# Multi-Messenger Physics with the Pierre Auger Observatory

The Pierre Auger Collaboration 1

###### Abstract

An overview of the multi-messenger capabilities of the Pierre Auger
Observatory is presented. The techniques and performance of searching for
Ultra-High Energy neutrinos, photons and neutrons are described. Some of the
most relevant results are reviewed, such as stringent upper bounds that were
placed to a flux of diffuse cosmogenic neutrinos and photons, bounds placed on
neutrinos emitted from compact binary mergers that were detected by LIGO and
Virgo during their first and second observing runs, as well as searches for
high energy photons and neutrons from the Galactic center that constrain the
properties of the putative Galactic PeVatron, observed by the H.E.S.S.
collaboration. The observation of directional correlations between ultra-high
energy cosmic rays and either high energy astrophysical neutrinos or specific
source populations, weighted by their electromagnetic radiation, are also
discussed. They constitute additional multi-messenger approaches aimed at
identifying the sources of high energy cosmic rays.

## Keywords:

UHECR, high energy neutrinos, high energy photons, high energy neutrons,
multi-messenger astrophysics, compact binary mergers

##  1 Introduction

With the discovery of neutrinos from SN1987A [1, 2] arriving four hours before
the light detected by conventional telescopes it became clear that there was a
lot to learn from examining any type of particles and radiation coming from
astrophysical objects, and that neutrino detectors could give early alerts
that would facilitate the observation of the evolution of such transients from
the earliest stages. Besides the different wavelengths of traditional
astronomy, neutrinos, cosmic rays, very high energy gamma rays, and
gravitational waves provide complementary information to study the most
energetic objects of the Universe. While the SN1987A event might be said to
mark the onset of multi-messenger astronomy, the term was only introduced in
the late 1990s (see, e.g. [3, 4]), and the final boost to the field took place
quite recently with the emergence of both neutrino [5] and gravitational-wave
astronomy [6]. Indeed the discovery of gravitational waves from the merging of
a neutron star binary system by LIGO and Virgo [7] triggered a spectacular
series of observations in the full electromagnetic spectrum from radio to the
very high energy gamma rays [8] and searches for neutrino fluxes with ANTARES,
IceCube, and the Pierre Auger Observatory [9]. The combined effort marks an
unprecedented leap forward in astrophysics revealing many aspects of the
Gamma-Ray Burst (GRB) induced by the merger and its subsequent kilonova. More
recently an energetic neutrino candidate was detected in IceCube in
coincidence with the powerful blazar TXS-0506+056 during a flare in the very-
high-energy gamma-ray band, incompatible with a chance correlation at the
$3\sigma$-level [10]. After scanning archival data, possible evidence for
enhanced neutrino emission (quantified at $3.5\sigma$-level) was also found
from this direction, during an independent 5-month period between September
2014 and March 2015 [11]. It is now quite clear that there is much potential
in the combined analysis of data from different experiments and multi-
messenger astronomy has excellent prospects of making many more significant
contributions in the near future.

The Pierre Auger Observatory is the largest and most precise detector of
ultra-high energy air showers, such as those regularly induced by cosmic rays.
While cosmic rays are expected to have curved trajectories in their path to
Earth and thus lose time correlation with light emission, at the highest
energies they could still keep directional information on the as-yet
unidentified sources where they are produced. The study of the arrival
directions of these particles has revealed deviations from isotropy [12, 13]
that, combined with other particle searches, could be of relevance for multi-
messenger astronomy. Indeed spatial correlations have been searched for
between the arrival directions of the highest energy cosmic rays and classes
of objects that have been proposed as sources in the very high-energy regime
such as Active Galactic Nuclei (AGN) and Starburst Galaxies (SBG) [14].
Notably, it has also been shown that it is possible to detect neutral
particles of sufficiently high energy with the Pierre Auger Observatory. By
looking at the depth development of the showers, it is relatively easy to
identify neutrinos which interact in the lower layers of the atmosphere.
Photons can also be discerned from the background of cosmic rays because the
produced showers have a reduced number of muons and they develop deeper than
cosmic rays in the atmosphere. Finally, there is no known possibility to
separate neutron-induced showers from the charged cosmic rays on the basis of
the shower development but, since neutrons are directional, it is in principle
possible to identify sources of nearby neutrons by looking at an excess from
given directions or to exploit potential time and directional correlations.
This procedure could also be applied to any type of neutral particles that
induce a shower in the atmosphere such as photons.

In this article, we review the capability of the Observatory to search for
signals of such particles and discuss the contributions that have been made.

##  2 The Pierre Auger Observatory

The Pierre Auger Observatory is designed to detect extensive air showers
produced by primary cosmic rays above 0.1 EeV. It is located near the city of
Malargüe, Argentina, at a latitude of $35.2^{\circ}$S, a longitude of
$69.2^{\circ}$W and at an approximate altitude of 1400 m above sea level, or
equivalently an atmospheric depth $X_{\mathrm{ground}}=880$ g cm-2. It
comprises a surface detector (SD) array of 1660 water-Cherenkov stations
deployed over a triangular grid of 1.5 km spacing and a system of 27
telescopes grouped in four sites forming the fluorescence detector (FD). The
telescopes are erected at the periphery of the Observatory to observe the
atmosphere over the full area of 3000 km2 covered by the SD array. The SD
stations sample the density of the secondary particles of the air shower at
the ground and are sensitive to the electromagnetic, muonic and hadronic
components. The FD observes the longitudinal development of the air shower by
detecting the fluorescence and Cherenkov light emitted during the passage of
the secondary particles of the shower in the atmosphere. Unlike the SD, the
fluorescence telescopes are operated only during clear and moonless nights,
for an average duty cycle of about 14 %. The hybrid feature of the Auger
Observatory combining these two well-established techniques has proved to be
extremely rewarding in making it the most precise instrument to reconstruct
the energy, mass, and direction of Ultra-High Energy Cosmic Rays (UHECRs).
Details about the different detector components and their performances can be
found in [15, 16].

Besides its general hybrid capabilities, a feature that makes the Pierre Auger
Observatory a very versatile and powerful multi-messenger observatory is the
specific design of the SD stations. They are constructed as cylinders of 3.6 m
diameter and 1.2 m height filled with 12 tonnes of purified water, each.
Charged particles entering a station induce emission of Cherenkov light which
is reflected at the walls by a diffusive Tyvek liner, and collected by three
9-inch photomultiplier tubes (PMT) at the top surface and in optical contact
with the water. The PMT signals are sampled by flash analog-to-digital
converters (FADC) with a time resolution of 25 ns [16]111The recorded sequence
of signals every 25 ns is referred to as the signal trace.. This provides good
discrimination of electrons and muons entering the detector station from the
top, a feature which is important to identify muon-poor air showers induced by
photons, not only with the FD but also with the SD. Moreover, the detector
stations also provide a large cross section for inclined and up-going air
showers, a feature that is of key-importance for the detection of neutrino-
induced air showers. Both of these aspects will be discussed in more detail
below.

The modular structure of the surface detector array and fluorescence
telescopes allowed the data taking to start in 2004 in a partial
configuration. In 2008 the detector was completed and since then data have
been taken, practically without interruption. Once installed, the local
stations work practically without external intervention. There are two types
of trigger conditions: a local trigger at the level of an individual station
(second order or T2 trigger), and a global trigger (third order or T3 trigger)
to register events. The T2 trigger condition is satisfied when either the
signal exceeds the equivalent of 3.2 Vertical Equivalent Muons (VEM) in at
least one time bin of the signal trace in the FADC – the so-called “threshold
trigger” – or when it exceeds a much lower threshold (0.2 VEM) in at least 13
bins within a 3 $\mu$s time window (i.e., 120 bins) – the so-called “Time-
over-threshold (ToT) trigger.” The ToT condition was designed to trigger on
signals broad in time, characteristic of the electromagnetic component
dominant over the first 1000 g cm-2 of the extensive air shower, and it is
crucial for neutrino identification as explained below. The data acquisition
system receives the local T2 triggers and builds a global T3 trigger requiring
a relatively compact configuration of at least three local stations compatible
in time, each satisfying the ToT trigger, or four triggered stations within a
time window slightly larger than light travel time with any type of T2 trigger
[15]. With the completed array, the global T3 trigger rate is about two events
per minute, one third being showers with energies above $3\times 10^{17}$ eV.

##  3 Neutrinos

Neutrinos can travel in straight paths from the confines of the Universe, are
capable of going through large matter depths without absorption, and are
excellent messengers from extragalactic sources, where protons or nuclei are
thought to be accelerated to high energies. Neutrinos are also produced from
cosmic-ray interactions with the cosmic microwave background. Besides pointing
to the position of their sources, they can also provide valuable information
concerning hadronic acceleration processes, composition, the local environment
at the sources, and their evolution with redshift [17].

The idea of using inclined showers to search for neutrino interactions is old
[18], and the large potential of the Pierre Auger Observatory for neutrino
detection was already recognized in its design stages [19]. Inclined showers
induced by cosmic rays were observed in Haverah Park [20], in the early days
of extensive air shower arrays. It was at the beginning of the 2000s when the
muon patterns of the showers at ground level were sufficiently understood to
allow shower reconstruction [21] (see also [22]). These showers traverse large
atmospheric depths, and their electromagnetic component gets almost completely
absorbed before the shower particles reach ground level. As a result, for
zenith angles exceeding $\sim 60^{\circ}$, the shower front reaching the SD
detector stations consists mainly of muons that typically have energies
between 20 and 200 GeV and travel tens of km without decaying. These muons are
little affected by interactions, except for continuous energy loss and
deflections in the Earth’s magnetic field [21]. As their radius of curvature
is a few thousands km, they do not deviate much from their initial
trajectories, keeping the timing of the shower front sharp.

As opposed to cosmic rays, inclined neutrinos can interact deep in the
atmosphere because their interaction length exceeds the matter depth of the
atmosphere for any zenith angle, $\theta$. The resulting “young” showers are
rich in electrons and photons at ground level. These showers induce signals
typically spread over much wider time intervals than cosmic rays with the same
$\theta$ because of multiple scattering of the electrons and photons in the
shower front. The broader digitized signal traces recorded at the particle
detectors of the SD array resemble those produced by vertical air showers and
it is thus relatively easy to single them out from the sharply arriving fronts
produced by inclined protons or nuclei. Tau neutrinos interact in the Earth
just below the surface and produce tau leptons that escape into the
atmosphere, decaying in flight and producing up-going air showers [23, 24].
For neutrino energies exceeding 100 PeV, the combined conversion and exit
probability is maximal for nearly horizontal (“Earth-skimming”) directions
that exit with an elevation angle between $1^{\circ}$ and $2^{\circ}$ above
the horizontal direction [25]. These showers develop close and almost parallel
to the ground. As the shower particles, particularly electrons and photons,
spread laterally they can reach the SD stations, inducing characteristic
signals which are also easily identified because of their broader time spread.

###  3.1 Selection and identification

The event selection, the neutrino identification, and the exposure
calculations are performed separately for Earth-skimming (ES) and down-going
(DG) showers [26]. To select ES showers, all triggered events involving at
least three stations are considered. The ellipsoidal signal pattern at the
ground is required to have a large eccentricity (characterized by a ratio of
major and minor axes greater than $5$), and the direction of the major axis
indicates the azimuthal angle of the event. The apparent speed of the signal
at the ground, measured for each pair of stations and averaged over all of the
pairs, must be in the interval $[0.967,1.034]c$ 222Actual values used are
[0.29, 0.31] m ns-1, and the r.m.s. spread must be less than $0.267c$ 3330.08
m ns-1. For the DG selection [27] a minimum of four triggered stations is
required to reduce noise from random triggers, and two zenith angle groups are
made:“Low” (DGL) and “High” (DGH) respectively for
$60^{\circ}<\theta<75^{\circ}$ and $75^{\circ}<\theta<90^{\circ}$. The
selection of DGL showers is just made requiring the reconstructed zenith angle
to be in the $58.5^{\circ}$-$76.5^{\circ}$ range allowing for uncertainties.
It is obtained fitting a plane to the positions of the triggered stations and
the start time of the signals. For the selection of DGH showers, the
reconstructed $\theta$ must exceed $75^{\circ}$, the apparent average speed of
the signal at the ground must be smaller than $1.044c$ 4440.313 m ns-1, its
spread must be smaller than $0.08c$, and the eccentricity must exceed a value
of 3.

The ES (DG) neutrino identification has been optimized using extensive shower
simulations for different energies, arrival directions, decay (interaction)
depths, and impact parameters, with standard software such as AIRES [28] and
CORSIKA [29] and including the SD response of the individual detector stations
using specific software [30, 15] based on GEANT4 [31]. A single variable has
been chosen for the identification procedure in both searches. Its
distribution for neutrino simulations is compared to that of regular showers
obtained from a small sample of the data assumed to be exclusively cosmic
rays. To select neutrino candidates, a cut is made on this variable at a value
that would just yield one cosmic-ray in 50 years according to the extrapolated
background distribution. For instance, in the ES case, this variable is the
average area over peak ($AoP$) value of the signal traces of each event. The
area is the integrated charge of the signal trace and the peak is its maximum
value, both normalized to one for vertical throughgoing muons used for on-line
calibration. The cut obtained is $AoP>1.83$, ensuring sufficient time spread
in the signal to reject cosmic-ray showers [26]. Events with just three
stations are further required to have $AoP$ exceeding 1.4. For DG showers, a
Fisher discriminant approach combines several variables based on the $AoP$ of
a few selected stations. For the DGH case, the multivariate Fisher method [32]
uses nine variables obtained with the $AoP$ values of the four earliest
stations and a tenth variable accounting for the asymmetry in the time
distribution of the earliest and latest stations of the event. For the DGL
case, the Fisher variable combines five or six variables obtained from the
$AoP$ of the four or five stations closest to the shower core and their
product. For DGL showers, $75\%$ of the triggers are also required to be of
special “ToT” kind [26]. For optimization purposes of DG neutrinos, further
subdivisions have been made using the number of triggered stations for the DGH
set and the reconstructed $\theta$ for the DGL set. The condition for a
neutrino candidate is slightly different in each subset.

Figure 1: Left: Distribution of the Fisher variable for the DGH search (see
text) for simulations compared to a small fraction of the data and assumed to
be due to cosmic rays. The cut is made at a value of the Fisher variable of
3.28. Right: Limits to the point source fluxes as a function of equatorial
declination obtained from the non-observation of ES and DGH neutrino
candidates up to March 31st 2017 (from [33]).

###  3.2 Efficiency and Aperture

The effective aperture of the detector (aperture from now on) is defined so
that when it is multiplied by the neutrino spectral flux, it gives the energy
distribution of the detection rate. For point sources in the DG case, this
aperture is simply calculated integrating the interaction and detection
probabilities over the SD area. Three separate apertures are calculated
according to interaction type and neutrino flavor: 1) neutral current for all
flavors and charged current muon neutrinos, where the shower is produced by
the nuclear debris (typically carrying about $20\%$ of the neutrino energy);
2) charged current for electron neutrinos where all the energy is converted to
the air shower ($80\%$ induced by the electron and the rest by nuclear
fragments); 3) charged current for tau neutrinos where the shower can be
produced either by the nuclear debris ($20\%$ of the energy) or, deeper in the
atmosphere, by the tau decay ($80\%$ of the energy). The three apertures can
be added under the assumption of equal fluxes for each neutrino flavor, as
expected because of neutrino flavor oscillations in the characteristic long
baselines provided by extragalactic distances to plausible sources [34, 35].

On the other hand, only tau neutrinos can be efficiently detected in the ES
mode. When muon neutrinos convert to high energy muons that exit the Earth,
practically all of them also exit the atmosphere without decaying because of
their longer lifetime and when electron neutrinos convert to electrons the
shower develops rapidly in the Earth before exiting. In the ES case, the
aperture is calculated as an integral over the Earth’s area (transverse to the
flux direction), weighted by the probability of a tau neutrino interacting and
producing a tau which subsequently exits the Earth, decays in the atmosphere
and induces a shower that triggers the SD and is effectively selected and
identified. This is done in two steps. The differential probability of a tau
lepton of given energy exiting the Earth has been calculated as a function of
$\theta$ using simulations of tau neutrino interactions in rock that include
regeneration [36]. The tau-exit probability must be integrated over decay
distance weighted by the survival probability, the decay probability per unit
distance and the probability of detection with the SD. The detection
probability for both DG and ES showers includes trigger, selection and
identification and it is also obtained from the simulations.

The exit and detection probabilities in the aperture calculation depend
strongly on $\theta$. A given source located at right ascension $\alpha$, and
declination $\delta$ in equatorial coordinates, is observed with a zenith
angle that oscillates with time according to:

$\cos\theta(t)=\sin\lambda\sin\delta+\cos\lambda\cos\delta\sin(2\pi
t/T-\alpha).$ (1)

Here $\lambda=-35.2^{\circ}$ is the latitude of the detector, $t$ is the
sidereal time, and $T$ the duration of a sidereal day. Naturally, the aperture
is strongly dependent on source position and time. At any given instant the
field of view of the Observatory for each of the ES, DGH and DGL channels is
limited to the bands corresponding to the zenith angle range of the channel,
as displayed in Figs. 2 (left) and 3 for specific examples discussed below.

Typically, the search for neutrinos from point sources is performed on a pre-
selected time interval chosen to match plausible emission times according to
theoretical models describing the mechanisms acting within the sources. The
expected event rate at the detector is calculated by integrating the aperture
over the neutrino flux and, if no candidate events are found in a given time
interval, a $90\%$ C.L. upper limit to the flux normalization is obtained
matching the expected number of detected events over the time interval to 2.39
[37]. An assumption about the energy spectrum of the flux has to be made. It
is customary to again assume a canonical $E_{\nu}^{-2}$. In this case, $90\%$
of the detected events at the Observatory are between 100 PeV and 25 EeV [26].
The most effective channel for neutrino detection is ES because the target
mass provided by the Earth is large compared to the atmosphere and also
because the threshold energy is low. For the ES channel algorithms have been
found to identify events with just three triggered stations. It has been shown
that Earth-skimming events can only be observed for zenith angles between
$90^{\circ}$ and $95^{\circ}$, corresponding to declination angles
$-55^{\circ}<\delta<60^{\circ}$. The exposure is maximal for
$\delta\simeq-53^{\circ}$ and $\delta\simeq 55^{\circ}$. The apertures
provided by the DGH and DGL are progressively smaller because, as the zenith
angle decreases, the atmosphere offers less target mass in which the neutrinos
can interact and be identified. As the search is effectively performed only
for $\theta$ between $60^{\circ}$ and $95^{\circ}$, the field of view of the
Observatory for neutrinos is limited to equatorial declinations between
$\sim-85^{\circ}$ and $\sim 60^{\circ}$.

When the pre-selected time interval is short, much less than a day, the
aperture can be very different depending on the source position and the
observation time. The instantaneous aperture reaches its maximum if the source
position is just below the horizon with $\theta$ between $90^{\circ}$ and
$95^{\circ}$. While the source is in the ES field of view, the instantaneous
aperture for neutrinos of energies above 100 PeV exceeds that of other
neutrino telescopes. Also, depending on declination, the source can be inside
the ES field of view for different lengths of time. For $\delta=0^{\circ}$
there are two transit periods per day into the ES field of view of about 25
minutes each. For the optimal declination positions ($\delta\simeq-53^{\circ}$
and $\delta\simeq 55^{\circ}$), near the extremes in declination of the ES
band, the total transit time per day is more than four hours, as can be seen
in Figs. 2 (left) and 3.

The selection and identification procedures have been applied to the data
registered by the Observatory. As no neutrino candidates have been found,
upper limits to the ultra-high energy (UHE) neutrino fluxes have been
obtained. Upper limits to the diffuse flux with important implications for
some models of UHECR production were first obtained in the ES channel [38, 16]
and subsequently extended to include the DGH [39] and the DGL [26] channels.
Upper limits to point source fluxes were obtained for the ES and DGH channels
[40]. Upper limits on neutrino fluxes from point sources are shown in Fig.1
(right) as a function of declination for the search period up to 31 March
2017. Articles with further updates and details on both diffuse and point
source searches are to appear soon.

###  3.3 Correlated searches of neutrinos

Figure 2: Left: Field of view of the Observatory exemplified at the instant of
detection of the black hole coalescence event GW150914 by Advanced LIGO [41].
The band limits from top to bottom correspond to lines of zenith angles
$95^{\circ}$, $90^{\circ}$, $75^{\circ}$ and $60^{\circ}$, used to separate
the neutrino search into ES, DGH and DGL channels. The black contours give the
$90\%$ C.L. region of the reconstructed position of the BH merger as obtained
by LIGO observations. Right: Upper limit at $90\%$ C.L. to the neutrino
spectral fluence in the 100 PeV to 25 EeV range as a function of declination
(see text), for the detection of black hole merger GW150914. The blue band is
the $90\%$ C.L. of the reconstructed source declination, illustrating the
limited precision level achieved using just detections at two LIGO sites.

The detection of gravitational waves from the merging of binary systems has
marked the birth of gravitational wave astronomy. The first signals from the
merging of two Black Hole (BH) systems GW150914 [6] and GW151226 [42] were
reported in 2016. The luminosity distances of the two events were deduced to
be $410_{-180}^{+160}$ and $440_{-190}^{+180}$ Mpc and the radiated energies
$3.0_{-0.5}^{+0.5}$ and $1.0_{-0.2}^{+0.1}$ solar masses respectively. They
triggered the search for the emission of electromagnetic radiation and
neutrinos [43, 44], even though the black hole systems are not expected to
emit any other type of radiation unless matter debris and magnetic fields can
be found in their neighbourhood, possibly remaining from the BH progenitors
[45, 46].

Neutrinos were searched for with the Pierre Auger Observatory in two periods
of time: $\pm 500$ s around the UTC times at which the mergers were observed
and one day following their occurrence [44]. These intervals were motivated by
the association of the merging of compact objects to Gamma-Ray Bursts (GRBs)
[47, 48, 49]. Neutrinos have been postulated to be produced by accelerated
cosmic rays interacting with the GRB photons in the prompt phase and with the
lower energy photons of the afterglow. The 1000 s time window is an upper
bound of the duration of the prompt GRB phase [50, 51] while the 1-day window
is a conservative bound of the duration of the afterglow [49].

Figure 3: Field of view of the Observatory in Earth-skimming and down-going
channels at the instant of the detection of neutron star merger GW170817. The
small red contour marks the event localization obtained by the Ligo-Virgo
collaborations [41] and the black cross is the position of NGC 4993, later
correlated to the event by optical telescopes [52].

No neutrino candidates were observed in coincidence with these events in
either of the time windows. Assuming a flux $k^{GW}E_{\nu}^{-2}$, where
$k^{GW}$ is a normalization constant, $90\%$ C.L. limits on the neutrino
emission in the EeV from these events were reported as fluence limits. The
spectral fluence is defined as $E_{\nu}^{2}\phi(E_{\nu})\Delta t$. Here $\phi$
is the spectral flux and $\Delta t$ is the chosen time interval over which the
emission is assumed to take place and during which it is also assumed to be
constant. In these conditions the spectral fluence is related to the total
energy emitted in neutrinos. For the assumed spectrum it is trivial to convert
the spectral fluence to an upper limit for the total energy emitted in
neutrinos in a given energy band, which can be compared to the energy radiated
in gravitational waves. The reported limits were obtained for a one-day
period. Due to the poor localization of these events the upper limits, shown
in Fig.2 (left) for GW150914, are reported as a function of declination. A
similar limit was obtained for GW151226 since the distances to both mergers
are quite similar. The most restrictive limit assuming emission during a
single day for GW150914 (GW151226) is obtained for $\delta=-53^{\circ}$
($\delta=55^{\circ}$) and would correspond to a total energy radiated in UHE
neutrinos smaller than $7.7\times 10^{53}$ ($9.7\times 10^{53}$) erg when
integrated from 100 PeV to 25 EeV. This corresponds to $14.3\%$ ($44.1\%$) of
the radiated energy in gravitational waves. The limits found by IceCube and
ANTARES apply to lower neutrino energies and give more restrictive limits on
the total energy radiated in these neutrinos [43].

Models have been proposed where black hole mergers accelerate UHECR [46], and
the energy budget of these collapsing events could account for all of the
UHECR with a modest fraction of the gravitational wave energy (of order $3\%$)
going to UHECR acceleration. Assuming that a similar fraction goes into UHE
neutrinos, about 0.5 events could have been expected at the Observatory in
coincidence with GW150914 in the most optimistic scenario. The upper limits
were obtained averaging the instantaneous aperture over a day. If the emission
time was shorter than one day, more stringent limits would be obtained
provided the source was in the neutrino field of view during the neutrino
emission, especially if it was in the ES field of view. Due to the poor
localization of both events, it is not possible to know if the events were in
the neutrino field of view at the detection time. Given the $90\%$ C.L.
contour of GW151226 (GW150917) the overlap with the field of view of the
neutrino search is $68.9\%$ ($13\%$) for a time window of 1000 s. The overlap
over a whole sidereal day is $100\%$ in both cases. The detection of black
hole mergers closer to us and with more directional precision could provide
more stringent constraints for such models.

Figure 4: Upper limits at $90\%$ C.L. on the neutrino spectral fluence from
GW170817 for a 1000 s period centered at (14 day after) the time of the event
in the top (bottom) panel. The bounds in the top panel are compared to
predictions of models of prompt and extended emission (EE) [51] in the case of
exact alignment of the line of sight to the rotation axis and for selected
off-axis viewing angles. The bounds obtained for a 14-day period on the bottom
panel are compared to models of longer lived emission [53]. All models have
been scaled to 40 Mpc, the distance to the host galaxy NGC 4993. (Modified
from [9]).

The observation of a neutron star merger GW170817 by the LIGO-Virgo
collaboration [54] less than two seconds before the short GRB 170817A detected
by Fermi-LAT [55, 56] from a coincident direction, has had outstanding
implications. The succesful alert systems triggered observations and detection
in practically all bands of the electromagnetic spectrum in an unprecedented
way, marking the onset of a new era of Multimessenger Astronomy [8]. Early
optical observations allowed a precise localization slightly off-centered in
the galaxy NGC 4993 at equatorial coordinates $\alpha(J2000.0)=13^{\rm
h}09^{\rm m}48^{\rm s}$,
$\delta(J2000.0)=-23^{\circ}22^{\prime}53.^{\prime\prime}343$ [52]. Searches
for neutrino events were also performed at neutrino telescopes and at the
Pierre Auger Observatory [9]. At the time of the GW detection, the source was
located at a zenith angle of $91.9^{\circ}$ at the Observatory site, just
below the horizon and extremely close to the sweet-spot for Earth-skimming
neutrinos, see Fig. 3. When considered in a time interval of $\pm 500$ s about
the detection ($93.3^{\circ}<\theta<90.4^{\circ}$), the EeV exposure is larger
than that of dedicated neutrino telescopes and provides the most stringent
upper limit to the neutrino fluence at 90% C.L. in the 100 PeV to 25 EeV
interval [9], complementary to IceCube and ANTARES, as illustrated in Fig. 4.
The analysis was also made for a longer time window of 14 days matching
predictions from longer-lived emission processes [57, 58] and also displayed
in Fig. 4. Neutrino bounds obtained with the Pierre Auger Observatory and
other neutrino telescopes are within the range predicted in some models of GRB
from neutron star mergers. IceCube bounds allowed constraints to be placed on
the orientation angle relative to the jet axis (assumed to be coincident to
the rotation axis) for one of the most optimistic models.

Acceleration of hadrons in astrophysical objects inevitably leads to photon
and neutrino production with similar energy fluences. On September 17th of
2017, a 290 TeV neutrino was detected at the IceCube telescope [59] in
Antarctica and subsequent correlated gamma-ray observations pointed for the
first time to a powerful blazar in a flaring state, TXS 0506+056, as a
candidate source of the neutrino [10]. This result was further supported by a
neutrino burst from the same direction of the sky obtained in a time window of
about 158 days when searching for time correlations from the same direction
[11]. A search for possible coincident signals at the Auger Observatory was
also performed when the results were made public. This source is $21\%$ of the
sidereal day in the field of view of the neutrino search but it was not in the
field of view at the exact time of the neutrino detection. No neutrinos were
found [60]. The details of this search and the bounds obtained are to be
reported in a separate article.

##  4 Photons

###  4.1 Photon identification

Ultra-high energy (UHE) photons are among the possible particles contributing
to the flux of cosmic rays and may arise from a number of processes. Firstly,
UHE photons are expected from the GZK-process just like UHE neutrinos and can
be used to probe the parameter space of UHECR sources. Secondly, they may be
produced by hadronic interactions of cosmic rays within the sources or within
their local environment. In these cases, the photons are produced on average
with around 10 % of the energy of the primary incident proton. Thirdly, a
large flux of UHE photons has been predicted in top-down models with UHECR
originating from the decay of supermassive particles such as topological
defects or Super Heavy Dark Matter (SHDM) particles (see e.g. [61, 62]).

As opposed to neutrinos, photons undergo interactions with the extragalactic
background light (EBL) inducing electromagnetic cascades. This process makes
photons sensitive to the extragalactic environment (e.g. EBL and magnetic
fields) but it also limits the volume from which EeV photons may be detected.
It is small compared to the GZK-sphere, but large enough to encompass the
Milky Way, the Local Group of galaxies, and possibly Centaurus A, given an
attenuation length of about 4.5 Mpc at EeV energies [63]. The secondary
electrons and positrons created by pair production in the photon fields can
again interact with background photons via inverse-Compton scattering,
resulting in an electromagnetic cascade that ends at GeV–TeV energies.
Comparing these expected diffuse GeV-TeV fluxes measured with instruments
sensitive in this energy range provides another important cross-link to
$\gamma$-astronomy (see e.g. [64]).

The search for UHE photon primaries is based on the different development and
particle content of electromagnetic and hadronic air showers. The induced
electromagnetic cascades develop slower than hadronic ones so that the shower
maximum $X_{\mathrm{max}}$ is reached closer to the ground. Proton and photon
simulated showers have average $X_{\mathrm{max}}$ values that differ by about
200 g cm-2 in the EeV energy range. This difference is enhanced at energies
above 10 EeV because of the Landau-Pomeranchuk-Migdal (LPM) effect [65, 66].
Above 50 EeV, photons also have a non-negligible probability to convert in the
geomagnetic field [67, 68, 69] producing a bunch of low-energy electromagnetic
particles, called “pre-shower”, entering the atmosphere. The
$X_{\mathrm{max}}$ of the pre-showered cascades is smaller than for non-
converted ones and the separation between the average $X_{\mathrm{max}}$ for
photons and proton correspondingly reduced. The fluorescence detector (FD)
with its high resolution of about 15 g cm-2 and systematic uncertainties of
less than about 8 g cm-2 for $X_{\mathrm{max}}$ [70, 71] is an ideal
instrument to discriminate photon- from hadron-induced air showers with high
sensitivity even in single events. Thus the FD is able to provide strong
constraints on the photon fraction of UHECR.

Figure 5: Left: Photon identification with a Boosted Decision Tree for signal
(photon, blue), background (proton, red) and data (black). For simulations,
both the training and the test samples are shown. The cut at the median of the
photon distribution is indicated by the dashed line. QGSJET-II-04 is used as
high-energy hadronic interaction model. Right: Compilation of upper limits on
the integral photon fluxes from [72]. Blue arrows: Integral photon upper
limits from the nine year hybrid data sample assuming a photon flux following
$E^{-2}$ and with no background subtraction. The limits obtained when the
detector systematic uncertainties are taken into account are shown as
horizontal segments (light blue) delimiting a dashed-filled box at each energy
threshold; Black arrows: Nine year SD data sample [73]. Previous data from
Auger as well as data from TA, AGASA, Yakutsk, and Haverah Park are included
for comparison. The lines and shaded regions give the predictions for top-down
models and GZK photon fluxes, respectively, assuming different parameters
(references can be found in [72]).

It is also possible to search for photon showers using the SD because the
shower development and nature of the primary cosmic ray affect the content and
the shape of the distribution of particles at the ground as a function of the
distance from the shower axis (Lateral Distribution Function, LDF). Photon-
induced showers generally are expected to have a steeper LDF compared to
hadron primaries because of the sub-dominant role played by the flatter muonic
component. The high-energy effects (LPM and pre-showering) affect the average
muon content of air showers by typically less than 20-50 % dependent on zenith
angle, which is small compared to the factor 5 to 8 difference in muons with
regards to hadronic showers [74, 69]. However, the different stage of shower
development at the ground (relative to $X_{\mathrm{max}}$) leads to a
modification of the observed LDF. Given the steeper LDF and the muon-driven SD
triggers, the footprint at the ground, and consequently the number of
triggered stations, $N_{\mathrm{stat}}$, is typically smaller for
electromagnetic showers [39]. These features can be combined into a single
observable $S_{b}$, that discriminates the photon- and hadron-induced air
showers [75]. Discrimination on a shower by shower basis is less efficient
compared to the FD, but given the much larger statistics of the SD, strong
constraints on the photon fluxes can be provided. For hybrid observations, the
discriminating observables $X_{\mathrm{max}}$, $S_{b}$, and
$N_{\mathrm{stat}}$ are injected into a Boosted Decision Tree (BDT) algorithm
that takes into account also the energy and angular dependencies of the
discriminating observables. To identify photons, a cut is defined at the
median of the BDT response distribution for simulated photons, as depicted in
Fig. 5 (left). We note that the discrepancy between the data and the proton
simulations is in agreement with the current experimental indications of a
composition varying from light to heavier in the EeV range [76] and with the
muon deficit observed in simulations with respect to the Auger data [77, 75].
Applying the cut in this conservative way, the signal efficiency remains
constant independently of the composition and hadronic model assumptions.
Events having a BDT response larger than the median cut are selected as
“photon candidates”. This yields a background contamination of $\sim 0.14$ %
for proton showers using QGSJET-II-04. This background level overestimates the
one expected in data because the composition is generally heavier than that of
pure protons and because the interaction models underestimate the muon number
in EAS, making the showers look more photon-like. If the multivariate analysis
is performed with a mixture of 50 % proton and 50 % iron as input to the
training phase, the background contamination reduces to $\sim 0.04$ % with the
main contribution coming from the smaller values of the simulated
$X_{\mathrm{max}}$. At present, such probabilities are to be considered
systematic uncertainty to the background of potential photon candidates, i.e.
such background events would dilute the photon limits.

###  4.2 Photon searches

Figure 5 (right) shows, as a result, the upper limits on the integral photon
flux derived from nine years (Jan. 2005 - Dec. 2013) of hybrid and SD data
compared to results from other experiments [72]. Eight candidates were found
in the first two energy intervals. These events are close to the applied
photon cut and detailed simulations of hadronic showers will need to be
performed to judge the probability that they are caused by UHECR rather than
by photons. For now, they are conservatively considered background rather than
signal so that the upper limits of the first two energy bins become less
stringent. The differential flux limit between 10-30 EeV is found to be
$6.80\cdot 10^{-11}$ GeV cm-2 s-1 sr-1 at 90 % C.L. Some top-down scenarios
proposed to explain the origin of trans-GZK cosmic rays (dashed lines) are
illustrated and rejected by previous bounds on the photon flux. A recent
super-heavy dark matter proposal (SHDM II) developed in the context of an
inflationary theory is shown as a long-dashed line [62]. Constraints on the
lifetime-and-mass parameter space of SHDM particle will be imposed by current
and future limits on the photon flux, as obtained for example in [78].

A prime interest in the search for photons is to identify the first UHE photon
point sources or – in case of non-observations – to provide relevant upper
limits thereby constraining the source characteristics. As already mentioned,
the horizon is limited to only a few Mpc which reaches out to Cen A but not
much farther [79]. If the energy spectra of TeV $\gamma$ sources measured by
atmospheric Cherenkov telescopes [80] extend to EeV energies, it is plausible
that photons and neutrons from these sources are detectable also by the Auger
Observatory. Sources that produce particle fluxes according to an $E^{-2}$
energy spectrum inject equal energy into each decade. Thus, a measured energy
flux of 1 eV cm-2 s-1 in the TeV decade, as is found for a number of Galactic
sources, would result in the same energy flux in the EeV decade if the
spectrum continues to such high energies and energy losses during propagation
are negligible. A source of specific interest is the galactic center for which
the H.E.S.S.-collaboration measured a gamma-ray flux up to about 50 TeV
without any observation of a cutoff or a spectral break, suggesting that the
Galactic center hosts a peta-electron volt accelerator, called “PeVatron”
[81].

It is still debated whether these photons are produced in hadronic processes.
An interesting test for this is provided by a search for neutrons from this
direction, because neutrons would necessarily be produced in charge exchange
interactions of protons at the source. This will be discussed in section 5.
The ratio between photon and neutron emissivities from p–p collisions at the
same primary proton energy depends primarily on the spectral index of the
protons at the source. Assuming a continuation of the parent proton spectrum
with a spectral index $\Gamma_{p}\lesssim 2.4$ well beyond the energies
observed in the photon spectrum, the photon emissivities are expected to
dominate over those of neutrons [82].

Figure 6: Gamma-ray spectrum from the Galactic center region as measured by
the H.E.S.S. collaboration (red points) [81]. The measured photon flux is
extrapolated into the EeV range, given the quoted spectral index and its
uncertainties (blue shaded region). The Auger limit [79] is indicated by a
green line (the green band reflects again the spectral uncertainties of the
gamma-ray spectrum). A spectral index with cutoff energy
$E_{\mathrm{cut}}=2.0\cdot 10^{6}$ TeV is indicated by the dashed line.

To search for photons from a list of predefined target sources [79], more
relaxed cuts are applied to the observed events [83]. For each candidate
source direction an optimized cut in the multivariate output distribution is
determined which depends on the expected number of isotropic background events
in that direction. This number is calculated by applying a scrambling
technique that takes into account detector efficiencies and aperture features.
For each target direction, a top-hat counting region of $1^{\circ}$ is assumed
[83]. Averaging over all considered target directions, the multivariate cut is
expected to retain 81.4 % of photons while rejecting 95.2 % of background
hadrons. In none of the sources and source classes could EeV photons be
detected. As an example, the result for the Galactic center is illustrated in
Fig. 6. Conservatively, the extrapolation of the H.E.S.S. data to EeV energies
does not take into account the increase of the p–p cross-section by about 40 %
relative to TeV energies. The current upper limit of 0.044 eV cm-2 s-1 (95 %
C.L., $\Gamma=2$) at energies above 0.2 EeV can already constrain the allowed
parameter space for a flux continuation to EeV energies.

The observation of photon fluxes from individual sources or from stacked sets
of targets would have proved that EeV protons are being accelerated at those
considered sources within the galaxy or its neighborhood. However, the null
results leave open the possibility that protons observed at EeV energies are
of extragalactic origin. This is in fact suggested by the large scale
anisotropies observed with the Auger Observatory [12]. Moreover, the absence
of detectable photon fluxes, as reported here, does not exclude the production
of EeV protons within the Galaxy because the derived flux limits are time-
averaged values. EeV photons might be produced in transient sources, such as
gamma-ray bursts or supernovae, or be aligned in jets not pointing to us.
Extending the searches to bursting sources of EHE photons is a goal of ongoing
multi-messenger analyses within Auger, briefly addressed in Section 7.

##  5 Neutrons

Like photons and neutrinos, neutrons travel in straight lines, undeflected by
magnetic fields. They produce air showers that are indistinguishable from
those produced by protons. Thus, a flux of neutrons from a discrete source
would cause an excess of cosmic-ray events around the direction to the source,
clustered within the angular resolution of the Observatory. Since free
neutrons undergo $\beta$-decay with a mean lifetime of about 880 s at rest,
the mean travel distance for relativistic neutrons is 9.2 kpc$\times
E_{\mathrm{n}}$/EeV. The distance from Earth to the Galactic center is about
8.3 kpc, and the radius of the Galaxy is approximately 15 kpc. Thus, sources
in part of the Galactic disk, including the Galactic center, should be
detectable via neutrons above 1 EeV. Above 2 EeV, the volume for detectable
neutron emitters includes most of the Galaxy.

Neutron production is governed by charge exchange interactions of high energy
cosmic-ray protons with ambient photons, protons, or nuclei leading to the
creation of a $\pi^{+}$-meson. The $\pi^{+}$ takes the positive charge of the
proton and a leading neutron emerges with most of the energy that the proton
had. The production of neutrons via creation of $\pi^{+}$-mesons is
necessarily accompanied by photons originating from decay of similarly
produced neutral pions. However, photons resulting from the decay of neutral
pions acquire only a small fraction of the proton energy, so that the
production of neutrons exceeds the hadronic production of photons of the same
energy, provided the accelerated proton spectrum falls approximately like
$1/E^{2}$ or more steeply with energy. This makes searches of high energy
neutrons a highly relevant and sensitive probe for Galactic hadronic cosmic
ray accelerators.

Similarly to the targeted search of EeV photon sources, discussed in Section
4, a flux of neutrons from astrophysical sources in the Galaxy can be detected
in the Pierre Auger Observatory as an excess of cosmic-ray air showers
arriving from the direction of the sources. To avoid the statistical penalty
for making many trials, classes of objects were tested in combinations as
“target sets”. Those target sets include msec pulsars (PSR), pulsar wind
nebulae (PWN), microquasars, and magnetars (the full list of targets can be
found in [84]). In addition, a search for a neutron flux from the Galactic
center and from the Galactic plane was performed. Within a target set, each
candidate source is weighted in proportion to its electromagnetic flux, its
exposure to the Auger Observatory, and its flux attenuation factor due to
neutron decay.

None of the searches provided evidence for a neutron flux from any class of
candidate sources. Based on the first nine years of data (Jan. 1, 2004 to Oct.
31, 2013), the upper limits on the energy flux from these candidate sources,
including the Galactic center, are mostly at a value of 0.10-0.15 eV cm-2 s-1
[84], which is about a factor of 10 below the energy fluxes detected from TeV
gamma-ray sources in the Galaxy [80] and at about the level of sensitivity
reached with the targeted photon searches, discussed above. If those sources
were accelerating protons in the same environment to EeV energies with the
$1/E^{2}$ dependence expected for Fermi acceleration, then the energy flux of
neutrons in the EeV energy decade would even exceed the energy flux in TeV
gamma rays.

Similarly, the energy flux of neutrons from the Galactic plane could be
derived to less than 0.56 eV cm-2 s-1 (95 % C.L.). This provides a stringent
constraint on models for continuous production of EeV protons in the Galaxy
because the injection rate of protons into the Galactic disk must be
sufficiently strong to balance their escape from the Galaxy. The concomitant
neutron emission rate is model dependent. It could exceed the proton emission
rate if protons are magnetically bound to the sources and only the produced
neutrons escape, yielding EeV protons by their later decays. More likely,
however, the neutron luminosity at any fixed energy is less than the proton
luminosity. Based on an estimate of the proton emission rate, the limits on
the neutron flux from the Galactic plane could be used to derive an upper
limit on the ratio $\eta$ = neutron luminosity/proton luminosity. It was found
to be $\eta_{\mathrm{UL}}\simeq 0.006$ [84], which is a significant constraint
on models for continuous production of EeV protons in the Galaxy.

##  6 Directional correlations of UHECRs with neutrinos and source candidates

Even the most energetic cosmic rays can be subject to significant deflection
and corresponding energy-dependent time delays. The strength and properties of
the extragalactic magnetic field (EGMF) causing these effects remain largely
unknown, and UHECR observed from bursting and continuous sources have already
helped to provide constraints to the EGMF (see e.g. [85, 86, 87]). In section
3.3 we have discussed the time-correlated observation of neutrinos from
merging binary systems. In fact, black hole mergers are also expected to
produce UHECR and may provide sufficient luminosity to power the UHECR
acceleration up to the highest energies [46]. On the other hand, 100 EeV
protons from an extragalactic bursting source located at a distance of 30 Mpc
are expected to arrive with time delays relative to photons and neutrinos by
$\mathcal{O}(1-1000)$ yrs [85] and may even reach out to $10^{6}$ yrs in
extreme cases, so that UHECR could not be included in those combined
directional and temporal searches in a straightforward way.

###  6.1 Search for UHECR-Neutrino Correlations

Purely directional correlations between high-energy neutrinos and UHECR still
provide an interesting target of opportunity because UHECRs accelerated in
astrophysical sources are naturally expected to produce high-energy photons
and neutrinos in interactions with the ambient matter and radiation. If
neutrinos result from the decays of pions produced in $p\gamma$ or $pp$
processes, they would carry about 3–5 % of the proton energy. Hence, neutrinos
observed with energies of 30 TeV to 2 PeV, such as observed by IceCube, would
have been produced by protons with energies in the 1 – 100 PeV range. These
energies are much smaller than those registered by the Auger Observatory, and
it is possible that the sources that produce the PeV neutrinos do not
accelerate CRs up to ultra-high energies. In that case no correlations would
be expected. On the other hand, the sources that produce the UHECR can also be
expected to produce lower energy CRs. If the observed neutrinos come from the
same sources as the UHECR, some degree of correlation in the arrival
directions of the highest energy cosmic rays and the observed neutrinos could
be expected depending on composition and on the strength of magnetic fields.
It should also be noted that neutrinos can reach us from cosmological
distances while the UHECR horizon is limited to only a few 100 Mpc. Thus, only
a small fraction of the detected neutrinos could be expected to have
originated from visible UHECR sources so that correlations, if they were
found, cannot be very prominent.

After some initial cross-correlation efforts by the ANTARES collaboration [88,
89] different analysis strategies have been devised in an intercollaborative
effort of IceCube and Auger [90] that were more recently joined also by the
ANTARES and Telescope Array (TA) collaborations [91, 92].

In a classical cross-correlation method, the number of observed UHECR-neutrino
pairs is counted as a function of the maximal angular separation and then
compared to the averaged background, which is calculated once for an isotropic
neutrino flux and once for an isotropic UHECR flux. Applying this analysis to
the latest combined Auger+TA UHECR and IceCube neutrino data sets, it was
found that a maximum departure from the expectation for an isotropic CR flux,
keeping the arrival directions of the neutrinos fixed, occurs at an angular
distance of $1^{\circ}$ for tracks and $22^{\circ}$ for cascades, with post-
trial p-values of $0.48$ and $5.4\cdot 10^{-3}$, respectively [92]. In
parallel, an unbinned likelihood method was used. It is based on a stacking
point-source analysis applied to the UHECRs, where the neutrino arrival
directions are smeared with a symmetric 2D-Gaussian and stacked as the signal
template. The width of the Gaussian is the quadratic sum of the UHECR
reconstruction uncertainty and the magnitude of the magnetic deflection, which
is assumed to be described by
$\sigma_{\mathrm{MD}}=6^{\circ}/E_{\mathrm{CR}}$[100 EeV]. Again, the analysis
is applied to track- and cascade-like high-energy neutrino candidates
separately. Assuming an isotropic flux of neutrinos, the smallest post-trial
p-values are found to be $2.2\cdot 10^{-2}$ and $1.7\cdot 10^{-2}$ for the
track- and cascade-like events, respectively.

These results are interesting, but the level of correlation is still too small
to arrive at a conclusion. It will be interesting to see how the signal
evolves as more data are accumulated by the neutrino and UHECR observatories
involved. In addition, new improved analysis strategies are being developed
making, for example, better use of the measured UHECR composition and energy
so that deflections in the EGMF and Galactic magnetic field can be more
appropriately accounted for.

###  6.2 Association of UHECR with Source Populations

A related multi-messenger approach aimed at identifying the sources of UHECR
from their directional information is to search for correlations with
catalogue based astronomical objects. The basic concept is related to the
studies presented in the previous section, only that the tracers are specific
source populations rather than samples of high energy neutrinos. A set of
source candidates that was studied in [14] includes nearby radio-loud AGNs and
SBGs555We note that the potential of SGBs to accelerate particles to the
highest energies has been questioned, recently [93, 94]., selected as possible
sources to accelerate CRs to the highest energies [95, 96]. Individual objects
found within a given catalogue are expected to contribute differently to the
total UHECR flux observed at Earth. Since the CR luminosity of individual
sources is not known, we have chosen their observed electromagnetic emission
as a proxy. In case of the 17 radio loud $\gamma$AGN found within 250 Mpc in
the 2FHL catalogue from Fermi-LAT [97], the measured integral $\gamma$-flux
from 50 GeV to 2 TeV was used as a proxy for the UHECR flux. Similarly, 23
(out of a total of 63) SBGs within 250 Mpc and with a flux larger than 0.3 Jy
were taken from [98] and weighted with their continuum radio emission at 1.4
GHz. In both cases, attenuation of UHECRs from distant objects in the photon
background fields was accounted for using a data-driven scenario that
reproduces the composition and spectral constraints obtained by the
Observatory [99]. Within this Ansatz, the relative contributions of NGC 4945,
NGC 253, and M83 are expected to be about 38, 33, 13 % of the total UHECR flux
from SBGs observed at the Auger Observatory. The relative contributions seen
at the Observatory from the $\gamma$AGN would be dominated by the Cen A Core
(75 %), followed by M87 (15 %) and Mkn 421 (8 %).

Figure 7: Top row: observed excess maps for $E_{\mathrm{CR}}>39$ EeV and
$E_{\mathrm{CR}}>60$ EeV. Bottom row: model excess maps for the SGB (left) and
$\gamma$AGN model (right) at the same energy thresholds as above. The color
scales indicate the number of events per smearing beam (see inset). The model
flux map corresponds to a uniform full-sky exposure. The supergalactic plane
is shown as a solid gray line. An orange dashed line delimits the field of
view of the array. (From [14]).

To test these models against the observed UHECR sky maps, a maximum-likelihood
analysis was performed with two free parameters aimed at maximizing the degree
of correlations of the model maps with UHECR events: a) the fraction of an
isotropic component contributing to the total UHECR flux in addition to the
source population being tested, and b) the width of a 2D Gaussian smearing
around the position of the source candidates. The first free parameter can be
interpreted as the contribution from more distant or less luminous sources,
and the second parameter effectively accounts for the random scattering of
UHECR in the EGMF. This analysis is repeated for a sequence of energy
thresholds applied to the UHECR events and the test statistic (TS) as a
function of the two parameters is analyzed. The results are shown in Fig. 7.
It was found that the SBG-Model rejects the hypothesis of an isotropic sky
best at $E_{\mathrm{CR}}>39$ EeV and yields a post-trial significance of
$4.0\sigma$. Here, the SBGs contribute about 10 % to the total flux, and the
smearing angle is $12.9^{\circ}$. In the case of the $\gamma$AGN model, the
best rejection of isotropy is reached at $E_{\mathrm{CR}}>60$ EeV at a level
of $2.7\sigma$ with a $\gamma$AGN fraction to the total flux of about 7 % and
a smearing angle $6.9^{\circ}$. Interestingly, the smearing angle decreases
with increasing energy as expected from the larger rigidities of the UHECR in
the $\gamma$AGN model.

The results are very promising and demonstrate departures from a pure
isotropic sky with more and more structure becoming visible, starting at above
8 EeV with the observation of a large-scale dipole [12], indications of higher
order moments above 16 EeV, [13], blurry spots above 40 EeV, and hints of
sharper spots above 60 EeV. There are indications that both SGB and AGN, as
well as a mixture of both or similar populations could play a significant role
in the production of UHECR. However, it is still too early to conclude about
this long standing problem. Future analyses, possibly profiting from full-sky
observations with TA, may involve a better modelling of the Galactic magnetic
field, account for the change of the composition as a function of energy, and
possibly introduce better proxies for the unknown UHECR luminosities, so that
$5\sigma$ detections are well within reach.

##  7 Conclusions and Prospects

The Pierre Auger Observatory was designed as a multi-purpose observatory for
the high-energy Universe with multi-messenger observations being foreseen from
the beginning. In this review, we have summarized a few prominent examples
that demonstrate the unprecedented sensitivities to UHE photons, neutrinos,
and neutrons. Observations of photons in the so far dark Universe at
$E_{\gamma}\gtrsim 10^{17}$ eV or of a neutrino at these energies would be a
major breakthrough by itself. However, the non-observation of point sources
and diffuse fluxes of photons and neutrinos allowed the derivation of upper
bounds that constrain models very effectively. The bounds to diffuse photon
and neutrino fluxes have ruled out the top-down models of UHECRs origin
motivated by particle physics and also started to constrain the GZK-effect as
a dominant process for explaining the observed flux suppression of the most
energetic UHECR [33]. Many TeV $\gamma$-sources are observed at energy fluxes
of the order of 1 eV cm-2 s-1. Such sources would be visible to the Auger
Observatory as strong photon and Galactic neutron sources if their energy
spectrum would continue with a Fermi-like energy distribution up to about
$10^{17}$ eV. Again, their absence suggests that their maximum source energy
does not reach out to the threshold energy of the Observatory and/or that
their spectrum is significantly softer than $dJ/dE\propto E^{-2}$.

Point source searches now routinely include also mergers of compact binaries
alerted by gravitational wave interferometers. The most spectacular event so
far was the neutron star merger GW170817 at a distance of about 40 Mpc. Within
the predefined $\pm 500$ s search window, the Auger Observatory reached a
neutrino flux sensitivity above 100 PeV that was over an order of magnitude
higher than of any other neutrino observatory presently operated. Again, the
absence of neutrinos at Auger, IceCube and ANTARES allowed constraining the
jet properties of the neutron star merger. The third observation run O3 is
about to start with many more events being expected in the near future. To
accommodate for this, mechanisms are set up to automatically react to GW or
other astrophysical alerts and search for neutrinos and photons.

While receiving alerts from a worldwide network of observatories, it is also
possible to send alerts from the Observatory. The Auger Observatory is both a
triggering and a follow-up partner in the Astrophysical Multi-messenger
Observatory Network (AMON) [100], which establishes and distributes alerts for
immediate follow-up by subscribed observatories with private or Gamma-Ray
Coordinates Network666https://gcn.gsfc.nasa.gov notices. AMON registers all
vertical ($\theta\leq 60^{\circ}$), high-quality, Auger SD events, with energy
above 3 EeV with a cadence of at most a few minutes. AMON establishes clusters
of two or more Auger events received correlated in time and arrival direction
as alerts. These are excesses of events expected from neutral (photon or
neutron) UHECRs as discussed in Section 4 and Section 5. Unlike the
aforementioned neutron and photon searches, this alert channel is sensitive to
ultra-high energy neutral transient emission which is less well constrained in
the time-integrated searches. For each Auger alert, AMON also receives and
transmits the background rate, in other words, the significance of each of the
alerts, which depends on the energy and declination of each event. Thus high-
significance alerts can be preferentially followed-up. To improve the overall
significance of alerts and to enhance future photon and neutrino searches in
general, additional improved hardware triggers have been implemented to the
existing surface detector array of the Observatory. Besides enhancing the
sensitivity to photons and neutrinos, they allow reducing their detection
thresholds. Thus, photon alerts for individual ultra-high energy photon
candidates from a selection of events which utilizes observables based on the
new triggers will be transmitted in near-real time and available for immediate
follow-up using the AMON infrastructure. Finally, the infilled array, covering
an area of 23.5 km2 with a 750 m detector spacing, will provide acceptance for
neutrinos and photons below the present detection threshold, though with
reduced total exposure. It may also be used to generate specific alerts and
there are plans to extend the neutrino search to the FD [101].

Presently, the Pierre Auger Observatory is being upgraded to AugerPrime
primarily to improve the mass composition measurements and particle physics
capabilities with the surface detector array [102, 103, 104]. For this
purpose, 3.8 m2 plastic scintillation detectors will be installed on each of
the existing surface detector stations and the current readout electronics
will be replaced by a faster and more powerful one. The new electronics will
facilitate three times faster sampling of the signals (120 MHz instead of 40
MHz), will have more monitoring features implemented, and will have more
sensitive triggers installed for low-energy showers and those initiated by
photons and neutrinos. Moreover, the infilled array will be furnished with
large area underground muon detectors [105, 106] and each surface detector
station will be amended by a radio antenna to provide improved shower
information for inclined air showers [107]. AugerPrime will be operated into
2025 and will improve the statistics for composition information and
composition enhanced astronomy by about a factor of 10. Clearly, all of these
features will open up many new possibilities for improved searches of photons
and neutrinos. This suit of enhancements will further strengthen the prominent
role of the Pierre Auger Observatory as a multi-messenger observatory for the
next decade.

## Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any
commercial or financial relationships that could be construed as a potential
conflict of interest.

## Author Contributions

All authors contributed to the original material and writing of the
manuscript. KHK and EZ coordinated this review.

### Acknowledgments

The successful installation, commissioning, and operation of the Pierre Auger
Observatory would not have been possible without the strong commitment and
effort from the technical and administrative staff in Malargüe. We are very
grateful to the following agencies and organizations for financial support:

Argentina – Comisión Nacional de Energía Atómica; Agencia Nacional de
Promoción Científica y Tecnológica (ANPCyT); Consejo Nacional de
Investigaciones Científicas y Técnicas (CONICET); Gobierno de la Provincia de
Mendoza; Municipalidad de Malargüe; NDM Holdings and Valle Las Leñas; in
gratitude for their continuing cooperation over land access; Australia – the
Australian Research Council; Brazil – Conselho Nacional de Desenvolvimento
Científico e Tecnológico (CNPq); Financiadora de Estudos e Projetos (FINEP);
Fundação de Amparo à Pesquisa do Estado de Rio de Janeiro (FAPERJ); São Paulo
Research Foundation (FAPESP) Grants No. 2010/07359-6 and No. 1999/05404-3;
Ministério da Ciência, Tecnologia, Inovações e Comunicações (MCTIC); Czech
Republic – Grant No. MSMT CR LTT18004, LO1305, LM2015038 and
CZ.02.1.01/0.0/0.0/16_013/0001402; France – Centre de Calcul IN2P3/CNRS;
Centre National de la Recherche Scientifique (CNRS); Conseil Régional Ile-de-
France; Département Physique Nucléaire et Corpusculaire (PNC-IN2P3/CNRS);
Département Sciences de l’Univers (SDU-INSU/CNRS); Institut Lagrange de Paris
(ILP) Grant No. LABEX ANR-10-LABX-63 within the Investissements d’Avenir
Programme Grant No. ANR-11-IDEX-0004-02; Germany – Bundesministerium für
Bildung und Forschung (BMBF); Deutsche Forschungsgemeinschaft (DFG);
Finanzministerium Baden-Württemberg; Helmholtz Alliance for Astroparticle
Physics (HAP); Helmholtz-Gemeinschaft Deutscher Forschungszentren (HGF);
Ministerium für Innovation, Wissenschaft und Forschung des Landes Nordrhein-
Westfalen; Ministerium für Wissenschaft, Forschung und Kunst des Landes Baden-
Württemberg; Italy – Istituto Nazionale di Fisica Nucleare (INFN); Istituto
Nazionale di Astrofisica (INAF); Ministero dell’Istruzione, dell’Universitá e
della Ricerca (MIUR); CETEMPS Center of Excellence; Ministero degli Affari
Esteri (MAE); México – Consejo Nacional de Ciencia y Tecnología (CONACYT) No.
167733; Universidad Nacional Autónoma de México (UNAM); PAPIIT DGAPA-UNAM; The
Netherlands – Ministry of Education, Culture and Science; Netherlands
Organisation for Scientific Research (NWO); Dutch national e-infrastructure
with the support of SURF Cooperative; Poland – National Centre for Research
and Development, Grant No. ERA-NET-ASPERA/02/11; National Science Centre,
Grants No. 2013/08/M/ST9/00322, No. 2016/23/B/ST9/01635 and No. HARMONIA
5–2013/10/M/ST9/00062, UMO-2016/22/M/ST9/00198; Portugal – Portuguese national
funds and FEDER funds within Programa Operacional Factores de Competitividade
through Fundação para a Ciência e a Tecnologia (COMPETE); Romania – Romanian
Ministry of Research and Innovation CNCS/CCCDI-UESFISCDI, projects PN-
III-P1-1.2-PCCDI-2017-0839/19PCCDI/2018, PN-III-P2-2.1-PED-2016-1922, PN-
III-P2-2.1-PED-2016-1659 and PN18090102 within PNCDI III; Slovenia – Slovenian
Research Agency; Spain – Ministerio de Economía, Industria y Competitividad
(FPA2017-85114-P and FPA2017-85197-P), Xunta de Galicia (ED431C 2017/07), ¿
Junta de Andalucía (SOMM17/6104/UGR), Feder Funds, RENATA Red Nacional
Temática de Astropartículas (FPA2015-68783-REDT) and ¿ María de Maeztu Unit of
Excellence (MDM-2016-0692); USA – Department of Energy, Contracts No. DE-
AC02-07CH11359, No. DE-FR02-04ER41300, No. DE-FG02-99ER41107 and No. DE-
SC0011689; National Science Foundation, Grant No. 0450696; The Grainger
Foundation; Marie Curie-IRSES/EPLANET; European Particle Physics Latin
American Network; European Union 7th Framework Program, Grant No.
PIRSES-2009-GA-246806; and UNESCO.

### Full author list and affiliations

A. Aab75, P. Abreu67, M. Aglietta50,49, I.F.M. Albuquerque19, J.M. Albury12,
I. Allekotte1, A. Almela8,11, J. Alvarez Castillo63, J. Alvarez-Muñiz74, G.A.
Anastasi42,43, L. Anchordoqui82, B. Andrada8, S. Andringa67, C. Aramo47, H.
Asorey1,28, P. Assis67, G. Avila9,10, A.M. Badescu70, A. Bakalova30, A.
Balaceanu68, F. Barbato56,47, R.J. Barreira Luz67, S. Baur37, K.H. Becker35,
J.A. Bellido12, C. Berat34, M.E. Bertaina58,49, X. Bertou1, P.L. Biermannb, J.
Biteau32, S.G. Blaess12, A. Blanco67, J. Blazek30, C. Bleve52,45, M.
Boháčová30, D. Boncioli42,43, C. Bonifazi24, N. Borodai64, A.M. Botti8,37, J.
Bracke, T. Bretz39, A. Bridgeman36, F.L. Briechle39, P. Buchholz41, A.
Bueno73, S. Buitink14, M. Buscemi54,44, K.S. Caballero-Mora62, L.
Caccianiga55, L. Calcagni4, A. Cancio11,8, F. Canfora75,77, J.M. Carceller73,
R. Caruso54,44, A. Castellina50,49, F. Catalani17, G. Cataldi45, L. Cazon67,
M. Cerda9, J.A. Chinellato20, J. Chudoba30, L. Chytka31, R.W. Clay12, A.C.
Cobos Cerutti7, R. Colalillo56,47, A. Coleman86, M.R. Coluccia52,45, R.
Conceição67, A. Condorelli42,43, G. Consolati46,51, F. Contreras9,10, F.
Convenga52,45, M.J. Cooper12, S. Coutu86, C.E. Covault80, B. Daniel20, S.
Dasso5,3, K. Daumiller37, B.R. Dawson12, J.A. Day12, R.M. de Almeida26, S.J.
de Jong75,77, G. De Mauro75,77, J.R.T. de Mello Neto24,25, I. De Mitri42,43,
J. de Oliveira26, F.O. de Oliveira Salles15, V. de Souza18, J. Debatin36, M.
del Río10, O. Deligny32, N. Dhital64, M.L. Díaz Castro20, F. Diogo67, C.
Dobrigkeit20, J.C. D’Olivo63, Q. Dorosti41, R.C. dos Anjos23, M.T. Dova4, A.
Dundovic40, J. Ebr30, R. Engel36,37, M. Erdmann39, C.O. Escobarc, A.
Etchegoyen8,11, H. Falcke75,78,77, J. Farmer87, G. Farrar85, A.C. Fauth20, N.
Fazzinic, F. Feldbusch38, F. Fenu58,49, L.P. Ferreyro8, J.M. Figueira8, A.
Filipčič72,71, M.M. Freire6, T. Fujii87,f, A. Fuster8,11, B. García7, H.
Gemmeke38, A. Gherghel-Lascu68, P.L. Ghia32, U. Giaccari15, M. Giammarchi46,
M. Giller65, D. Głas66, J. Glombitza39, F. Gobbi9, G. Golup1, M. Gómez
Berisso1, P.F. Gómez Vitale9,10, J.P. Gongora9, N. González8, I. Goos1,37, D.
Góra64, A. Gorgi50,49, M. Gottowik35, T.D. Grubb12, F. Guarino56,47, G.P.
Guedes21, E. Guido49,58, R. Halliday80, M.R. Hampel8, P. Hansen4, D. Harari1,
T.A. Harrison12, V.M. Harvey12, A. Haungs37, T. Hebbeker39, D. Heck37, P.
Heimann41, G.C. Hill12, C. Hojvatc, E.M. Holt36,8, P. Homola64, J.R.
Hörandel75,77, P. Horvath31, M. Hrabovský31, T. Huege37,14, J. Hulsman8,37, A.
Insolia54,44, P.G. Isar69, I. Jandt35, J.A. Johnsen81, M. Josebachuili8, J.
Jurysek30, A. Kääpä35, K.H. Kampert35, B. Keilhauer37, N. Kemmerich19, J.
Kemp39, H.O. Klages37, M. Kleifges38, J. Kleinfeller9, R. Krause39, D.
Kuempel35, G. Kukec Mezek71, A. Kuotb Awad36, B.L. Lago16, D. LaHurd80, R.G.
Lang18, R. Legumina65, M.A. Leigui de Oliveira22, V. Lenok37, A. Letessier-
Selvon33, I. Lhenry-Yvon32, O.C. Lippmann15, D. Lo Presti54,44, L. Lopes67, R.
López59, A. López Casado74, R. Lorek80, Q. Luce32, A. Lucero8, M. Malacari87,
G. Mancarella52,45, D. Mandat30, B.C. Manning12, P. Mantschc, A.G. Mariazzi4,
I.C. Mariş13, G. Marsella52,45, D. Martello52,45, H. Martinez60, O. Martínez
Bravo59, M. Mastrodicasa53,43, H.J. Mathes37, S. Mathys35, J. Matthews83, G.
Matthiae57,48, E. Mayotte35, P.O. Mazurc, G. Medina-Tanco63, D. Melo8, A.
Menshikov38, K.-D. Merenda81, S. Michal31, M.I. Micheletti6, L. Middendorf39,
L. Miramonti55,46, B. Mitrica68, D. Mockler36, S. Mollerach1, F. Montanet34,
C. Morello50,49, G. Morlino42,43, M. Mostafá86, A.L. Müller8,37, M.A.
Muller20,d, S. Müller36,8, R. Mussa49, L. Nellen63, P.H. Nguyen12, M.
Niculescu-Oglinzanu68, M. Niechciol41, D. Nitz84,g, D. Nosek29, V. Novotny29,
L. Nožka31, A Nucita52,45, L.A. Núñez28, F. Oikonomou86,i, A. Olinto87, M.
Palatka30, J. Pallotta2, M.P. Panetta52,45, P. Papenbreer35, G. Parente74, A.
Parra59, M. Pech30, F. Pedreira74, J. Pȩkala64, R. Pelayo61, J. Peña-
Rodriguez28, L.A.S. Pereira20, M. Perlin8, L. Perrone52,45, C. Peters39, S.
Petrera42,43, J. Phuntsok86, T. Pierog37, M. Pimenta67, V. Pirronello54,44, M.
Platino8, J. Poh87, B. Pont75, C. Porowski64, R.R. Prado18, P. Privitera87, M.
Prouza30, A. Puyleart84, S. Querchfeld35, S. Quinn80, R. Ramos-Pollan28, J.
Rautenberg35, D. Ravignani8, M. Reininghaus37, J. Ridky30, F. Riehn67, M.
Risse41, P. Ristori2, V. Rizi53,43, W. Rodrigues de Carvalho19, J. Rodriguez
Rojo9, M.J. Roncoroni8, M. Roth37, E. Roulet1, A.C. Rovero5, P. Ruehl41, S.J.
Saffi12, A. Saftoiu68, F. Salamida53,43, H. Salazar59, G. Salina48, J.D.
Sanabria Gomez28, F. Sánchez8, E.M. Santos19, E. Santos30, F. Sarazin81, R.
Sarmento67, C. Sarmiento-Cano8, R. Sato9, P. Savina52,45, M. Schauer35, V.
Scherini45, H. Schieler37, M. Schimassek36, M. Schimp35, F. Schlüter37, D.
Schmidt36, O. Scholten76,14, P. Schovánek30, F.G. Schröder88,37, S.
Schröder35, J. Schumacher39, S.J. Sciutto4, M. Scornavacche8, R.C. Shellard15,
G. Sigl40, G. Silli8,37, O. Sima68,h, R. Šmída87, G.R. Snow89, P. Sommers86,
J.F. Soriano82, J. Souchard34, R. Squartini9, D. Stanca68, S. Stanič71, J.
Stasielak64, P. Stassi34, M. Stolpovskiy34, A. Streich36, F. Suarez8,11, M.
Suárez-Durán28, T. Sudholz12, T. Suomijärvi32, A.D. Supanitsky8, J. Šupík31,
Z. Szadkowski66, A. Taboada37, O.A. Taborda1, A. Tapia27, C. Timmermans77,75,
C.J. Todero Peixoto17, B. Tomé67, G. Torralba Elipe74, A. Travaini9, P.
Travnicek30, M. Trini71, M. Tueros4, R. Ulrich37, M. Unger37, M. Urban39, J.F.
Valdés Galicia63, I. Valiño42,43, L. Valore56,47, P. van Bodegom12, A.M. van
den Berg76, A. van Vliet75, E. Varela59, B. Vargas Cárdenas63, D. Veberič37,
C. Ventura25, I.D. Vergara Quispe4, V. Verzi48, J. Vicha30, L. Villaseñor59,
J. Vink79, S. Vorobiov71, H. Wahlberg4, A.A. Watsona, M. Weber38, A. Weindl37,
M. Wiedeński66, L. Wiencke81, H. Wilczyński64, T. Winchen14, M. Wirtz39, D.
Wittkowski35, B. Wundheiler8, L. Yang71, A. Yushkov30, E. Zas74, D.
Zavrtanik71,72, M. Zavrtanik72,71, L. Zehrer71, A. Zepeda60, B. Zimmermann37,
M. Ziolkowski41, Z. Zong32, F. Zuccarello54,44

1

Centro Atómico Bariloche and Instituto Balseiro (CNEA-UNCuyo-CONICET), San
Carlos de Bariloche, Argentina

2

Centro de Investigaciones en Láseres y Aplicaciones, CITEDEF and CONICET,
Villa Martelli, Argentina

3

Departamento de Física and Departamento de Ciencias de la Atmósfera y los
Océanos, FCEyN, Universidad de Buenos Aires and CONICET, Buenos Aires,
Argentina

4

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

5

Instituto de Astronomía y Física del Espacio (IAFE, CONICET-UBA), Buenos
Aires, Argentina

6

Instituto de Física de Rosario (IFIR) – CONICET/U.N.R. and Facultad de
Ciencias Bioquímicas y Farmacéuticas U.N.R., Rosario, Argentina

7

Instituto de Tecnologías en Detección y Astropartículas (CNEA, CONICET,
UNSAM), and Universidad Tecnológica Nacional – Facultad Regional Mendoza
(CONICET/CNEA), Mendoza, Argentina

8

Instituto de Tecnologías en Detección y Astropartículas (CNEA, CONICET,
UNSAM), Buenos Aires, Argentina

9

Observatorio Pierre Auger, Malargüe, Argentina

10

Observatorio Pierre Auger and Comisión Nacional de Energía Atómica, Malargüe,
Argentina

11

Universidad Tecnológica Nacional – Facultad Regional Buenos Aires, Buenos
Aires, Argentina

12

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

13

Université Libre de Bruxelles (ULB), Brussels, Belgium

14

Vrije Universiteit Brussels, Brussels, Belgium

15

Centro Brasileiro de Pesquisas Fisicas, Rio de Janeiro, RJ, Brazil

16

Centro Federal de Educação Tecnológica Celso Suckow da Fonseca, Nova Friburgo,
Brazil

17

Universidade de São Paulo, Escola de Engenharia de Lorena, Lorena, SP, Brazil

18

Universidade de São Paulo, Instituto de Física de São Carlos, São Carlos, SP,
Brazil

19

Universidade de São Paulo, Instituto de Física, São Paulo, SP, Brazil

20

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

21

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

22

Universidade Federal do ABC, Santo André, SP, Brazil

23

Universidade Federal do Paraná, Setor Palotina, Palotina, Brazil

24

Universidade Federal do Rio de Janeiro, Instituto de Física, Rio de Janeiro,
RJ, Brazil

25

Universidade Federal do Rio de Janeiro (UFRJ), Observatório do Valongo, Rio de
Janeiro, RJ, Brazil

26

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

27

Universidad de Medellín, Medellín, Colombia

28

Universidad Industrial de Santander, Bucaramanga, Colombia

29

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

30

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

31

Palacky University, RCPTM, Olomouc, Czech Republic

32

Institut de Physique Nucléaire d’Orsay (IPNO), Université Paris-Sud, Univ.
Paris/Saclay, CNRS-IN2P3, Orsay, France

33

Laboratoire de Physique Nucléaire et de Hautes Energies (LPNHE), Universités
Paris 6 et Paris 7, CNRS-IN2P3, Paris, France

34

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

35

Bergische Universität Wuppertal, Department of Physics, Wuppertal, Germany

36

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

37

Karlsruhe Institute of Technology, Institut für Kernphysik, Karlsruhe, Germany

38

Karlsruhe Institute of Technology, Institut für Prozessdatenverarbeitung und
Elektronik, Karlsruhe, Germany

39

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

40

Universität Hamburg, II. Institut für Theoretische Physik, Hamburg, Germany

41

Universität Siegen, Fachbereich 7 Physik – Experimentelle Teilchenphysik,
Siegen, Germany

42

Gran Sasso Science Institute, L’Aquila, Italy

43

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

44

INFN, Sezione di Catania, Catania, Italy

45

INFN, Sezione di Lecce, Lecce, Italy

46

INFN, Sezione di Milano, Milano, Italy

47

INFN, Sezione di Napoli, Napoli, Italy

48

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

49

INFN, Sezione di Torino, Torino, Italy

50

Osservatorio Astrofisico di Torino (INAF), Torino, Italy

51

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

52

Università del Salento, Dipartimento di Matematica e Fisica “E. De Giorgi”,
Lecce, Italy

53

Università dell’Aquila, Dipartimento di Scienze Fisiche e Chimiche, L’Aquila,
Italy

54

Università di Catania, Dipartimento di Fisica e Astronomia, Catania, Italy

55

Università di Milano, Dipartimento di Fisica, Milano, Italy

56

Università di Napoli “Federico II”, Dipartimento di Fisica “Ettore Pancini”,
Napoli, Italy

57

Università di Roma “Tor Vergata”, Dipartimento di Fisica, Roma, Italy

58

Università Torino, Dipartimento di Fisica, Torino, Italy

59

Benemérita Universidad Autónoma de Puebla, Puebla, México

60

Centro de Investigación y de Estudios Avanzados del IPN (CINVESTAV), México,
D.F., México

61

Unidad Profesional Interdisciplinaria en Ingeniería y Tecnologías Avanzadas
del Instituto Politécnico Nacional (UPIITA-IPN), México, D.F., México

62

Universidad Autónoma de Chiapas, Tuxtla Gutiérrez, Chiapas, México

63

Universidad Nacional Autónoma de México, México, D.F., México

64

Institute of Nuclear Physics PAN, Krakow, Poland

65

University of Łódź, Faculty of Astrophysics, Łódź, Poland

66

University of Łódź, Faculty of High-Energy Astrophysics,Łódź, Poland

67

Laboratório de Instrumentação e Física Experimental de Partículas – LIP and
Instituto Superior Técnico – IST, Universidade de Lisboa – UL, Lisboa,
Portugal

68

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

69

Institute of Space Science, Bucharest-Magurele, Romania

70

University Politehnica of Bucharest, Bucharest, Romania

71

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

72

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

73

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

74

Instituto Galego de Física de Altas Enerxías (I.G.F.A.E.), Universidad de
Santiago de Compostela, Santiago de Compostela, Spain

75

IMAPP, Radboud University Nijmegen, Nijmegen, The Netherlands

76

KVI – Center for Advanced Radiation Technology, University of Groningen,
Groningen, The Netherlands

77

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

78

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

79

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

80

Case Western Reserve University, Cleveland, OH, USA

81

Colorado School of Mines, Golden, CO, USA

82

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

83

Louisiana State University, Baton Rouge, LA, USA

84

Michigan Technological University, Houghton, MI, USA

85

New York University, New York, NY, USA

86

Pennsylvania State University, University Park, PA, USA

87

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

88

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

89

University of Nebraska, Lincoln, NE, USA

—–

a

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

b

Max-Planck-Institut für Radioastronomie, Bonn, Germany

c

Fermi National Accelerator Laboratory, USA

d

also at Universidade Federal de Alfenas, Poços de Caldas, Brazil

e

Colorado State University, Fort Collins, CO, USA

f

now at the Hakubi Center for Advanced Research and Graduate School of Science
at Kyoto University

g

also at Karlsruhe Institute of Technology, Karlsruhe, Germany

h

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

i

now at European Southern Observatory, Garching bei München, Germany

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