The quest for the sources of ultra-high energy
cosmic rays: results and perspectives

Antonella Castellina
INAF - Osservatorio Astrofisico di Torino
INFN - Sezione di Torino
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High and lowluminosity
Gamma-ray bursts

Active galactic Nuclei
Magnetars

Candidates of UHECR sources
Tidal disruption event

Starburst Galaxies...and many others2Constraints on UHECR sources453

Q ⇠ 10 /erg/M pc /yr

measured
UHECR emissivity
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The GZK cutoff• the vast majority of events above ~60 EeV come from distances D< 200 Mpc (~652 Mly)
the contribution of distant sources is thus eliminated:the higher the energy, the smaller the size of the collection region
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UHECR detectors
tinolecans!

2mk700

Pierre Auger Observatory

2m00 k30

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UHECR detectors…and their exposure

tinolecans!

2mk700

Pierre Auger Observatory

2m00 k30

6Detecting extensive air showers

7Energy spectrum

Phys.Rev.Lett. 125 (2020) 121106
Phys.Rev. D102 (2020) 062005
Subm. to Eur.Phys.J. C (2021)

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Energy spectrum
New feature around
1.4 1019 eVSuppression4.7 1019 eV

Flux recovery?
…foreground source ofprotons

Phys.Rev.Lett. 125 (2020) 121106
Phys.Rev. D102 (2020) 062005
sub. to Eur.Phys.J. C (2021)

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Auger vs TA: overall agreement?

(common declination band)• good agreement in the common declination•band (-150<δ<24.80) within systematics
declination dependence of the spectrum in thehighest energy region:- not seen in Auger (except for dipole)- seen in TA at ~4.3σ level, not explained byany systematic effects. Astrophysical effect?
Y.Tsunesada (PAO+TA working group), ICRC 202110

Mass composition

phXmax i = hXmax
i + fE hlnAi

2(Xmax ) = h2
2
sh i + fE lnA

FDNµA ⇠ A✓

Nµp ⇠E/A✏⇡d◆✓E✏⇡d◆= Nµp A1

SDRD

Air shower+hadronic interaction models are required
to convert Nμ and Xmax to A
model uncertainty = maximum contribution to systematics- lack of data on multiparticle production in very forward phase-space in hadron-nucleus interactions at UHE
increasing with energy (far from the tested region)11

Mass composition

Phys.Rev. D90 (2014) 122005
Phys.Rev. D96 (2017) 122003+ ICRC2021

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Auger vs TA: overall agreement?

R.Abbasi et al. (PAO+TA working group), JPS Cons.Proc. 9 (2016) 010016
A.Yushkov et al. (PAO+TA working group), EPJ Web of Cons. 210 (2019) 010009

Non trivial comparison !

• TA•
•

and Auger
measurements of Xmax agree
within systematics up to 1019 eV: the TA
distributions in this range are as compatible to pureprotons as they are to AugerMix
no comparison has been performed above 10 EeV up
to now by TA choice
WG active again: stay tuned !
18.2

19.0

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𝛄

The UHECR Sky,ν

messengers for which directional astronomy is possible

arrival directions and energy are measurable
charged CRs
further complications : magnetic fields deflections, mass composition
Large Scale Anisotropies

Intermediate/Small scale anisotropies• Galactic : diffusion and escape of GCRs
• high rigidity, nearby sources• Transition from Galactic to Extra-Galactic• clustering of events from the same source• extra-Galactic: small dipole due to our motion • correlation with a population of sources14

Large Scale Anisotropy

Dipolar distribution consistent with nearby galaxy stellar mass distribution (2MRS)AUGERScience 357 (2017) 1266; Astrophys. J. 891 (2020) 142;
R. De Almeida, PoS(ICRC2021) 335

D.Harari et al., PRD92 (2015) 06314;
Ding et al., arXiv2101.04564

AUGER&TA : FULL SKY SEARCH• no need to make assumptions on higher moments• results compatible with Auger-only, but smaller
uncertainties (can be reduced with more exposure in
Northern hemisphere in the future)• only significant moment is the dipole
P.Tinyakov et al. (PAO+TA working group), PoS(ICRC2021) 375

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The UHECR Sky

All catalogs: most significant signal at E>38-41 EeV on top-hat scale 23-270 with signal fraction 6-15%
Significance compatible with linear growth within expected varianceObservation (discovery) level (≳5σ) expected within few years
J.Biteau et al. (Auger Coll.,) PoS(ICRC2021) 307

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Auger&TA: full sky search

Post-trial signi cance
2.9σ
4.2σfiA.di Matteo et al. (Auger-TA working group) PoS(ICRC2021) 308

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What can we learn from data
The composition of theprimary beam is mixed
and getting heavier
above ~ 2 1018 eV
Peter’s cycle ∝ E/Zorspallation ∝ E/A ?
Pure/light composition
excluded in ankle region
Transition from Gal. toEGCR below the ankle

Combined fitof energy spectrum andXmax distribution
Example from a simplified
astrophysical model+Well established suppression ofthe flux: sources must be nearby(<200 Mpc). Propagation (GZKhorizon) and/or source (maximumacceleration energy) effects ?
Limits on the local density of sources
Ltot ~ 6 1044 erg Mpc -3 yr -1
ns ~ 10-6 — 10-4 Mpc -3‣ Observation of a dipolar anisotropy at large scale at >8 EeV : UHECR above this energy are extragalactic‣ Hints of correlation with SBGs
‣ Strong limits on the flux of neutrinos and gammas: can’t have new physics, rather ordinary matter

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The Transition regionp➡ Bounds on the dipolar component imply thatthe observed light component must be extragalactic
the ankle is a feature of the EG spectrum, the transition must be below its energy➡ Upper limits can be respected if mix of Galactic heavier nuclei+EG protons (around EeV)

-Auger Coll., Astrop.J.Lett.762 (2013) L1319

The measured spectrum and compositionof UHECR can be described by• the interplay of two EG populations of sources: aHE one, with hard spectrum, and a LE one with asofter spectrum (plus possibly a secondary Galacticcomponent)➡Homogenous sources➡Different sources
dNs /dEmax ⇠ EmaxdNinj /dE ⇠ E ↵E.Guido, PoS(ICRC2021) 311• Considering both the escaping and injected spectra(interactions at the source)

SBGs

A. Condorelli, PoS(ICRC2021) 959+1

Rodrigues, PoS(ICRC2021) 1321

! dNCR /dE ⇠ E ↵20

Composition-sensitive anisotropy‣Verification of the mixed

composition above the ankle‣May not be related to theGMF‣Local source distribution ormass dependent horizons?
‣Still no independentconfirmation

E.Mayotte et al. PoS(ICRC2021) 321

OutlookSpectrum

T.Bister et al. PoS(ICRC2021) 368+Composition+Anisotropy
Combined fit

21Outlook (2025-2030)
AugerPrime

In the Infill region (61 WCD)➡ The UMD (Underground Muon Detector)

increase thecoverage to ~3000
km2 to increase thestatistics at UHE

S.Ogio, PoS(ICRC2019) 013

On each Water-Cherenkov station (WCD):➡ the SSD (Surface Scintillator Detector)➡ The RD (Radio Detector)➡ New electronics, wider dynamic range

A.Castellina, EPJ Web Conf., 210 (2019) 06002

a large exposure detector with composition
sensitivity above ~4 1019 eVTAx4➡ the SD array: +500 stations with 2 km spacing➡ the FD telescopes:+4 FD in the Northern site,
8 in the Southern site
➡ TALE hybrid: low energy extension of TAhybrid sensitivity down to 1016 eV22

Outlook: AugerPrime = FD+SD+RD+UMD
Vertical showers

Horizontal showers

μSignificance of distinguishing two different
realisations of Scenario 1 (maximum rigiditymodel) :- as it predicts, i.e. no protons at UHE- adding 10% protons

Hybrid:Erad from radio
muons from WCD>5σ in 5 years of operations
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SO
GC

Thank you !
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BACKUPSLIDES

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protons• adiabatic energy losses• pair production• photopion production• contribution of IR/UV/Opt photons irrelevant

nuclei• decrease in Lorentz factor✓adiabatic losses✓pair production losses

E=
mp A

• photodisintegration processes✓ Giant dipole resonance (GDR)✓ quasi-deuteron processes (QD)

• pion production✓ pion production[A x Ethr(p)]26Multimessenger information‣From blazars: (86+15)% of EGɣbck, but only <17% IceCube neutrinos‣UHEp can reproduce only the HE tail of the neutrino flux
‣Additional component needed27

Neutral messengers

JCAP 10 (2019) 022; JCAP 11 (2019) 004

PoS (ICRC2021) 373; PoS (ICRC2021) 449

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Mass composition• pure compositions excluded in the range [1018.5-1019 eV] at > 6σ
• mix of p+He excluded in the range [1018.5-1019 eV] at > 5σAuger Coll., Phys.Lett. B762 (2016) 288
A.Yushkov, PoS(ICRC2019) 482

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The Radio Detector of AugerPrime

Geomagnetic emission (~90%)

Charge-exchange (Askaryan effect, ~10%)

As it is not absorbed in atmosphere, the radioemission depends on the source distance andnot on the amount of traversed matterERD

∝ECR

2

σsyst(ERD) ~ 28% —>σsyst(ERadioCR) ~ 14% at 1 EeV

inclined air showers can be detectedby a sparse antenna grid (> 1 km
mutual distance)‣ lower E amplitude‣ larger footprint‣ less steep LDF≥5 stations with signals above bkg
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Muon deficit in hadronic interaction models

Auger Coll., Phys.Rev.Lett.126 (2021) 152002• Clear deficit of muons in the hadronic interaction models• First measurement of the muon intrinsic fluctuations: welldescribed by models• Working group ongoing work with data from 9experiments: significant slope of muon excess vs energy(wrt models) at ~8σ level
D.Soldin (WG), PoS(ICRC2021) 34931