,

J.F. Soriano85, R. Squartini10, M. Stadelmaier31, D. Stanca72, S. Staniˇc75, J. Stasielak69, P. Stassi35 , M. Straub41,

A. Streich38,8, M. Su´arez-Dur´an14, T. Sudholz13, T. Suomij¨arvi36, A.D. Supanitsky8, Z. Szadkowski70, A. Tapia28,

C. Taricco62,51, C. Timmermans80,79, O. Tkachenko40, P. Tobiska31, C.J. Todero Peixoto18, B. Tom´e71, Z. Torr`es35,

A. Travaini10, P. Travnicek31, C. Trimarelli56,45, M. Tueros4, R. Ulrich40, M. Unger40, L. Vaclavek32, M. Vacula32,

J.F. Vald´es Galicia67, L. Valore59,49, E. Varela63, A. V´asquez-Ram´ırez29, D. Veberiˇc40, C. Ventura26, I.D. Ver-

gara Quispe4, V. Verzi50, J. Vicha31, J. Vink82, S. Vorobiov75, C. Watanabe25, A.A. Watsonc, A. Weindl40,

L. Wiencke84, H. Wilczy´nski69, D. Wittkowski37, B. Wundheiler8, A. Yushkov31, O. Zapparrata14 , E. Zas78,

D. Zavrtanik75,76, M. Zavrtanik76,75, L. Zehrer75

1Centro At´omico Bariloche and Instituto Balseiro (CNEA-UNCuyo-CONICET), San Carlos de Bariloche, Argentina

2Centro de Investigaciones en L´aseres y Aplicaciones, CITEDEF and CONICET, Villa Martelli, Argentina

3Departamento de F´ısica and Departamento de Ciencias de la Atm´osfera y los Oc´eanos, FCEyN, Universidad de Buenos

Aires and CONICET, Buenos Aires, Argentina

4IFLP, Universidad Nacional de La Plata and CONICET, La Plata, Argentina

5Instituto de Astronom´ıa y F´ısica del Espacio (IAFE, CONICET-UBA), Buenos Aires, Argentina

arXiv:2210.12959v1 [astro-ph.HE] 24 Oct 2022

6Instituto de F´ısica de Rosario (IFIR) – CONICET/U.N.R. and Facultad de Ciencias Bioqu´ımicas y Farmac´euticas

U.N.R., Rosario, Argentina

7Instituto de Tecnolog´ıas en Detecci´on y Astropart´ıculas (CNEA, CONICET, UNSAM), and Universidad Tecnol´ogica

Nacional – Facultad Regional Mendoza (CONICET/CNEA), Mendoza, Argentina

8Instituto de Tecnolog´ıas en Detecci´on y Astropart´ıculas (CNEA, CONICET, UNSAM), Buenos Aires, Argentina

9International Center of Advanced Studies and Instituto de Ciencias F´ısicas, ECyT-UNSAM and CONICET, Campus

Miguelete – San Mart´ın, Buenos Aires, Argentina

10 Observatorio Pierre Auger, Malarg¨ue, Argentina

11 Observatorio Pierre Auger and Comisi´on Nacional de Energ´ıa At´omica, Malarg¨ue, Argentina

12 Universidad Tecnol´ogica Nacional – Facultad Regional Buenos Aires, Buenos Aires, Argentina

13 University of Adelaide, Adelaide, S.A., Australia

14 Universit´e Libre de Bruxelles (ULB), Brussels, Belgium

15 Vrije Universiteit Brussels, Brussels, Belgium

16 Centro Federal de Educa¸c˜ao Tecnol´ogica Celso Suckow da Fonseca, Nova Friburgo, Brazil

17 Instituto Federal de Educa¸c˜ao, Ciˆencia e Tecnologia do Rio de Janeiro (IFRJ), Brazil

18 Universidade de S˜ao Paulo, Escola de Engenharia de Lorena, Lorena, SP, Brazil

19 Universidade de S˜ao Paulo, Instituto de F´ısica de S˜ao Carlos, S˜ao Carlos, SP, Brazil

20 Universidade de S˜ao Paulo, Instituto de F´ısica, S˜ao Paulo, SP, Brazil

21 Universidade Estadual de Campinas, IFGW, Campinas, SP, Brazil

22 Universidade Estadual de Feira de Santana, Feira de Santana, Brazil

23 Universidade Federal do ABC, Santo Andr´e, SP, Brazil

24 Universidade Federal do Paran´a, Setor Palotina, Palotina, Brazil

25 Universidade Federal do Rio de Janeiro, Instituto de F´ısica, Rio de Janeiro, RJ, Brazil

26 Universidade Federal do Rio de Janeiro (UFRJ), Observat´orio do Valongo, Rio de Janeiro, RJ, Brazil

27 Universidade Federal Fluminense, EEIMVR, Volta Redonda, RJ, Brazil

28 Universidad de Medell´ın, Medell´ın, Colombia

29 Universidad Industrial de Santander, Bucaramanga, Colombia

30 Charles University, Faculty of Mathematics and Physics, Institute of Particle and Nuclear Physics, Prague, Czech

Republic

31 Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic

32 Palacky University, RCPTM, Olomouc, Czech Republic

33 CNRS/IN2P3, IJCLab, Universit´e Paris-Saclay, Orsay, France

34 Laboratoire de Physique Nucl´eaire et de Hautes Energies (LPNHE), Sorbonne Universit´e, Universit´e de Paris, CNRS-

IN2P3, Paris, France

35 Univ. Grenoble Alpes, CNRS, Grenoble Institute of Engineering Univ. Grenoble Alpes, LPSC-IN2P3, 38000 Grenoble,

France

36 Universit´e Paris-Saclay, CNRS/IN2P3, IJCLab, Orsay, France

37 Bergische Universit¨at Wuppertal, Department of Physics, Wuppertal, Germany

38 Karlsruhe Institute of Technology (KIT), Institute for Experimental Particle Physics, Karlsruhe, Germany

39 Karlsruhe Institute of Technology (KIT), Institut f¨ur Prozessdatenverarbeitung und Elektronik, Karlsruhe, Germany

40 Karlsruhe Institute of Technology (KIT), Institute for Astroparticle Physics, Karlsruhe, Germany

41 RWTH Aachen University, III. Physikalisches Institut A, Aachen, Germany

42 Universit¨at Hamburg, II. Institut f¨ur Theoretische Physik, Hamburg, Germany

43 Universit¨at Siegen, Department Physik – Experimentelle Teilchenphysik, Siegen, Germany

44 Gran Sasso Science Institute, L’Aquila, Italy

45 INFN Laboratori Nazionali del Gran Sasso, Assergi (L’Aquila), Italy

46 INFN, Sezione di Catania, Catania, Italy

47 INFN, Sezione di Lecce, Lecce, Italy

48 INFN, Sezione di Milano, Milano, Italy

49 INFN, Sezione di Napoli, Napoli, Italy

50 INFN, Sezione di Roma “Tor Vergata”, Roma, Italy

51 INFN, Sezione di Torino, Torino, Italy

52 Istituto di Astroﬁsica Spaziale e Fisica Cosmica di Palermo (INAF), Palermo, Italy

53 Osservatorio Astroﬁsico di Torino (INAF), Torino, Italy

54 Politecnico di Milano, Dipartimento di Scienze e Tecnologie Aerospaziali , Milano, Italy

55 Universit`a del Salento, Dipartimento di Matematica e Fisica “E. De Giorgi”, Lecce, Italy

56 Universit`a dell’Aquila, Dipartimento di Scienze Fisiche e Chimiche, L’Aquila, Italy

57 Universit`a di Catania, Dipartimento di Fisica e Astronomia “Ettore Majorana“, Catania, Italy

58 Universit`a di Milano, Dipartimento di Fisica, Milano, Italy

59 Universit`a di Napoli “Federico II”, Dipartimento di Fisica “Ettore Pancini”, Napoli, Italy

60 Universit`a di Palermo, Dipartimento di Fisica e Chimica ”E. Segr`e”, Palermo, Italy

61 Universit`a di Roma “Tor Vergata”, Dipartimento di Fisica, Roma, Italy

62 Universit`a Torino, Dipartimento di Fisica, Torino, Italy

63 Benem´erita Universidad Aut´onoma de Puebla, Puebla, M´exico

64 Unidad Profesional Interdisciplinaria en Ingenier´ıa y Tecnolog´ıas Avanzadas del Instituto Polit´ecnico Nacional (UPIITA-

IPN), M´exico, D.F., M´exico

65 Universidad Aut´onoma de Chiapas, Tuxtla Guti´errez, Chiapas, M´exico

66 Universidad Michoacana de San Nicol´as de Hidalgo, Morelia, Michoac´an, M´exico

67 Universidad Nacional Aut´onoma de M´exico, M´exico, D.F., M´exico

68 Universidad Nacional de San Agustin de Arequipa, Facultad de Ciencias Naturales y Formales, Arequipa, Peru

69 Institute of Nuclear Physics PAN, Krakow, Poland

70 University of L´od´z, Faculty of High-Energy Astrophysics, L´od´z, Poland

71 Laborat´orio de Instrumenta¸c˜ao e F´ısica Experimental de Part´ıculas – LIP and Instituto Superior T´ecnico – IST,

Universidade de Lisboa – UL, Lisboa, Portugal

72 “Horia Hulubei” National Institute for Physics and Nuclear Engineering, Bucharest-Magurele, Romania

73 Institute of Space Science, Bucharest-Magurele, Romania

74 University Politehnica of Bucharest, Bucharest, Romania

75 Center for Astrophysics and Cosmology (CAC), University of Nova Gorica, Nova Gorica, Slovenia

76 Experimental Particle Physics Department, J. Stefan Institute, Ljubljana, Slovenia

77 Universidad de Granada and C.A.F.P.E., Granada, Spain

78 Instituto Galego de F´ısica de Altas Enerx´ıas (IGFAE), Universidade de Santiago de Compostela, Santiago de Com-

postela, Spain

79 IMAPP, Radboud University Nijmegen, Nijmegen, The Netherlands

80 Nationaal Instituut voor Kernfysica en Hoge Energie Fysica (NIKHEF), Science Park, Amsterdam, The Netherlands

81 Stichting Astronomisch Onderzoek in Nederland (ASTRON), Dwingeloo, The Netherlands

82 Universiteit van Amsterdam, Faculty of Science, Amsterdam, The Netherlands

83 Case Western Reserve University, Cleveland, OH, USA

84 Colorado School of Mines, Golden, CO, USA

85 Department of Physics and Astronomy, Lehman College, City University of New York, Bronx, NY, USA

86 Michigan Technological University, Houghton, MI, USA

87 New York University, New York, NY, USA

88 University of Chicago, Enrico Fermi Institute, Chicago, IL, USA

89 University of Delaware, Department of Physics and Astronomy, Bartol Research Institute, Newark, DE, USA

90 University of Wisconsin-Madison, Department of Physics and WIPAC, Madison, WI, USA

—–

aLouisiana State University, Baton Rouge, LA, USA

balso at University of Bucharest, Physics Department, Bucharest, Romania

cSchool of Physics and Astronomy, University of Leeds, Leeds, United Kingdom

dFermi National Accelerator Laboratory, Fermilab, Batavia, IL, USA

enow at Graduate School of Science, Osaka Metropolitan University, Osaka, Japan

fMax-Planck-Institut f¨ur Radioastronomie, Bonn, Germany

gColorado State University, Fort Collins, CO, USA

hPennsylvania State University, University Park, PA, USA

Correspondence: spokespersons@auger.org

Abstract

The Pierre Auger Observatory, being the largest air-shower experiment in the world, oﬀers an unprece-

dented exposure to neutral particles at the highest energies. Since the start of data taking more than 18

years ago, various searches for ultra-high-energy (UHE, E&1017 eV) photons have been performed: either

for a diﬀuse ﬂux of UHE photons, for point sources of UHE photons or for UHE photons associated with

transient events like gravitational wave events. In the present paper, we summarize these searches and review

the current results obtained using the wealth of data collected by the Pierre Auger Observatory.

Keywords: photons; ultra-high energies; air showers; Pierre Auger Observatory; upper limits, transients

1 Introduction

The search for neutral particles—in particular photons and neutrinos—of cosmic origin at the highest energies

has been for many years one of the major scientiﬁc objectives of the Pierre Auger Observatory. From

the theory side, such searches are well-motivated: many models for the origin of ultra-high-energy (UHE)

cosmic rays predict at least some neutral particles as by-products, either directly at the sources or during

the propagation through the Universe (see, e.g., [1–5]). In fact, even though no UHE photons have been

unambiguously identiﬁed so far, the upper limits on their incoming ﬂux have been already used to severely

constrain so-called top-down models for the origin of UHE cosmic rays involving e.g. topological defects or

super-heavy dark matter (see, e.g., [6–9]). In addition, the recent observations of photons with energies up

to 1015 eV [10] further motivate searches for photons at even higher energies. An observation of such photons

would also be key in completing the multi-messenger approach aimed at understanding the most extreme

processes in the Universe, taking advantage of the fact that neutral particles directly point back at their

production site. However, one has to take into account that UHE photons, unlike neutrinos, interact with

the background photon ﬁelds permeating the Universe, reducing their attenuation length to about 30 kpc

around 1015 eV, which increases to the order of 10 Mpc around 1019 eV [11]. In the present paper, we review

the current state of such searches at the Pierre Auger Observatory. After a short introduction addressing the

speciﬁcities of air showers initiated by photons (Sec. 2), we brieﬂy describe the Pierre Auger Observatory

(Sec. 3). We then focus ﬁrst on the searches for a diﬀuse ﬂux of UHE photons using the diﬀerent detector

systems of the Observatory (Sec. 4), before we describe the searches for UHE photons from point sources and

transient events (Sec. 5). We close with a short outlook (Sec. 6), discussing the ongoing detector upgrade of

the Pierre Auger Observatory, dubbed AugerPrime.

2 Photon-induced air showers

When a UHE photon enters the Earth’s atmosphere, it may interact with a particle from the atmosphere,

for example a nitrogen nucleus, and induce an extensive air shower, much in the same way as a charged

cosmic ray does. Hence, a cosmic-ray observatory detecting air showers is also, by construction, a photon

observatory—and even a neutrino observatory, highlighting the importance of such observatories for multi-

messenger astrophysics. In fact, since the incoming ﬂux of UHE cosmic particles is so low (on the order

of one particle per square kilometer and year and less), measuring the extensive air showers they initiate

in the atmosphere with large detector arrays on ground is the only way to eﬃciently detect them. The

challenge lies in distinguishing air showers induced by photons from the vast background of air showers that

are initiated by charged cosmic rays, i.e., protons and heavier nuclei (for a review, see, e.g., [11]). The two

main diﬀerences between photon- and nucleus-induced air showers are shown schematically in Fig. 1. The

longitudinal development of an air shower, as a function of the slant depth X, is delayed for a primary photon

with respect to primary nuclei, due to the lower multiplicity of electromagnetic interactions (compared to

hadronic interactions) that dominate in a photon-induced air shower. The maximum of the shower devel-

opment within the atmosphere, Xmax, is reached later. For example, at a primary energy of 1019 eV, the

diﬀerence is about 200 g cm−2. Since the mean free path for photo-nuclear interactions is much larger than

the radiation length, only a small fraction of the electromagnetic component in a photon-induced shower

is transferred to the hadronic component and subsequently to the muonic component. Showers induced by

photons are thus characterized by a lower number of muons. On average, simulations show that photon-

induced showers have nearly one order of magnitude less muons than those initiated by protons or nuclei

of the same primary energy. In one way or another, all searches for UHE photons using air-shower data

exploit these two key diﬀerences: Xmax, for example, can be directly measured using the air-ﬂuorescence

technique. The number of muons cannot yet be directly measured using the current detector systems of the

Pierre Auger Observatory. However, one can measure the lateral distribution of secondary particles from

the air shower at ground level, which depends on both the number of muons and the longitudinal devel-

opment. In particular, the steepness of the lateral distribution is sensitive to the type of the primary particle.

The diﬀerence in Xmax between photon- and proton-induced air showers is ampliﬁed by the Landau-

Pomeranchuk-Migdal (LPM) eﬀect [12, 13], i.e., the suppression of the bremsstrahlung and pair production

cross sections at high energies. In addition, the preshower eﬀect [14–16] also has to be taken into account:

depending on its incident direction with respect to the local geomagnetic ﬁeld, a UHE photon can initiate an

electromagnetic cascade in the Earth’s magnetic ﬁeld even before entering the atmosphere—a preshower. The

observed air shower is then a superposition of the individual cascades of the photons and electrons/positrons

from the preshower, leading on average to a smaller measured Xmax than for non-preshowering photons of

the same primary energy.

3 The Pierre Auger Observatory

The Pierre Auger Observatory [17], located near the town of Malarg¨ue in the Argentinian Pampa Amarilla,

is the largest cosmic-ray observatory to date, oﬀering an unprecedented exposure to UHE photons. A key

feature of the Pierre Auger Observatory is the hybrid concept, combining a Surface Detector array (SD) with

a Fluorescence Detector (FD), see Fig. 2. The SD consists of 1600 water-Cherenkov detectors arranged on a

triangular grid with a spacing of 1500 m, covering a total area of about 3000 km2. The SD is overlooked by

24 ﬂuorescence telescopes, located at four sites at the border of the array. The SD samples the lateral shower

proﬁle at ground level, i.e., the distribution of particles as a function of the distance from the shower axis,

with a duty cycle of ∼100 %, while the FD records the longitudinal shower development in the atmosphere

above the SD. The FD can only be operated in clear, moonless nights, reducing the duty cycle to ∼15 %.

Through combining measurements from both detector systems in hybrid events, a superior accuracy of the

air-shower reconstruction can be achieved than with just one system [18]. In the western part of the SD

array, 50 additional SD stations have been placed between the existing SD stations, forming a sub-array

with a spacing of 750 m and covering a total area of about 27.5 km2. With this sub-array, air showers of

lower primary energy (below 1018 eV) with a smaller footprint on ground can be measured. To allow also for

hybrid measurements in this energy range, where air showers develop above the ﬁeld of view of the standard

FD telescopes, three additional High-Elevation Auger Telescopes (HEAT) have been installed at the FD site

Coihueco, overlooking the 750 m SD array. The HEAT telescopes operate in the range of elevation angles

from 30◦to 60◦, complementing the Coihueco telescopes operating in the 0◦to 30◦range. The combination

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SearchesforUltra-High-EnergyPhotonsatthePierreAugerObservatoryThePierreAugerCollaborationP.Abreu71,M.Aglietta53;51,I.Allekotte1,K.AlmeidaCheminant69,A.Almela8;12,J.Alvarez-Mu~niz78,J.Am-mermanYebra78,G.A.Anastasi53;51,L.Anchordoqui85,B.Andrada8,S.Andringa71,C.Aramo49,P.R.AraujoFerreira41,E.Arnone62...

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

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