The science of ultra-high energy cosmic rays after more than 15 years of operation of the Pierre Auger Observatory

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The science of ultra-high energy cosmic rays after

more than 15 years of operation of the Pierre Auger

Observatory

Olivier Deligny*, for the Pierre Auger Collaboration**

*Laboratoire de Physique des 2 Inﬁnis Ir`ene Joliot-Curie (IJCLab)

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

** Full author list: https://www.auger.org/archive/authors 2022 09.html

E-mail: *deligny@ijclab.in2p3.fr, ** spokespersons@auger.org

September 2022

Abstract. The Pierre Auger Observatory has been detecting ultra-high energy

cosmic rays (UHECRs) for more than ﬁfteen years. An essential feature of the

Observatory is its hybrid design: cosmic rays above 100 PeV are detected through the

observation of the associated air showers with diﬀerent and complementary techniques,

from surface detector arrays and ﬂuorescence telescopes to radio antennas. The

analyses of the multi-detector data have enabled high-statistics and high-precision

studies of the energy spectrum, mass composition and distribution of arrival directions

of UHECRs. The resulting picture is summarized in this contribution. While no

discrete source of UHECRs has been identiﬁed so far, the extragalactic origin of the

particles has been recently determined from the arrival directions above 8 EeV, and

the ring is closing around nearby astrophysical sites at higher energies. Also, the

established upper limits on ﬂuxes of UHE neutrinos and photons have implications on

dark matter and cosmological aspects that are also presented in this contribution.

The Pierre Auger Observatory. Ultra-high energy cosmic rays (UHECRs) are

the most energetic particles produced in nature, with energies in excess of 100 EeV.

Uncovering their origin is a persistent task since their discovery, mostly because of the

very small value of their intensity on Earth and because of the magnetic deﬂections they

experience en route to Earth. The Pierre Auger Observatory, located in the province of

Mendoza (Argentina) and covering 3000 km2, is the present ﬂagship experiment studying

UHECRs [1]. Two techniques of detection are combined to measure the extensive air

showers (EAS). A surface detector (SD) array, consisting of 1660 autonomously operated

water-Cherenkov detectors, provides a lateral sampling of the EAS at the ground level.

The detectors are arranged on a triangular grid of 1500 m spacing (SD-1500 m), except

for a denser inﬁll area of '30 km2, where the spacing is 750 m (SD-750 m). The

atmosphere above the SD array is overlooked by ﬂuorescence detectors (FD), which

consists of telescopes that detect the faint UV light emitted by nitrogen molecules

previously excited by the charged particles from the EAS. This technique provides the

arXiv:2210.05745v1 [astro-ph.HE] 11 Oct 2022

UHECRs at the Pierre Auger Observatory 2

Figure 1. Flux map at energies above 40 EeV with a top-hat smoothing radius of 25◦

in Galactic coordinates. The supergalactic plane is shown as a gray line. The blank

area is outside the ﬁeld of view of the Observatory. From [2].

opportunity of performing shower calorimetry by mapping the ionization content along

the shower tracks, and of measuring the primary energy on a nearly model-independent

basis. The FD can only operate during dark, moonless nights with a ﬁeld of view free

of clouds. Online and long-term performances of the detectors and data quality are

monitored continuously, and a set of high-quality devices installed in the Observatory

array monitor the atmospheric conditions during operation.

The SD operates with a quasi-permanent duty cycle and thus provides a harvest of

data. Yet, assessing the energy of the observed events requires assumptions about the

primary mass and the hadronic processes that control the cascade development. This

proves to be a diﬃcult task as the primary mass on an event-by-event basis is unknown

and the centre-of-mass energy reached at the LHC corresponds only to that of a proton

of '100 PeV colliding with a nitrogen nucleus. To circumvent these limitations, the

energies are obtained by making use of a subset of events detected simultaneously by

the FD and the SD. This “hybrid” approach allows a calorimetric estimate of the energy

for events recorded during periods when the FD cannot be operated.

Arrival directions. The identiﬁcation of UHECR sources relies primarily on

capturing in the arrival directions a pattern suggestive in an evident way of a class

of astrophysical objects. Such a capture is still eluding our grasp, but some recent

observations have conﬁrmed the long-lasting broad statement of the extragalactic origin

of UHECRs above the so-called ankle energy. On the one hand, an anisotropy at large

scales has been revealed above '8 EeV [3], the amplitude and the direction of which are

consistent with expectations drawn from sources distributed in a similar manner to the

extragalactic matter [3, 4]. On the other hand, at higher energies, the energy losses of

UHECRs limit the horizon of the highest-energy particles. For small-enough magnetic

deﬂections, the distribution of the arrival directions of UHECRs above '40 EeV could

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Thescienceofultra-highenergycosmicraysaftermorethan15yearsofoperationofthePierreAugerObservatoryOlivierDeligny*,forthePierreAugerCollaboration***LaboratoiredePhysiquedes2InnisIreneJoliot-Curie(IJCLab)CNRS/IN2P3,UniversiteParis-Saclay,Orsay,France**Fullauthorlist:https://www.auger.org/archive/auth...

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The science of ultra-high energy cosmic rays after more than 15 years of operation of the Pierre Auger Observatory

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