ICRC 2023 SATELLITE WORKSHOP

August 7, 2023
Chiba University, Japan

Multi-messenger studies at the
Pierre Auger Observatory

Eleonora Guido1 on behalf of the Pierre Auger Collaboration2
1 Center for Particle Physics Siegen, Universität Siegen, Germany
2 Observatorio Pierre Auger, Malargüe, Argentina

1

guido@hep.physik.uni-siegen.deCosmic messengers at ultra-high energies (UHE)✴ Messengers providing different information about the potential sources:• Cosmic rays

• Gamma-rays

Eleonora Guido for the Pierre Auger Collaboration
ICRC2023 Satellite Workshop• Neutrinos• Neutrons

Multi-messenger studies at the Pierre Auger Observatory✴ Gravitational waves

2

Cosmic messengers at ultra-high energies (UHE)✴ Messengers providing different information about the potential sources:• Gamma-rays• Cosmic rays

• Neutrinos• Neutrons✴ Gravitational waves

Cosmic rays (nuclei):• Accelerated by extreme astrophysical events• De ected by magnetic elds

Nucleus

Eleonora Guido for the Pierre Auger Collaborationfi

fl

ICRC2023 Satellite Workshop

Multi-messenger studies at the Pierre Auger Observatory2

Cosmic messengers at ultra-high energies (UHE)✴ Messengers providing different information about the potential sources:• Gamma-rays• Cosmic rays

• Neutrinos

Cosmic rays (nuclei):• Accelerated by extreme astrophysical events• De ected by magnetic elds

• Neutrons✴ Gravitational waves

Gamma-rays:• Propagate in straight lines• Easily absorbed at ultra-high energies

PhotonNucleus
Cosmo
genic photon

Eleonora Guido for the Pierre Auger Collaborationfi

fl

ICRC2023 Satellite Workshop

Multi-messenger studies at the Pierre Auger Observatory2

Cosmic messengers at ultra-high energies (UHE)✴ Messengers providing different information about the potential sources:• Gamma-rays• Cosmic rays

• Neutrinos

Cosmic rays (nuclei):• Accelerated by extreme astrophysical events• De ected by magnetic elds

Photon• Neutrons✴ Gravitational waves

Gamma-rays:• Propagate in straight lines• Easily absorbed at ultra-high energies

Neutrinos:• Not de ected and not absorbed• Low interaction rate → di cult to detect

Neutrino
Cosmogenic neutrino

Nucleus
Cosmo
genic photon

Eleonora Guido for the Pierre Auger Collaboration
ffi

fi

fl

fl

ICRC2023 Satellite Workshop

Multi-messenger studies at the Pierre Auger Observatory2

Cosmic messengers at ultra-high energies (UHE)✴ Messengers providing different information about the potential sources:• Gamma-rays• Cosmic rays

• Neutrinos

Cosmic rays (nuclei):• Accelerated by extreme astrophysical events• De ected by magnetic elds

Photon• Neutrons✴ Gravitational waves

Gamma-rays:• Propagate in straight lines• Easily absorbed at ultra-high energies

Neutrinos:• Not de ected and not absorbed• Low interaction rate → di cult to detect

Neutrons:• Propagate in straight lines• Not distinguishable from protons

Neutron

Neutrino

Cosmogenic neutrino

Nucleus
Cosmo
genic photon

Eleonora Guido for the Pierre Auger Collaboration
ffi

fi

fl

fl

ICRC2023 Satellite Workshop

Multi-messenger studies at the Pierre Auger Observatory2

Cosmic messengers at ultra-high energies (UHE)✴ Messengers providing different information about the potential sources:• Gamma-rays• Cosmic rays

• Neutrinos• Neutrons✴ Gravitational waves

Alerts crucial to study
transient events
Cosmic rays (nuclei):• Accelerated by extreme astrophysical events• De ected by magnetic elds

PhotonGamma-rays:• Propagate in straight lines• Easily absorbed at ultra-high energies

Neutrinos:• Not de ected and not absorbed• Low interaction rate → di cult to detect

Neutrons:• Propagate in straight lines• Not distinguishable from protons

Neutron

Neutrino

Cosmogenic neutrino

Nucleus
Cosmo
genic photon

Eleonora Guido for the Pierre Auger Collaboration
ffi

fi

fl

fl

ICRC2023 Satellite Workshop

Multi-messenger studies at the Pierre Auger Observatory2

Cosmic messengers at ultra-high energies (UHE)✴ Messengers providing different information about the potential sources:• Gamma-rays• Cosmic rays

• Neutrinos• Neutrons✴ Gravitational waves

Alerts crucial to study
transient events

They can be observed by the
Pierre Auger Observatory
Cosmic rays (nuclei):• Accelerated by extreme astrophysical events• De ected by magnetic elds

PhotonGamma-rays:• Propagate in straight lines• Easily absorbed at ultra-high energies

Neutrinos:• Not de ected and not absorbed• Low interaction rate → di cult to detect

Neutrons:• Propagate in straight lines• Not distinguishable from protons

Neutron

Neutrino

Cosmogenic neutrino

Nucleus
Cosmo
genic photon

Eleonora Guido for the Pierre Auger Collaboration
ffi

fi

fl

fl

ICRC2023 Satellite Workshop

Multi-messenger studies at the Pierre Auger Observatory2

Cosmic messengers at ultra-high energies (UHE)✴ Messengers providing different information about the potential sources:• Gamma-rays• Cosmic rays

They can be observed by the
Pierre Auger Observatory• Neutrinos• Neutrons✴ Gravitational waves

Alerts crucial to study
transient events

Neutral particles which pointback to their origin

Cosmic rays (nuclei):• Accelerated by extreme astrophysical events• De ected by magnetic elds

PhotonGamma-rays:• Propagate in straight lines• Easily absorbed at ultra-high energies

Neutrinos:• Not de ected and not absorbed• Low interaction rate → di cult to detect

Neutrons:• Propagate in straight lines• Not distinguishable from protons

Neutron

Neutrino

Cosmogenic neutrino

Nucleus
Cosmo
genic photon

Eleonora Guido for the Pierre Auger Collaboration
ffi

fi

fl

fl

ICRC2023 Satellite Workshop

Multi-messenger studies at the Pierre Auger Observatory2Extensive air showers

ig. 1 Logo of KCDC (https://kcdc.ikp.kit.edu).
Very low rate of particles at ultra-highenergies→ detection through extensive air showers (EAS)• UHE particles start interacting with atmospheric nuclei (N, O, Ar)→ cascadesof ionised+ electromagnetic
radiation
We describe
shortly
theparticles
evolutionof KCDC andgive finally• Cascades
by ground-based
detectors, like
Auger Observatory
n outlookonobserved
possibleuse-case analyses
forthethePierreavailableinteraction→ theprimary
particle
canKCDC
be inferredfrom theair shower characteristics
ata setandtypeonofthefutureof theproject
itself.
with air nuclei
A first release [1] of KCDC is running since November
013 with a positive response from the community and pubc users. Motivated by this success, we had several KCDC
pdates, the last major release, called NABOO 1.0, in Februry 2017 and NABOO 2.0 and 2.1 in October 2017 andMarch 2018, respectively. Presently we provide in differnt formats data from more than 433 million events fromhe three detector components KASCADE (representing the
riginal KASCADE Array), the Central Hadron Calorimeer, and the array of the extension KASCADE-Grande. With
Fig. 2 Photograph of the KASCADE array with its central dehe latest updates we provide as well simulations, separately
building (upper panel); Schematic view of an extensive airs
3
Eleonora
Guido
for
the
PierreAuger
Collaborationor the three detector components for direct
download
Multi-messenger
studies asat the Pierre Auger Observatory
ICRC2023 Satellite Workshop(EAS), where KASCADE is measuring the hadron, muon, andThe Pierre Auger Observatory
Ground-based experiment, hybrid detection technique (Surface Detector (SD) + Fluorescence Detector (FD))
Located in Argentina, close to Malargüe (~1400 m a.s.l.)
Largest observatory in the world for thedetection of ultra-high-energy particles

3000 km2

FD

SD~450 members from ~90
institutions in 17 countries
Eleonora Guido for the Pierre Auger Collaboration
ICRC2023 Satellite Workshop• Nucl. Instrum. Meth. A 798, 172–213

Multi-messenger studies at the Pierre Auger Observatory

4

The Pierre Auger Observatory
How do we detect cosmic ray showers

Surface Detector (SD)

Pierre Auger Observatory combines two independent techniques to detect andacterise cosmic ray showers: one is based on the surface detector, that collects
rmation on the charged particles reaching ground; the other is based on the fluorescencector - it detects light produced in the atmosphere due to the passing of the cascade
icles. While the surface detector is always in operation, this fluorescence detector
ects data only on dark, moonless nights. In this guide we will concentrate on theace detector, since the data made public by the Observatory were collected using theace detector.
Auger surface detector consists of more than 1600 water tanks, sketched in figure 6,
ed at about 1.5 km from each other, and which will sample the charged particles ofshower as they reach ground.

1660 water-Cherenkov tanks covering a ~3000 km2 area, with aspacing of 1.5 km (and a denser smaller array for lower energies)

Fluorescence Detector (FD)24 telescopes in 4 sites overlooking the SD (and 3 additional
ones for lower energies)

3 PMTs collecting the light produced within the tank

Collecting the nitrogen fluorescence light produced in theatmosphere

Sampling the shower particles reaching
the ground

Duty cycle: ~15%dE
Ecal =dX∫ dX

Duty cycle: ~100%signal [VEM]

Figure 6: Water tank of the Pierre Auger Observatory.2/Ndf: 10.8/ 11
candidates
shower particles reaching ground are detected in the tanks due to the Cherenkov e↵ect:saturated recoveredn a charged3particle travels at a speed above the speed of light in the medium (ther inside 10
the tank) - something that is not possible in vacuum (why?)saturated- they will emit
renkov radiation. This light will be detected by 3 photomultipliersnon-triggering(light detectors,

T). The photons are emitted while the particle crosses the tank (or until it is absorbed
accidentalwhich
he water in the tank)and many of them quickly reach the photomultipliers,
S(1000)vert them, by
2 photoelectric e↵ect, in a measurable electric current. The collected10
tric signal is proportional to the number of charged particles crossing the tank, and11

10

Lateral distributionfunction

1

500

1000

1500

2000

S(r) ∝ r β(r + rM)β+γ→ S(ropt)
Signal at the
2500
3000
3500
distance to axis [m] optimal distance: Dependence of the signal size on distance from the shower core.

Eleonora Guido for the Pierre Auger Collaborationfi

ICRC2023 Satellite Workshop

Calorimetric energy
datatotal light
Mie Cherenkov
direct Cherenkov
Rayleigh Cherenkov
multiple scattered

350
300
250
200
150
100

dE/dX [PeV/(g/cm2)]

detected light [photons/m2 /100ns]

Ongoing detector upgrade (AugerPrime)50

χ 2/Ndf= 42.45/44

40
30

Longitudinal
pro le

20
10

50
0
0

250

300

350

400

450

time slots [100 ns](a) Light at aperture.

Hybrid events = observed by both detectors

200

400

600

800

Xmax

1000

1200

1400

1600

slant depth [g/cm2 ](b) Energy deposit.

Depth of shower maximum

Figure 34: Example of a reconstructed shower profile.
Multi-messenger studies at the Pierre Auger Observatory5

20

photon MC (no preshower)

photon MC (preshower)

photon MC (preshower)

Data - burn sample

Data - burn sample

Events0.6

photon MC (no preshower)

Δ

LLDF

0.8

photon MC (no preshower)
photon MC (preshower)

102

Data - burn sample

15

Data - search sample

0.4

The Pierre Auger Observatory as a multi-messenger detector
0.2

10

10

0

−0.2

1

5

−0.4

10−1

0
−0.6

SEARCH FOR NEUTRINOS AND PHOTONS−0.8
1919.1 19.2 19.3 19.4 19.5 19.6 19.7 19.8 19.9 20
log (E /eV)

10−2
−4

Diffuse neutrino flux

10

−2

0

2

4

6

γ

8
Fisher(center); distribution of the Fisher discriminant and results of the application to data (right); for deta
see [2].

Diffuse photon flux

102

Auger HeCo + SD 750 m (2022), U.L. at 95 % C.L.
Auger Hybrid (2021), U.L. at 95 % C.L.
Auger SD 1500 m (2023), U.L. at 95 % C.L.
KASCADE-Grande (2017), U.L. at 90 % C.L.

10

EAS-MSU (2017), U.L. at 90 % C.L.
Telescope Array (2019), U.L. at 95 % C.L.
Telescope Array (2021), U.L. at 95 % C.L.

1

GZK proton I (Kampert et al. 2011)
GZK proton II (Gelmini, Kalashev & Semikoz 2022)
GZK mixed (Bobrikova et al. 2021)

0

Integral photon flux for E > E [km-2 sr-1 yr-1]

Energy spectrum

Mass composition

19.1 19.2 19.3 19.4 19.5 19.6 19.7 19.8 19.9 20
log (E /eV)

Figure 3: Results of the data unblinding for two search channels: ES channel (left); DG channel (right), in
Figure 1: Search for photons with energies above 1019 eV: distributions of the observables ! LDF (left) a
the sub-channel with #stations 2 [7, 11]; for details, see [5, 6]γ

10

COSMIC RAY MEASUREMENTS−5
19

CR interactions in Milky Way (Berat et al. 2022)−1

10

SHDM Ia (Kalashev & Kuznetsov 2016)
SHDM Ib (Kalashev & Kuznetsov 2016)
SHDM II (Kachelriess, Kalashev & Kuznetsov 2018)

10−2

10−3

10−4

10−5
1018

Arrival directions

E0 [eV]

1019

1020

Figure 4: Upper limits on the diﬀuse flux of neutrinos, both
integral (straightsolid(red,line)blue
as andwell
as diﬀerential
Follow-up
ofgravitational
wave
eventsObservatorygraycircles);shown are also upper limits published by other experiments as w
(curved solid line); shown are also upper limits from IceCube and ANITA
the expected
neutrino
fluxestheoretical assumptions and scenarios; for details, see [3].
as theandexpected
photon fluxes
under diﬀerent
under diﬀerent theoretical assumptions and scenarios, including ones based on Auger data (cf. [7]); fordetails, see, e.g., [5, 6].

Figure 2: Current upper limits on the integral photon flux determined from data collected by the Pierre Au

SD-only analysis is based on two observables, ! LDF and (see Fig. 1). The two observables rel
the measured total signals in the individual SD stations and the measured risetimes of the sign
to a data benchmark, describing the average of all of the SD data (assumed to be overwhelmin
redshift are strongly constrained and even excluded due to the non-observation of neutrinos so far.
constituted by primary nuclei). The use of the average behavior of all SD data in the two quantit
A more detailed study of the constraints on the characteristics ofremoves
the sources
UHE
cosmiconrays
the needoffor
assumptions
the composition of the background, which is not known
that can be derived from the limits on the neutrino flux is also shown
thishighest
conference
detail at
at theenergies.[7].
The two observables are combined in a Fisher discriminant analys
with the burnt sample—about 2 % of the full data sample which are not used in the final analysis
representing the background and photon simulations the signal (see also Fig. 1). The analy
3. Gravitational Wave Follow-Up Searches
is applied to SD data collected between 1 January 2004 and 30 June 2020. Overall, 16 eve
from the data sample pass the photon candidate cut, which is consistent with the expectation fr
The large exposure to UHE photons and neutrinos makes the Pierre
Auger Observatory
also anidealbackground.
No primary photon
could
therefore be unambiguously identified. The resulting up
on theevents.
integral photon
are shown in Fig. 2, together with upper limits determined fr
tool for follow-up studies to gravitational wave (GW) and other limits
transient
Alertsflux
received
experiments
as well onas thethrough the General Coordinates Network (GCN)1 are routinelyotherfollowed
up basedtheexpected
stand-photon fluxes under diﬀerent theoretical assumptions a

Eleonora Guido for the Pierre Auger Collaboration
ICRC2023 Satellite Workshop• Phys. Rev. Lett. 125, 121106
alone searches discussed in the previous section. To highlight the capabilities of the Observatory• PoS(ICRC2023)365
3Multi-messenger
studiesinstrument,at the Pierre
Auger
as a multimessenger
we mention
theObservatory
follow-up search for UHE neutrinos to
the
event
• PoS(ICRC2023)1488• PoS(ICRC2023)252
GW170817, a binary neutron star merger [8]. Here, stringent upper limits on the neutrino
fluence6

C O S M I C R AY S
NEUTRINOS
PHOTONSNEUTRONSCosmic rays

Cosmic rays as multi-messenger probes✴ Ultra-high-energy cosmic rays are the main target of the Pierre Auger Observatory✴ Combined t of the energy spectrum and mass composition measurements above 1017.8 eV:• Identical and uniformly distributed extragalactic sources• Two populations are necessary to describe the ankle feature

Predicted uxes
at Earth• Rigidity-dependent cutoffs• Propagation effects on the uxes are considered

Cosmogenic neutrinos (and gamma rays) are produced during the
propagation of UHECRs
It is possible to compute the ν uxes
associated to the best t results• Mixed mass composition + low rigidity cutoff

zmax=5at high-energy → low ν uxes

• For strong source evolutions expected

sensitivities of future detectors are reached
Eleonora Guido for the Pierre Auger Collaboration
fl

fl

fl

fi

fi

fl

ICRC2023 Satellite Workshop

zmax=1LE
Rcut> 1019.5

LE
Rcut= 1018.6• JCAP05(2023)024

Multi-messenger studies at the Pierre Auger Observatory

7

C O S M I C R AY S
NEUTRINOS
PHOTONSNEUTRONSNeutrinos

Neutrino identification at the Pierre Auger Observatory✴ The Pierre Auger Observatory is sensitive also to UHE neutrinos✴ They are probes to specific astrophysical scenarios and can be used to study transient and steady sources✴ They rarely interact with matter → can travel very long distances

How to distinguish neutrino-induced air showers? (from the background of hadron-induced ones)
1. Neutrinos may interact very deep in atmosphere→ even very inclined shower are still “young" at the groundlevel (electromagnetic component still present)

2. ντ may interact in the Earth crust producing a τ→ the lepton decays in the atmosphere and an upward-goingshower can be observed

Downward-going neutrinos

CR
Top of the atmosphere

Cosmic Ray (Inclined)

Top of the atmosphere

Neutrino (Inclined)

Upward-going neutrinos
Top of the atmosphere

EM component

EM component
Muons
Hadronic
component

EM component
Muons

Hadronic
component

Earth

Muons

Earth

Hadronic
component

Earth

Earth-Skimmingtau Neutrino

tau lepton[M. Niechciol]Eleonora Guido for the Pierre Auger Collaboration
ICRC2023 Satellite Workshop

Multi-messenger studies at the Pierre Auger Observatory

8

Neutrino identification at the Pierre Auger Observatory

Neutrinosǡǡċǡ

✴ The Pierre Auger Observatory is sensitive also to UHE neutrinos✴ They are probes to specific astrophysical scenarios and can be used to study transient and steady sourcesƔ ǢǢǡǡ✴ They rarely interact withmatter → can travel very long distancesƔǢ̦ǢŸǢıīƑǢǢǢǢǢHow to distinguish neutrino-induced air showers? (from the background of hadron-induced ones)
ǢǢǢǢǢ  ǢĺǢǢǢǢ Ǣ
1. Neutrinos may interact very deep in atmosphere→ even very inclined shower are still “young" at the groundƔ ǢǢǢǢǢǢĺǢlevel (electromagnetic component still present)Ɣ ǢǢǡǡǢǢǢǢǢǢǢƔ 2.Ǣ̦ǢŸǢıīƑǢǢǢǢǢ→ the lepton decays in the atmosphere and an upward-goingντ mayEarth crust producing a τ interact in theǢǢǢ
ǢǢǢǢǢ  ǢĺǢƔǢǢĺǢǢǢǢǢǢǢshower can be observedǢǢǢ Ǣ
ċǡ
ǢǢǢǢǢǢǗǢǢǢoƔ ǢǢǢǢǢǢĺǢθ
>60
Search for inclined showers() with a large electromagnetic component at the ground (SD data are used)ǢǢǢǢǢǢǢǢǢǢǢǢǢǢǢǢǢǢǢ
Broader signal in each SD stationǢǢ ǢǢǢ
footprint on theƔ ǢǢǢǢǢǢǝǝǢƭƮǢ (large Area-over-Peak ratio) ǢĺǢ
SD stationsƔǢǢĺǢǢǢǢǢǢǢǢĺǢǢǢǢǢǢǢ ǢǢǢǢǢǢǢǗǢǢǢ(large L/W ratio)ǢǢǢǢ
ǢĺǢǢǢǢǢǢǢǢǢǢǢǢ
ǢǢǡǡċǡƔ ǢǢǢǢǢǢǝǝǢƭƮǢǢǢơǢǢǢ ǢǢǢǢǢ 
ǢǢ
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Guido for the Pierre Auger Collaboration
Multi-messenger studies at the Pierre Auger ObservatoryǢǢ ǢǢǢǢ
ICRC2023 Satellite Workshop

ǝǢǢǢǢǢǢ ǢǢǢǢ9

Neutrinos

The neutrino diffuse flux
Downward-goingshowers✴ 60o < θ < 75o (high angle) → Events divided in 5 subsample basing on θrec✴ 75o < θ < 90o (low angle) → Events divided in 3 subsample basing on Nstat→ multivariate analysis: different observables in each subsample→ Different Fisher discrimination variables
Earth-skimming
showers

✴ 90

o

o

< θ < 95 → Discriminating observable: <AoP> over all the triggered stations in the event

Data unblinding (two examples)ES (1 Jan 2004 - 31 Dec 2021)

DGH /$%"%&'($ ∈ [2, 44] (1 Jan 2004 - 31 Dec 2021)

PRELIMINARYPRELIMINARY• Analysis recently updated with data until 31

December 2021
• Two search channels shown as an example• No candidate events have been identi edin any of the search channels• Neutrino candidate cut chosen to get a false-positive rate of 1 event per 50 years

Eleonora Guido for the Pierre Auger Collaboration• No candidate events have been identified in anyof the searchchannels
Multi-messenger
studies
at the Pierre Auger Observatory
fi

ICRC2023 Satellite Workshop• JCAP 10 (2019) 022• PoS(ICRC2023)1488

10

The total exposure Etot folded with a single-flavor flux of UHE neutrinos per u
area A, solid angle ⌦ and time, (E⌫ ) = d6 N⌫ /(dE⌫ d⌦ dA dt) and integrate
Neutrinos
he Thedata unblindingfortwosearchchannels:ES
channel(left);
DG
channel(right),ingives the expected number of events for that flux:
neutrino diffuse fluxZ#stations 2 [7, 11]; for details, see [5, 6]
Nevthave= beenEtot(EabovedE⌫ ) (E10⌫ ) 17⌫.
No neutrino candidate has been identified so far but upper limits
set
eV
E⌫

−2
Assumingadifferentialux
the upper
limit
k valueϕ=k⋅E2ν ,upperAssuming a di↵erential neutrino flux = k · E⌫ , anlimit
to totheat 90% C.L. is given by:
C.L. is obtained as
Feldman-Cousins
2.39
k90 = R, factor in absence2E⌫ E⌫ Etot (E⌫ ) dE⌫of background
Exposure

where 2.39 is the Feldman-Cousins factor [58] for non-observation of events in thThe
integrated upper
limit between
1017 eV and 2.5
1019 eV:
expected backgroundaccounting
for systematic
uncertainties[33,x 59].
The inte
2 di↵erential neutrino flux neede
represents the value of the normalizationofaE
k90 < 3.5 × 10−9 ⌫GeV cm−2 s−1 sr−1
⇠ 2.39 expected events.
• Auger
sets limitsuncertainty
comparablehavewith been
the IceCube
ones in the ca
Several sourcesof systematicconsidered
the exposure and limit.
The
uncertaintydue
tosimulations
includesthee↵ec• Maximum sensitivity at ~EeV (peaks of most cosmogenic
several neutrino interaction
generators,
showersimulations,hadronic
interaction
models)thinning level. These would modify the event rate for a (E⌫ ) / E⌫ 2 flux in eq. (4• Several models for the production of cosmogenic andastrophysical neutrinos are already constrainedfls on the diﬀuse flux of neutrinos, both integral (straight solid line) as well as diﬀerential
Cosmogenic models involving a pure-proton composition and a strong evolution of the sources–with
redshift are already excluded
11–
own are also upper limits from IceCube and ANITA and the expected neutrino fluxes
tical
assumptions
including ones based on Auger data (cf. [7]); for• JCAP 10 (2019) 022
Eleonora
Guido for theand
Pierrescenarios,
Auger Collaboration• PoS(ICRC2023)1488
Multi-messenger studies at the Pierre Auger Observatory• PoS(ICRC2023)1520]. ICRC2023 Satellite Workshop

11

Neutrinos from point-like sources

Neutrinos✴ The same sets of inclined events as in the diffuse flux search

are considered✴ At each instant, only neutrinos from a specific region of theoosky corresponding to 60 < θ < 95 can be detected.

✴ Same exposure calculation as in the analysis for diffuse

neutrinos except for the solid angle integration over the sky✴ A blind search is performed and no neutrino candidate isobserved• Assuming a differential ux ϕ = kPS ⋅ Eν−2, the upper limit to kPS(δ)

at 90% C.L. according to Feldman-Cousins is computed

Eleonora Guido for the Pierre Auger Collaborationfl

ICRC2023 Satellite Workshop

Figure 8. Pierre Auger Observatory upper limits (1 Jan 2004 - 31 Aug 2018) at 90% C.L. on t
normalization k PS of a single flavour point-like flux of UHE neutrinos dN/dE⌫ = k PS E⌫ 2 as a functiof the source declination . Also shown are the limits
for 11
IceCube(2008 - 2015) [38] and ANTAR• JCAP(2019) 004
12
(2007
2015)
[46].
Note
the
di↵erent
energyrangeswhere
thelimitsofeachobservatory
apply.
Multi-messenger studies at the Pierre Auger Observatory

Marcus Niechciol

Latest results from the searches for UHE photons and neutrinos at Auger

Neutrinos

Follow-up of gravitational waves✴ Routine follow-up of gravitational-wave (GW) events:→ search for neutrinos in the ES and DG channels in achosen time window around the eventent through, e.g., GCN→ no neutrinos found so far
most 15 minutes, both in the→ limits complementary to those of IceCube and ANTARES

ified

35(/,0,1$5<✴ Stacking analysis of 83 binary black holes (BBH) mergers

detected by LIGO and Virgo via gravitational waves✴ Two hypothetical emission periods:• Δ=24 h after the GW event• Δ=60 d after the GW event✴ ∝ E −2 spectrum and constant emission are assumed

Figure 5: Relative contribution
each BBH merger
to the24-hourfoundUHE-neutrino
luminosity
limit; the✴ Noofneutrinos
have
been
→ upper
limitsare set on
events are shown in chronological order with colors indicating the GW event catalog; for details, see [9].

vent

➡ GW170817 (binary neutronstar merger) as an example➡ In the FoV of the ES channel
at the time of the eventthe total energy emitted in UHE neutrino35(/,0,1$5<

24-hour time ➡ Limit per source at 90 %window
C.L.: ~2.3 × 1053 erg

eCube, Pierre Auger, Ligo Scientific and Virgo Colls., ApJL 850 (2017) L35]

C 2023Eleonora(Nagoya, Japan)
28 July2023 Auger
12Collaboration
Guido for the
PierreICRC2023 Satellite Workshop• ApJL
850upper(2017) limit
L35 on the total
Figure 6: Results of the stacking analysis in the 24-hour time window [9]. Solidline:• PoS(ICRC2021)968
energyemittedinUHEneutrinos.
Dashedlines:
partialresultswhen
onlyasubset of the available sources
Multi-messenger studies at the Pierre Auger Observatory• PoS(ICRC2023)1488
is taken into account.
• ApJ 952 (2023) 91

13

C O S M I C R AY S
NEUTRINOS
PHOTONSNEUTRONSPhotons

Photon identification at the Pierre Auger Observatory✴ The Pierre Auger Observatory is sensitive also to UHE photons✴ They can be produced either at the sources or during the propagation of UHE cosmic rays

→ constrain speci c astrophysical scenarios (e.g. GZK effect, top-down/bottom-up models for UHECRs production)✴ Neutral particles → as UHE neutrinos, they are used to study steady and transient sources

How to distinguish photon-induced air showers? (from the background of hadron-induced ones)
A photon-initiated shower is dominatedby EM interaction

Top of the atmosphere
EM component

Photon

Muons
Earth

→ deeper depth of shower maximum Xmax
→ less muons at the ground level

Photon(Vertical)
Hadronic
component

Top of the atmosphere
EM component

ProtonCosmic Ray(Vertical)
Hadronic
component

Muons
Earth

[M. Niechciol]Eleonora Guido for the Pierre Auger Collaborationfi

ICRC2023 Satellite Workshop

Multi-messenger studies at the Pierre Auger Observatory

14

Photons

E > 1019 eV

20

0.8

LLDF

Δ

The diffuse photon flux
photon MC (no preshower)
photon MC (preshower)

photon MC (no preshower)
photon MC (preshower)

0.6

Data - burn sample

Data - burn sample

15
0.4• Only the SD measurements are used

0.2

10

0

• Zenith angles between 30° and 60° (selection of fully

5

−0.2
−0.4

0
−0.6

How to distinguish SD photon events:• Slower rising signal in the single SD6 triggeredγ

1019.1 19.2 19.3 19.4 19.5 19.6 19.7 19.8 19.9 20
log (E /eV)10γ

Figure 4. Distribution of
Data - search sample
Fisher axis
candidate cut

stations → observable related to therisetime Δ
4photon MC
preshower) of the photon energy E“ for the burn(left) and LLDF (right)
as (noa function
MC (preshower)
sample and photon simulations. Preshowering photon
photons
are shown only in the energy range in which- burn sample
they represent102a fraction of the selected eventsDatalarger
than 3%. The bands represent one standard
Data - search sample
deviation of the photon distributions.

10

The photon
candidatemaximum with respect to non-preshowering ones of the same primaryenergy. This
results
in an underestimate of their energy of 30% using the look-up table
constructed
as described
cut
is
the
medianof the1in section 4.2.

2

• SLDF, Δ are are combined using a Fisher discriminant

analysis−0.8
1919.1 19.2 19.3 19.4 19.5 19.6 19.7 19.8 19.9 20
log (E /eV)

Events~
LLDF• Steeper LDF (less muons) → observable
SLDF
8

−5
19

photon test sample

0−2

16 events from the data sample pass the photon candidate cut−4−6 background)
−4
−2
0
2
(consistent with the expectation from

4.4
4

6
∼
Δ

Selection of photon candidates
10−2
−4

−2

0

2

4

6

8
Fisher

JCAP0510−1

JCAP05(2023)021

developed showers)

To combine the information contained in the two discriminating variables and define a criterion for the identification of photon candidate events, a burn sample is extracted from theJCAP05representing(2023) 021 a fair sample of the 15
Eleonora Guido for the Pierre Auger Collaboration
setin )a plane
way thatguaranteesof• events
Figure 6. (Left) Distribution in the ( ˜ ,L̃of theevents aoftime
thedistribution
search sample
compared
LDF
Multi-messenger
studies at theinstantaneous
Pierre Augerexposure
Observatory≠2overthe time
periodconsidered.
The5).
burn sample consists of 886
with
the
E
-weighted
distribution
fornon-preshowering
photons(same
contourlevelsas in figure
ICRC2023 Satellite Workshop
events, corresponding to ≥1.8% of the total selected events. The use of such a subset avoids

The diffuse photon flux

Photons

Universe 2022, 8, 579

7 of 20

The Astrophysical Journal, 933:125 (11pp), 2022 July 10

Abreu et al.

FD+SD are used (hybrid measurements) in the photons searches below 1019 eV1018 eV < E < 1019 eVStandard data set(E > 1018 eV)• Zenith angles below 60°• FD measurements:→ Larger depth of shower maximum Xmax
Figure 2. Normalized distributions of the three discriminating observables X , S , and N. The photon sample is shown in blue, the proton sample in red, and• Muon content given by the parameter Fμ (derived
fromthethe data sample
in black.
Only events with E > 2 × 10 eV are shown. The simulated samples are subdivided into a training sample used to train the MVA and a test
17

max

b

stationsγ

sample used to determine the separation power of the individual observables. Note that for illustrative purposes and to facilitate the comparison of the data
distributions to the ones obtained from the simulated samples, the latter were weighted with an EH3 spectrum instead of the EH2 one used in the MVA (see Section 4).

SD signals with air-shower universality concept)

Nstations are shown in Figure 2 for the simulated samples as wellas the data sample.
To combine the three discriminating observables, a multivariate analysis (MVA) is performed using the boosted
decision tree (BDT) method as implemented by the TMVA
package (Hoecker et al. 2007). To take into account energy andzenith angle dependencies, the photon energy Eγ and the zenith
angle θ are also included in theLow-energy
MVA. The MVA datais trainedmax
setusing two thirds of the simulated samples described before,
2 trained
x 1017eV)
while the remaining third is used to(Etest>theMVA
forconsistency and calculate the performance of the MVA withregard to photon/hadron separation. In Figure 2, the trainingSDand test subsamples are denoted by the markers and the shaded
bregions, respectively, for both the photon and the proton
samples. In the training and testing stages of the MVA, events
are weighted according to a power-law spectrum E−Γ with aspectral index Γ = 2, as in previous photon searches (see, e.g.,
Figure 3. Normalized distributions of the ﬁnal discriminator β. The photon
sampleis shownin blue,medianthe proton sample
red, andphoton
the data sampletestin black.
The photon candidatecut
is theofin thesampleAab et al. 2017a).
17
Only events with Eγ > 2 × 10 eV are shown. The simulated samples are
The distribution ofFigure
the output
from
the
BDT
β,
which
is
used
4. (Top left) scatter plot of Xmax andFµ , i.e.,observables
theandhybrid
searchsubdivided
into athetraining
sample used to usedtrain theinMVAa test sample
used foras the ﬁnal discriminator for separating photon-induced air
to determine the separation •powerof the fullanalysis.579
The dashed line denotes
Universe
8(blue)(2022)
photonsusing
air-shower
universality,forsimulatedprimary
photons
andprotons (red); The
16
showersfromthehadronic
background,
is
shown
in
Figure3
the median of the photon test sample, which is used as the photon candidate
Multi-messenger studies at the Pierre Auger Observatory cut. The inlay shows a zoom on the data distribution around the photon
contour
90%, 50%for both the simulated
andlines
the enclose
data samples(see andalso10%, respectively, of the events. (Top right) distributions of thecandidate cut.
Section 5). The photon and proton distributions are clearly2 x 1017 eV < E < 1018 eV• FD measurements:→ Larger depth of shower maximum X• SD measurements:→ Smaller number of triggered SD stations N→ Steeper LDF (less muons) → observable S

The observables are combined to obtain a discriminant
Eleonora Guido for the Pierre Auger Collaboration
ICRC2023 Satellite Workshop

the expected average background contamination.

The diffuse photon flux 6

Photons

Results

Since thenumberphotonis compatible
withthe background e
16 eVNo photon has been unambiguously
detectedsoof
farselected
but upper
limitscandidates
have been set
above 5 x 10
tion, upper limits (UL) on the integral photon flux at 95% confidence level (C.L.) areas:
Upper limit on the integral ux at 95% C.L.
Auger SD 433 m + UMD (2023), U.L. at 95 % C.L.

102

Auger HeCo + SD 750 m (2022), U.L. at 95 % C.L.
Auger Hybrid (2021), U.L. at 95 % C.L.
KASCADE-Grande (2017), U.L. at 90 % C.L.

10

EAS-MSU (2017), U.L. at 90 % C.L.

0.95
U L (E> E0 ) =Telescope Array (2019), U.L. at 95 % C.L.

> E0 )E (E > E0 |E)

Feldman-Cousins
upper limit for 0
background= E −2 ofwhere N 0.95 is the Feldman-Cousins upper limitIntegrated
at 95%exposure
CL on fortheE −Γ
numberGZK proton I (Kampert et al. 2011)
GZK proton II (Gelmini, Kalashev& Semikoz 2022) zero background events and E
candidates
assumingis the integrated exposure ab
Telescope Array (2021), U.L. at 95 % C.L.

1

0

Integral photon flux for E > E [km-2 sr-1 yr-1]

Auger SD 1500 m (2023), U.L. at 95 % C.L.

N 0.95 (EGZK mixed (Bobrikova et al. 2021)

CR interactions in Milky Way (Berat et al. 2022)
SHDM Ia (Kalashev & Kuznetsov 2016)

10−1

SHDM Ib (Kalashev & Kuznetsov 2016)
SHDM II (Kachelriess, Kalashev & Kuznetsov 2018)• Auger set the most stringent limits above ~1017 eV• Top-down models are already disfavoured• GZK predictions still not constrained

–9–

−2

10

→ slightly lowering the limits would put some constraints• Improvement are expected in the next future (AugerPrime)

10−3

10−4

Ongoing analysis in the energy range 50 - 200 PeV
1018

1017

E0 [eV]

Eleonora Guido for the Pierre Auger Collaborationfl

ICRC2023 Satellite Workshop

1019

1020

→ upper limits extended to lower energies

Multi-messenger studies at the Pierre Auger Observatory• Universe 8 (2022) 579• PoS(ICRC2023)1488• PoS(ICRC2023)191

17

Table 1. Combined Unweighted probabilities P andNote. In addition, information on the most signifi
and expected (Exp) events and the corresponding
events column indicate the number of events needed
computed at 95% confidence level. The last two coldiscrete distribution of p-values arising in isotropic s

Photons

Photons from point-like sourcesClass✴ Goal: Identifying the first UHE photon point sources (or constraining their characteristics)

No.

Pw

P

R.A.

Decl.

[◦ ]

[◦ ]

msec PSRs

67

0.57

0.14

286.4

4.0

γ-ray PSRs

75

0.97

0.98

312.8

-8.5

LMXB

87

0.13

0.74

258.1

-40.8

HMXB

48

0.33

0.84

285.9

-3.2

H.E.S.S. PWN

17

0.92

0.90

266.8

-28.2

H.E.S.S. other

16

0.12

0.52

258.3

-39.8

regionH.E.S.S. UNID

20

0.79

0.45

257.1

-41.1

→ the continuation of such spectra to EeV energy could be observed by Auger

Microquasars

13

0.29

0.48

267.0

-28.1

Magnetars

16

0.30

0.89

257.2

-40.1

Gal. Center

1

0.59

0.59

266.4

-29.0

LMC

3

0.52

0.62

84.4

-69.2

Cen A

1

0.31

0.31

201.4 7-43.0✴ Photons are attenuated by the interactions with background radiation→ sources within few Mpc (including Centaurus A)✴ Atmospheric Cherenkov telescopes (e.g. HESS) observed gamma-ray sources in the TeV

source candidates)

For each target direction we use a top-hat counting region of 1◦ . Details of this multivariate cut selection ando
18.5
are given in Aab et al. (2014c). Av• Selected events: hybrid events,, 17.3 counting procedureeraging over all 364 target directions, the multivariate• 5 mass-sensitive observables used to train a BDT cut is expected to retain 81.4% of photons while rejecting 95.2% of background hadrons. After applying thethe total number of recorded events from all of the• A combined p-value is associated to each targetcut,
targets is reduced from 11,180 to 474.
Each target set is tested with and without statistical→ no evidence of EeV photon (statistical signi cancealways
lower
than
3
)
weights. The weight wi is assigned to each target i in atarget set proportional
to thespectra
measured electromagnetic→ upper limits are set → constraints on the extrapolation
of
TeV
to
EeV
flux fi in the catalog and proportional to the directionalenergies (e.g. Ecut < 2 EeV for the Galactic center) photon exposure !i of the Pierre Auger Observatory
based on Settimo & The Pierre Auger Collaboration(2012). Relative attenuation differences from candidate
sources of the same class can be neglected given an interaction length between 90 and 900 kpc of primary
Eleonora Guido for the Pierre Auger Collaboration
Multi-messenger
studies
at thePierreAuger
Observatory
photonsin theenergyrange
considered(see Figure 1).
ICRC2023 Satellite Workshop
The sum of weights in each set is normalized to 1 (see

θ < 60 10

eV < E < 10

P

eV

fi

fi

σ

E2 x flux [TeV cm-2 s-1]

• Sources grouped in 12 target sets to have more signi cant signals (364 individual
10-11

netic field were !B" > 1 nG, since in this case t
Galactic center
trons would be largely deflected.
10-12Since there is a close connection between hadro
duction processes for photons and neutrons, any
10-13date source of neutrons is also a candidate source
tons. As a consequence this analysis adopts the
10-14
tic point source target sets defined in Aab et al.
butadds the new H.E.S.S. unidentified sources r
H.E.S.S. measurement
confidence
bandof the best-fit
spectraet al.
(2015).
The
Galactic source clas
in1σ Deil-15
10
Auger photon GC limit (Γ=2.32 ± 0.11)
millisecond
pulsars(msec
H.E.S.S. extrapolation(Γ =2.32± 0.11)PSRs), γ-ray pulsars
H.E.S.S. extrapolation
Γ =2.32
and Ecut = 2.0X-ray
EeV)
PSRs),
low-mass (andhigh-mass
binaries (L10-16
and HMXBs),2 H.E.S.S.
Pulsar 5Wind 6 Nebulae
(P
3
10
10
10
10-1
10
104
107
1
10
other H.E.S.S. identified and unidentified
sources
E [TeV]quasars, magnetars, and the Galactic center. To
Figure 2.
Photon flux as a function of energy fromindependent
targetsets a candidate source that a
the Galactic
centerMeasured data by H.E.S.S.
• ApJL
837region.
L25 (2017)18 ex
in twoor moreis kept onlyin theare indicated,
as wellas the sets
extrapolated
photonflux mostatEarth in theset.
EeVBecause
range, given
the quoted observable
spectral indicesthe maximum
distance(Abramowski et al. (2016); conservatively the extrapolation

Photons

selection; look at time windows of ±500 s and+1 daythe time of the merger• No photon candidate
hasaroundbeen observed• Noupper
coincident
photons
werespectral
identified
in theSD C.L.
• For each GW event
limit onthe photonuence
at 90%
data set (above 1019 eV) for any of the 10 GW events:
determine upper limits on the spectral fluence

γ -flux spectral index α ∈ [-2.3,-1.7]±500 s

50
45
40
35
30

GW200311

GW200224

GW200208

GW190814

GW190701

GW170818

25

8.5
8
7.5
7
6.5
6

+1 daydirectional uncertainty of GW source
γ -flux spectral index α ∈ [-2.3,-1.7]

GW190728→ only 10 GW events
overlapwith the 10
eldGW
of viewof passthe SDmeasuredby LIGO/Virgo:
eventsthe during
one of the two time windows

55

GW190517

within the photon
horizon,events butvery well localised,…)• Focuson farther
close and/or
well-localized
GW events

directional uncertainty of GW source

GW150914•
Photondetection
efficiency
betterthanfor✴ The SD data above 1019 eV are usedneutrinos,
but
identification
more
difficult✴ Same method used for the search of the diffuse photon flux above 1019 eV(larger hadronic background)✴ Two time windows: Δ=1000 s starting 500 s before the GW event• Only Δfollow-up
selected
GWthe
alertsto=24 h starting
500 s after
GW event
reducethe rateof false-positive
detections✴ Selection of GW
events basedon localization
quality anddistance (events

60

GW170817✴ Goal: search for UHE photons from the sources of gravitational waves (GW)

spectral fluence upper limit / MeV cm-2

Follow-Up Search for UHE Photons

spectral fluence upper limit / MeV cm-2

Follow-up of gravitational wave events[Pierre Auger Coll., accepted for publication in ApJ[P. Ruehl (Pierre Auger Coll.), PoS (ICRC 2021) 973• PoS(ICRC2021)973
19• ApJ 952 (2023) 91

Eleonora Guido for the Pierre Auger Collaborationfi

ICRC2023 Satellite Workshop

flMulti-messenger studiesMarcus
at the Niechciol
Pierre Auger(PierreObservatory
Auger Collaboration) / ICRC 2023 (Nagoya, Japan)

28 July 2023

14

C O S M I C R AY S
PHOTONSNEUTRINOS
NEUTRONSNeutron search for source targets

Neutrons✴ Also UHE neutrons are not deflected by magnetic fields and may point back to their sources✴ Mean travel distance before decaying is 9.2 kpc En/EeV → neutrons above 1 EeV from sources in the Galactic disk can bedetected✴ Neutron-induced air showers cannot be distinguished from proton-initiated ones

→ search for an excess in given directions (as in the targeted search of EeV photon sources)

No evidence for a neutron flux from any target sets of sources→ upper limits (95% C.L.)• Energy flux upper limit, assuming an−2 spectrum• Analysis performed with the events of the SD array• Plan to perform an updated blind search for a neutron flux from any

direction

Eleonora Guido for the Pierre Auger Collaboration𝐸

ICRC2023 Satellite Workshop

Multi-messenger studies at the Pierre Auger Observatory• PoS(ICRC2023)246

20

Summary
The Pierre Auger Observatory is sensitive not only to UHECRs, but also to UHE photons, neutrinos and neutrons✴ Photons can be discriminated from hadrons because they initiate showers with reduced muon content and deeper Xmax

→ they are searched with both the SD and the FD✴ Neutrinos produce showers that develop deep in atmosphere → large electromagnetic component at the ground (“young”

showers)→ search for inclined events with the SD (electromagnetic component of hadron showers is almost completely absorbed)✴ No candidate events → stringent upper limits on the diffusive fluxes (and on the fluxes from point-like steady sources)✴ Follow-up of gravitational wave events has not led to the observation of candidates so far → upper limits✦ Cosmic rays can be used as multi-messenger probes:→ Predictions of cosmogenic neutrinos associated to astrophysical scenarios for UHECRs are compared to the ν upper limits→ possible constraints on source properties (e.g. cosmological evolution, rigidity cutoff at the sources,…)✦ Neutrons can be detected by looking for an excess of particles from a a given direction→ analysis recently updated but no excess observed so far
Eleonora Guido for the Pierre Auger Collaboration
ICRC2023 Satellite Workshop

Multi-messenger studies at the Pierre Auger Observatory

21

dyin a
mic

s to
the

om:
s in

2.7 The Radio Upgrade

OutlookAn engineering array of radio detectors (AERA) has beenrunning in the Pierre Auger Observatory for a few years.
With the collected data, it was possible to demonstrate thefeasibility of radio detection with a grid of antennas at✴1500 m mutual distances, and measure horizontal showers (✓ = 60
84 ), which illuminate an area of several✴km2 on the ground [20].
Based on these results, a full radio upgrade of the Pierre
Auger Observatory has been proposed [21], where each
WCD of ✦the SD will be equipped with a radio antenna
mounted on its top surface, as shown in Fig.5.
✦
The new detectors
will operate together with the upgraded
SD, forming a unique setup to measure the properties of
17.5
cosmic raysabove
10
eV. For horizontal showers, they✦
will nicely complement the information on the particle
type we will get from the WCD+SSD, thereby increasing
the exposure for mass-sensitive investigations.

The current upper bounds will be lowered thanks to increasing exposure and number of transient event
Expected improvements with AugerPrime upgrade → improved sensitivity of the SD to different primaries
New electronics → improved resolution and larger dynamic range
SD stations equipped with radio antennas → detection of radio signals in air showers

Auger
Upgrade:mass/charge
sensitivityuptohighestenergyPlastic scintillator on top of each SD station (Surface Scintillator Detectors)→ different sensitivity to the muonic/electromagnetic components
Scintillator detector on top of each water tank

✴ SSD are more sensitive to electrons/positrons✴ WCD are more sensitive to muonsAuger Upgrade: mass/charge sensitivity up to highest energy→ Enhanced discrimination power of primary particles

Scintillator detector on top of each water tank

beThe
n to

Scintillator (3.8 m2)
22

Scintillation detector (SSD)20

water-Cherenkov detector (WCD)

18
16
14
12
10
8
6

e

27
el).
t of
enon-

Scintillation detector (SSD)

Cherenkov light in water
Figure 5. Sketch of the radio antenna mounted atop
Scintillator (3.8 m2) the
WCD+SSD.

4
2
0

0

100

200

300

400

500

600

700

22
20

water-Cherenkov detector (WCD)

18

Auger with its unique sensitivity will continue to monitor the UHE sky and contribute to multi-messenger studies
16

14

3 Expected physics performances

Eleonora
formaintheaimPierre
AugerisCollaboration
As discussedGuido
above, theof AugerPrimethat ofCherenkov
lightin waterproviding event-by-event composition-sensitive measureICRC2023
Satellite Workshop
ments by increasing the ability to recognize and separate

Both signals used to derive number of muons and electromagnetic particles
12
10

8
6

Multi-messenger studies at the Pierre Auger Observatory
4

2
0

0

100

200

300

400

500

600

700

14

22

Thank you for your attention

Eleonora Guido for the Pierre Auger Collaboration
ICRC2023 Satellite Workshop

Multi-messenger studies at the Pierre Auger Observatory

22

Multi-messenger astronomy
Combining the information from any particle and radiation coming from astrophysical objects→ complementary insight on the most energetic events in the Universe• Sources can be studied through different wavelengths of theelectromagnetic spectrum

Electromagnetic
wavelengths• SN1987A (neutrinos from SN): onset of multi-messenger astronomy• Neutrino astronomy & observation of gravitational waves → recent

boost of multi-messenger studies:✦ 2017: measurements of the electromagnetic spectrum emission incoincidence with the rst neutron star merger observed by LIGO
and Virgo

Neutrinos
Cosmic rays

✦ 2017: IceCube observed a high-energy neutrino (~290 TeV) in

coincidence with a aring gamma-ray blazar.

Gravitational waves

Additional
messengers✦ 2021: IceCube reported the association of a high-energy neutrino

with a tidal disruption event✦ 2023: rst map of neutrinos emissions in our Galaxy
Eleonora Guido for the Pierre Auger Collaboration
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ICRC2023 Satellite Workshop

Multi-messenger studies at the Pierre Auger Observatory• Bionta et al., 1987
• Hirata et al., 1987
• IceCube Coll. 2023

• Abbott et al., 2017a• Aartsen et al., 2018a• R. Stein et al., 20211Preliminary Design Report

The Pierre Auger Observatory
SD: water-cherenkov tanks (WCD) :3) How do we detect cosmic ray showers

1661 covering 3000 km2•
Communication
antennas

Solar panels•

The Pierre Auger Observatory combines two independent techniques to detect andcharacterise cosmic ray showers: one is based on the surface detector, that collects
information on the charged particles reaching ground; the other is based on the fluorescence
detector - it detects light produced in the atmosphere due to the passing of the cascadeparticles. While the surface detector is always in operation, this fluorescence detector
collects data only on dark, moonless nights. In this guide we will concentrate on thesurface detector, since the data made public by the Observatory were collected using thesurface detector.
~100%
duty cycle
The Auger surface detector consists of more than 1600 water tanks, sketched in figure 6,
placed atabout 1.5 km
fromeachwaterother, andwhich will
samplethe charged particlesof
3 PMT
looking
into
thecollect
theCherenkov
light producedbythe shower as they reach ground.

the particles (mainly electrons and muons)Electronics

Plastic scintillator

Battery box

Plastic tank with 12 tons ofultra-pure water
Figure 6: Water tank of the Pierre Auger Observatory.

• AugerPrime: additional plastic scintillator on each tank→improved
information
onaretheprimary
particles
The shower particles
reaching grounddetectedin the tanks
due to the Cherenkov e↵ect:when a charged particle travels at a speed above the speed of light in the medium (thewater inside the tank) - something that is not possible in vacuum (why?) - they will emit
light will
be detected by 3 photomultipliers (light detectors,
8 Cherenkov radiation. This
Eleonora
Guido
PMT). The photons are emitted while the particle crosses the tank (or until it is absorbed

the exposure for mass-sensitive investigations.

de:Themass/charge
sensitivityuptohighestenergyAugerPrime data set

llator detector
on topAugerof each Observatorywater tank
The Pierre
is undergoing an upgrade called “AugerPrime"
Scintillation detector (SSD)✴ Improved new electronics, addition small photomultiplier

Figure9. Angularcorrelation of Auger data modified according t
Figure 4. Top: Two AMIGA modules duringassembly.
Bottom:the Moon and
Jupiter aboveScintillator
one of the Fluorescence
Detectors
in for details). The relative excesses of pairs of event✴ Plastic scintillator on top of each SD station(Surface
Detectors)
catalog"(see text
the Pierre Auger Observatory.

sitivity up to highest energy

ank

SSD

data set (left), the selection deprived of light elements (center) and→ different sensitivity to the muonic/electromagnetic components

Scintillator (3.8 m2)
22

Scintillation detector (SSD)

20
18
16
14

2.6 The extended duty cycle of the Fluorescencewater-Cherenkov detector (WCD)
Detector
Figure 5. Sketch of the radio antenna mounted atop the
WCD+SSD.

The Auger Fluorescence Detector (FD) is composed of 27
10
telescopes overlooking the SD area (Fig.4, bottom panel).
8
6
They are used to observe the longitudinal development of
4
Cherenkov light in water
air showers in the atmosphere, thus providing mass sen20
a modelindependent energy recon0100200sitive
300observables
400
500 and 600
700
3 Expected physics performances
struction.
The FD duty cycle is currently limited to about 15 %, be22
As discussed above, the main aim of AugerPrime is that of20
ing operations limited to dark and moonless nights. The
✴ SSD
are
more sensitive
to electrons/positronswater-Cherenkov
detector (WCD)
providing event-by-event composition-sensitive measure18
current criteria for FD measurements require the Sun to16
ments by increasing the ability to recognize and separate
14✴ WCD
be more than 18particles
below the horizon, the Moon to be bearemoresensitive
Bothsignalsused
to derive
numbertoof muons
muons and electromagnetic
the di↵erent components of air showers. The goodness of
12
low the horizon for more than 3 hours and the illuminated10
the separation between two di↵erent primaries i and j can
8→ Enhanced
discrimination power of primary
fraction of particles
the Moon to be below 70%.
14
be quantified by defining a merit factor
6
4
A significant increase of about 50% in the duty cycle can2✴ Machine|S i S j |learning
techniquesarealso
particularly
suitabletobeobtainedbyextendingtheFD
operations
to
times
with
0
0
100
200
300
400
500
600
700
f MF = p(1)
larger night sky background, relaxing the second and third
2 (S ) + 2 (S )ijcombine different data setsrequirements. By reducing the supplied high voltage, the[The Pierre Auger Collaboration, AugerPrime: the Pierre
PMT gains can indeed be reduced by a factor of 10.
The most direct technique to evaluate the muon comAuger Observatory Upgrade, EPJ Web of Conferences, 2019]
Preliminary tests show that the PMTs
operated10.
at reduced
exploits on
the thedi↵erentof the(left)
WCDandand nFigureExpectedponent
sensitivity
fluxresponsesof photons
13
Eleonora Guido
umber of muons and electromagnetic particles
12

conservative estimates based only on the increase of statistics.

The Pierre Auger Observatory
FD: fluorescence telescopes• 24 in 4 sites overlooking the SD, covering an elevation up to 30°→ E>1018 eV• 3 additional telescopes covering the elevation range between 30° and 58° (HEAT) → E>1017 eV• Each FD site covers 180° x 30° in azimuth and elevation• They collect the nitrogen fluorescence light produced in the atmosphere• ~15% duty cycle (FD operate only on clear moonless nights)
9Eleonora Guido

The Pierre Auger Observatory
Surface Detector (SD)

Fluorescence Detector (FD)

Sampling the secondary particles reaching the ground

Measuring the fluorescence light produced by the de-excitationof atmospheric nuclei

Duty cycle: ~100%detected light [photons/m2 /100ns]

Hybrid events = observed by both detectors

signal [VEM]

10.1051/epjconf/20135304009

2/Ndf: 10.8/ 11
candidates
saturated recovered
saturated
non-triggering

103

data
total light
Mie Cherenkov
direct Cherenkov
Rayleigh Cherenkov
multiple scattered

350
300
250
200
150
100

dE/dX [PeV/(g/cm2)]

Duty cycle: ~15%
50
40

20
10

50
0
0

250

300

350

400

200

450

600

800

(a) Light at aperture.

1200

1400

1600

slant depth [g/cm2 ](b) Energy deposit.
( dX )( Xmax − X0 )X − X0dE
dEshower profile.
Figure 34: Example of a reconstructed
=
⋅

1000

1500

2000

Xmax − X0λXmax − X0
⋅ exp
.
(
)
λ

Estimator S(ropt) = shower size at a
distanceFinally,
ropt fromthecore
the calorimetric energy of the shower is obtained by integrating equa-

Lateral distributionfunction
500

1000

Xmax

dX

2500

3000

3500

distance to axis [m]maxdEEcal‘invisible=dXtion (8) and the total energy is estimated by correcting for theenergy’carried∫dX
β+γS(r) ∝awayr β(r +r)→ S(r) energy muons [115]. An example of the measured light atby neutrinos
andopthighM

SD1500: ropt = 1000 m
SD750 (Infill): ropt = 450 m
e 38: Dependenceof the signal size on distance from the shower core.

Calorimetricaperture and the reconstructed light contributions, and energy
deposit energy
profile is shownin Figs. 34(a) and 34(b).
10

fi

400

time slots [100 ns]

102

1

Longitudinal
pro le30

accidental

S(1000)10

χ 2/Ndf= 42.45/44

Eleonora Guidointegrations on the CMB background, so that the attenuation of light nuclei is much fasterthan that of protons, qualitatively, the faster the lighter nucleus. For heavy nuclei like iron,
10
UHECRspropagation
the factor remains below 10 even at energies as high as 100 EeV. The photodissociation
occurs then on infrared background photons with energies about an order of magnitude
Consider the propagation effects → infer source properties from the measured uxes
higher than the typical CMB photon energy. Since these photons are less abundant than the
CMBEnergy
photons,
the attenuation
heavy
18 eV : is relatively slow and is comparable to thatloss processes
occurringoffor
E > 10nuclei
of protons. In any case, for both light and heavy nuclei, the result of 1adE
photodissociation is3• Adiabatic energy losses (expansion of the Universe)
−
=
H
(1
+
z)
Ωm + ΩΛ0most often a nucleon and a lighter nucleus with the same factor, (
which
in
turn
is
subject
E dt )ad
to further photodisintegration.
• Interactions
nuclei withandbackgroundphotons(EBL,CMB)studied in detail both for
The resulting
UHECRofspectrum
composition
have
been
protons [8] as well as for heavierPhoto-pionnuclei [9],production
and specific benchmark
scenarios
N+γ→N+
π 0 / N + can
π ± be obtained−by numerical simulations with Pairany productionof several existingcodes [10].
N + γpropagation→ N + e + + e public(A, Z) + γ → (A − n, Z − n′) + nN

Photo-disintegrationenergy loss lengths for protons

energy loss lengths for iron-56
10000

cE(dE/dt) 1 [Mpc]

1000

100

10

1
1x101

redshiftpair production (CMB)
pion production (CMB)
pion production (EBL)
total

100

10

1x101

1x10 01x10 1

1x10

1
1x101

redshiftpair production (CMB)
pion production (CMB)
pion production (EBL)
disintegration (CMB)
disintegration (EBL)
total
1x101

1x10 01x10 11x10

log10(E/eV)40

￼log10(E/eV)

Eleonora Guido

1000

fl

cE(dE/dt) 1 [Mpc]10000Note. In addition, information on the most signifi
and expected (Exp) events and the corresponding
events column indicate the number of events needed
computed at 95% confidence level. The last two codiscrete distribution of p-values arising in isotropic

Photon search

Photons from point-like sourcesClass✴ Goal: Identifying the first UHE photon point sources (or constraining their characteristics)✴ Photons are attenuated by the interactions with background radiation→ sources within few Mpc (including Centaurus A)✴ Atmospheric Cherenkov telescopes (e.g. HESS) observed gamma-ray sources in the TeV

region→ the continuation of such spectra to EeV energy could be observed by Auger• Sources grouped in 12 target sets to have more signi cant signals (364 individual

No.

Pw

P

R.A.

Decl.

[◦ ]

[◦ ]

msec PSRs

67

0.57

0.14

286.4

4.0

γ-ray PSRs

75

0.97

0.98

312.8

-8.5

LMXB

87

0.13

0.74

258.1

-40.8

HMXB

48

0.33

0.84

285.9

-3.2

H.E.S.S. PWN

17

0.92

0.90

266.8

-28.2

H.E.S.S. other

16

0.12

0.52

258.3

-39.8

H.E.S.S. UNID

20

0.79

0.45

257.1

-41.1

Microquasars

13

0.29

0.48

267.0

-28.1

Magnetars

16

0.30

0.89

257.2

-40.1

Gal. Center

1

0.59

0.59

266.4

-29.0

LMC

3

0.52

0.62

84.4

-69.2

Cen A

1

0.31

0.31

201.4

-43.0

source candidates)o

17.3• Selected events: hybrid events, θ < 60 , 1018.5

eV < E < 10eV

• 5 mass-sensitive observables used to train a BDT• A combined p-value P is associated to each target→ no evidence of EeV photon (statistical signi cance always lower than 3σ)→ upper limits are set → constraints on the extrapolation of TeV spectra to EeV
energies (e.g. Ecut < 2 EeV for the Galactic center)fi

ICRC2023 Satellite Workshop

Multi-messenger studies at the Pierre Auger ObservatoryfiEleonora Guido for the Pierre Auger Collaboration

netic field were !B" > 1 nG, since in this case ttrons would be largely deflected.
Since there is a close connection between hadro
duction processes for photons and neutrons, andate source of neutrons is also a candidate source
tons. As a consequence this analysis adopts thetic point source target sets defined in Aab et al.
but adds the new H.E.S.S. unidentified sources r
in Deil et al. (2015). The Galactic source cla
millisecond pulsars (msec PSRs), γ-ray pulsar
PSRs), low-mass and high-mass X-ray binaries (and HMXBs), H.E.S.S. Pulsar Wind Nebulae (10other H.E.S.S. identified and unidentified sources
quasars, magnetars, and the Galactic center. T

19.6 19.7 19.8 19.9 20
log (E /eV)
γ

10

19

−4
−2
0
2
6
8
three out of 3000
showerspass
the4 photon
selections
and are misclassified, in agreemen
Fisherthe expected average background contamination.
Photon search

19.1 19.2 19.3 19.4 19.5 19.6 19.7 19.8 19.9 20
log (E /eV)
10

10

γ

The
diffusephotonflux
or photons with energies above 1019 eV: distributions of the observables ! LDF (left) and

6 ofResultstion of the Fisher discriminant and results
the application to data (right); for details,

17 eV
Since thenumberphotonis compatible
withthe background ex
No photon has been unambiguously
detectedsoof
farselected
but upper
limitscandidates
have been set
above 2 x 10

tion, upper limits (UL) on the integral photon flux at 95% confidence level (C.L.) are dAuger HeCo + SD 750 m (2022), U.L. at 95 % C.L.
as:Auger Hybrid (2021), U.L. at 95 % C.L.
Upper limit on the integral ux at 95% C.L.

102

10

EAS-MSU (2017), U.L. at 90 % C.L.

0.95
U L (E

Telescope Array (2019), U.L. at 95 % C.L.
Telescope Array (2021), U.L. at 95 % C.L.

1

GZK proton I (Kampert et al. 2011)
GZK proton II (Gelmini, Kalashev & Semikoz 2022)> E0 ) =Feldman-Cousins
upper limit for 0
backgroundN 0.95 (E > E0 )E (E > E0 |E

)

−Γ
−2
0.95 is the Feldman-Cousins upper limit atIntegrated
E=E
exposure
for
where
N
95%
CL
on
the
numberof
GZK mixed (Bobrikova et al. 2021)
CR interactions in Milky Way (Berat et al. 2022)
candidates
assuming
zero background events and E is the integrated exposure abo
SHDM Ia (Kalashev & Kuznetsov
2016)

0

Integral photon flux for E > E [km-2 sr-1 yr-1]

Auger SD 1500 m (2023), U.L. at 95 % C.L.
KASCADE-Grande (2017), U.L. at 90 % C.L.

10−1

SHDM Ib (Kalashev & Kuznetsov 2016)

SHDM II (Kachelriess, Kalashev & Kuznetsov 2018)

10−2

–9–

10−3

10−4

10−5
1018

E0 [eV]

1019

1020

Eleonora Guido for the Pierre Auger Collaboration
upper
limits on the integral photon flux determinedfrom datacollected
byAugerthe Pierre
Auger
Multi-messenger
studies
at the Pierre
Observatoryfl

ICRC2023 Satellite Workshop

9Follow-up of gravitational wave events✴ Goal: search for UHE photons and neutrinos from the sources ofgravitational waves (GW)
✴ Two time windows: Δ=1000 s starting 500 s before the GW event

Δ=24 h starting 500 s after the GW event• The ±500 s window: upper limit on the duration of the prompt phase of GRBs, when typically PeV neutrinos are thought to be

produced in interactions of accelerated cosmic rays and the gamma rays within the GRB itself.
• The 1-day window after the GW event: conservative upper limit on the duration of GRB afterglows, where ultrahigh-energy

neutrinos are thought to be produced in interactions of UHECRs with the lower-energy photons of the GRB afterglow.

Eleonora Guido for the Pierre Auger Collaboration
ICRC2023 Satellite Workshop

Multi-messenger studies at the Pierre Auger Observatory

11

Neutrino search

The neutrino diffuse flux

The exposure of the SD of Auger needs to be calculated for the periodof data taking:• Monte Carlo simulations of neutrino-induced showers.
• The same selection and identi cation criteria applied to the data werealso applied to the results of these simulations• The identi cation ef ciencies for each channel were obtained as thefraction of simulated events that trigger the Observatory and pass theselection procedure and identi cation cuts
• An integration over the whole parameter space, detection area, andtime gives the exposure[The Pierre Auger Collaboration, JCAP10(2019)022]

Eleonora Guido for the Pierre Auger Collaboration
fi

fi

fi

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ICRC2023 Satellite Workshop

Multi-messenger studies at the Pierre Auger Observatory

14

)022

hannel⌫⌧orth-skimmingdi↵use fluxes
ownward-going ⌫e + ⌫µ + ⌫⌧0.86
Relative contribution
0.79
0.21

The neutrino diffuse flux

Neutrino search

JCAP10(2019)022

osureE
foldedwithasingle-flavor
flux
of
UHE
neutrinos
per
unit
energy,tot
relative
contributionof folded
the threeneutrino
flavors
to theevent
rate neutrinos
in Auger per unit energy, area A, solid angle Ω and time, φ(Eν) and
The total
exposurewithasingleavor
ux
of
UHE
6 N /(dE d⌦ dA dt) and integrated in energy
2and time,
angle
⌦
(E
)
=
d
⌫ contribution⌫
E⌫ . Bottom:
relativeto⌫numberthe rateof
inevents
the Earth-skimming⌫ /integrated
in energy
gives
the expected
for that ux
cted
numberof events for that flux:ing (DG)
channels.
Z
Nevt =
Etotof(Ethe(E⌫ ) dEshown(4.1)
pect to the reference
calculationin figure 5. The⌫ ) exposure⌫.
E⌫↵erent models of ⌫⌧ cross-section and ⌧ energy-loss a↵ects mainly the−2rate between 28% and 34%.
responding
range
of variationuxofϕthe
event
=
k
⋅
E
Assumingadifferential
the upper
limit
k value of k at 90%
2ν ,upper↵erential neutrino flux = k · E⌫ , anlimit
to tothend theatObservatory
is not
90% C.L. is given
by:accounted for explicitly in the calculation ofd as
Feldman-Cousins
Differential upper limits to the normalization of thenstead taken as a systematic uncertainty
increase the event
2.39 that wouldfactor
in absence
R
k
=
,
(4.2)
90 total uncertainty,
diffuseux:
integrating
thedenominator
in
bins
of
⇠ 20% [48, 49]. The
obtained by adding
these
bands
2
of background
E
E
(E
)
dE
⌫
tot
⌫
⌫
width 0.5 in log (Eν) .
from 28% to 39%, andEis⌫ incorporated in the value of the limit itself
Exposure
ianFeldman-Cousins
extension [59] of the
Feldman-Cousins
approach [58].
hefactor[58] for non-observationof events in the absence ofr 90% The
C.L.
integrated
is: is:integrated
upper
limit
groundaccounting
forlimit
systematic
uncertainties [33, 59]. The integrated limit

value of the normalization
2
value of the normalizationof a E⌫2 di↵erential
needed to predictof aneutrino
differential flux
ux needed
91
1k90 < 4.4 ⇥ 10 GeV cm s sr ,(4.3)
to predict ∼ 2.39 eventsed events.

2 yrsystematic
1 sr 1 . It uncertainty
17 eV–
ourceshave
considered
in 10
the
calculation ofeV km ofmostly applies
in beenthe energy
intervalandlimit.
uncertainty
ich ⇠90% Theof thetotal eventdue
ratetois simulations
expected in includes
the case theof ae↵ectsE⌫ 2 of using
2 flux due too interaction
generators,
shower simulations,hadronicmodels andativecontributions
to the expected
rate of events
for a E⌫interaction
2 fluxvors
andto theES andDGchannelsareadisplayed
1.inForThese
would
modifytheeventrate for(E⌫ ) / inE⌫tableeq.such(4.1) between

fl

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utrinos contribute to ⇠ 86% of the total event rate, and in particular
Eleonora Guido for the Pierre Auger Collaboration
te the rate of ⌫⌧ events over the downward-going⌫⌧ . The contribution
Multi-messenger
studies at the Pierre Auger Observatory
ICRC2023 Satellite Workshop
is smaller than 15% in this case.

[The Pierre Auger Collaboration, JCAP10(2019)022]

14