Arrival Direction Studies of Ultra-High-Energy Cosmic Rays Detected by the Pierre Auger Observatory

Lago, Bruno Lazarotto

doi:10.5281/zenodo.17279345

Arrival Direction Studies of Ultra-High-Energy Cosmic Rays Detected by the Pierre Auger Observatory

Lago, Bruno Lazarotto

Publisher: Zenodo
DOI: 10.5281/zenodo.17279345
Repository: zenodo.org
Language: en

Abstract

The origin of ultra-high-energy cosmic rays remains an enigmatic phenomenon, and studies about their arrival direction play an important role in the attempt to solve this issue. The Pierre Auger Observatory, located in Argentina, stands as the world's largest cosmic ray observatory exhibiting an effective area spanning 3,000 km² and an accumulated exposure of 135,000 km² yr sr over 19 years of operation. These studies are performed using the events reconstructed with the surface detector of the Observatory over the entire field of view which covers about 85% of the sky. To analyze large angular scales in the distribution of arrival directions, we employ a combined Fourier analysis in right ascension and azimuth above the full trigger efficiency. The “east–west” method is used for energies at which the efficiency is small. For studies on small and intermediate angular scales, our approach involves blind searches for overdensities, investigations around the Centaurus region, and likelihood analyses using astrophysical catalogs. At large scales, we observe a first-harmonic modulation in right ascension above 8 EeV with a significance of 6.9𝜎. This suggests an extragalactic origin above this energy. Above ~ 40 EeV the intermediate-angular-scale analyses reveal an excess near Cen A with a significance of 4.0𝜎, while catalog-based searches yield a 3.8𝜎 signal from nearby starburst galaxies. These results provide crucial insights into the origins and distribution of ultra-high-energy cosmic rays, advancing our understanding of cosmic ray astrophysics and underscoring the unique capabilities of the Pierre Auger Observatory in unraveling cosmic mysteries. Presented on behalf of the Pierre Auger Collaboration

Full text

Maximum likelihood method [3]: Sky model: isotropic + catalog •Fisher radius Θ, (Ψ = 1.59Θ) •Signal fraction, 𝛼. Hybrid detector: 1660 Cerenkov tanks (SD) 27 Fluorescense telescopes (FD) Location: Malargue, Argentina Latitude: −35.2∘, Area: 3000 km2 Under upgrade with scintilator detectors and radio antennas Scan: Energy threshold: 32 EeV to 80 EeV Top-hat window radius, Ψ:1∘to 30∘ Method [2]: Binomial probability of measuring the number of events, 𝑁𝑜𝑏𝑠 , inside the window, compared to the mean number of events expected from isotropic simulations, 𝑁𝑒𝑥𝑝. Pierre Auger Observatory [1] Arrival Direction Studies of Ultra-High-Energy Cosmic Rays Detected by the Pierre Auger Observatory Bruno Lago for the Pierre Auger Collaboration –CEFET/RJ –UnED Petrópolis SD FD Rayleigh analysis in right ascention (𝑬 ≥ 𝟐 𝐄𝐞𝐕) [4]: 𝑎𝛼=2 𝒩σ𝑖=1 𝑁𝜔𝑖cos 𝛼𝑖;𝑏𝛼=2 𝒩σ𝑖=1 𝑁𝜔𝑖sin 𝛼𝑖 𝑟 𝛼= 𝑎𝛼 2+ 𝑏𝛼 2; tan 𝜑𝛼=𝑏𝛼 𝑎𝛼; 𝒩 = σ𝑖𝜔𝑖 East-west method (𝑬 < 𝟐 𝐄𝐞𝐕) [5]: 𝑎𝐸𝑊 =2 𝑁σ𝑖=1 𝑁cos 𝛼0(𝑡𝑖− 𝜉𝑖); 𝑏𝐸𝑊 =2 𝑁σ𝑖=1 𝑁sin 𝛼0(𝑡𝑖− 𝜉𝑖); 𝑟 𝛼=𝜋 cos 𝛿 2 sin 𝜃 𝑟𝐸𝑊 ; 𝜑𝛼= 𝜑𝐸𝑊 + 𝜋/2. Blind Search for overdensities Analysis 𝑬𝒕𝒉 ( 𝐄𝐞𝐕 ) 𝚿(∘) 𝒑-value Post - trial 𝒑-value Overdensity 38 27 1 .8 × 10−8 0.02 Cen A 38 27 1 .1 × 10−7 3 .0 × 10−5 Catalog based searches Large-AngularScale Searches Dataset SD data from Jan. 2004 to Dec. 2022 Small and intermediate-angular-scale: •Energy 𝐸 > 32 EeV and zenith 𝜃 < 80∘ Large angular-scale: •𝐸 > 4 EeV and zenith 𝜃 < 80∘ •0.25 < 𝐸/ eV < 4 and 𝜃 < 60∘ •0.03 < 𝐸/ eV < 0.25 and 𝜃 < 55∘ Angular resolution: •Better than 0.9∘for 𝐸 > 10 EeV •Up to 1.6∘for lower energies Statistical uncertainty in 𝑬: •∼7% for 𝐸 > 10 EeV •Up to 20% for lower energies Systematic uncertainty on the absolute energy scale is 𝟏𝟒% Catalog 𝚿 (∘) 𝜶 (%) TS Post - trial 𝒑-value All galaxies 24 −8 +15 14 −6 +8 18 .5 6 .3 × 10−4 Starburst 25 −7 +13 9−4 +7 23 .4 6 .6 × 10−5 All AGNs 25 −7 +12 7−3 +4 20 .5 2 .5 × 10−4 Jetted AGNs 23 −7 +8 6−3 +3 19 .2 4 .6 × 10−4 𝑬𝒕𝒉 =𝟑𝟖 𝐄𝐞𝐕 Flux above 8 EeV, smoothed (top-hat, Ψ = 45∘) Significance 6.9𝜎 For the bin 8 𝐸𝑒𝑉, 16 𝐸𝑒𝑉 : The significance is 5.7𝜎 References: [1] Pierre Auger collaboration, Nucl. Instrum. Meth. A 798 (2015) 172. [2] Pierre Auger collaboration, Astrophys. J. 935 (2022)170. [3] Pierre Auger collaboration, Astrophys. J. Lett. 853 (2018) L29. [4] Pierre Auger collaboration, Science 357 (2017)1266. [5] Pierre Auger collaboration, Astrophys. J. 891 (2020)142.