Searching for multi-messenger signals with the Pierre 1 Auger Observatory

. The Pierre Auger Observatory [1], primarily designed for the de-6 tection of ultra-high energy (UHE) cosmic rays, has been proven to be also 7 an excellent tool in multi-messenger searches. With its unprecedented expo-8 sure to UHE particles, it is exploited to set stringent upper limits on the di ff use 9 flux of UHE photons and neutrinos and to look for neutral particles associated 10 with steady sources and transient events, such as gravitational waves. All these 11 searches can provide key information to investigate the most energetic phenom-12 ena in the Universe and answer some of the most important still-open questions 13 in astrophysics.


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Multi-messenger astronomy aims at combining the information from any particle and radia-16 tion coming from astrophysical objects to get a deeper insight on their nature. Such studies 17 underwent a significant boost after the first detection of gravitational waves and the rise of 18 neutrino astronomy [2,3]. 19 The Pierre Auger Observatory, the largest experiment in the world for the detection of 20 ultra-high energy particles, exploits the properties of air showers to identify the ones induced 21 by UHE photons and neutrinos among the hadronic background. Since many models for the 22 origin of UHE cosmic rays predict the production of neutral particles either at the sources or 23 during the propagation through the Universe, the current upper limits have already set strong 24 constraints on them. Moreover, neutral particles directly point back to their production site, 25 which allows their association with specific sources or events. An overview of the on-going 26 multi-messenger activities within the Pierre Auger Collaboration is here presented. 27 2 Searches for photons 28 In many models UHE photons originate from the decay of neutral pions produced by the 29 interactions of protons with the Cosmic Microwave Background (CMB) photons and/or with 30 photon fields within the source environment. However, since UHE photons also interact 31 during their propagation and may be eventually absorbed, only distances to about a few tens 32 of kpc can be explored with photons around 10 15 eV, which rises to few Mpc around 10 19 eV.

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The identification of photon-induced air showers relies on the fact that they are dominated 34 by electromagnetic interactions, hence they exhibit a deeper maximum development in the 35 atmosphere X max , which is discriminated with the Fluorescence Detector (FD), and a smaller 36 amount of muons at the ground, which is associated with a slower-rising signal in the single 37 SD stations. Both the deeper X max and the lower number of muons are reflected in a steeper

A Targeted Search for Point Sources of EeV Photons with the Pierre Auger Observatory
The targeted search for point sources of UHE photons follows the same analysis logic as the blind search summarized before, albeit with a larger dataset (January 2005 to December 2013). To reduce the statistical penalty of looking at all directions in the visible sky, the targeted search is restricted to 12 predefined target classes, containing 364 targets in total. Since the attenuation length of photons in the energy range considered here (10 17.3 to 10 18.5 eV, same as before) varies between 90 and 900 kpc [43], these target classes contain    figure 6). We note, however, that the possibility of pure proton (or iron) primaries in the energy range of interest is disfavored by recent results on the composition of UHECR [12, 13, 66-68]. Instead, a gradually increasing fraction of heavier primaries is observed with increasing energy up to at least E ⇠ 5 ⇥ 10 19 eV [66]. In addition to this, adopting a simple astrophysical model fitting the energy spectrum and the mass composition suggests that the observed flux suppression is primarily an e↵ect of the maximum rigidity of the sources of UHECR rather than only the e↵ect of energy losses in the CMB and EBL [73,74]. In consequence, cosmogenic neutrino fluxes would be reduced much further and may escape detection for the foreseeable future [21,22,75]. Thus, fluxes of cosmogenic neutrinos provide an independent probe of source properties and of the origin of the UHECR flux suppression at the highest energies.
In table 2, we show the expected number of events in the present lifetime of the Observatory for several cosmogenic neutrino models and the associated Poisson probability of observing no events. Scenarios assuming sources that accelerate only protons and that have -13 -   Observatory. The search for ES showers is performed considering the average signal Area 118 over Peak over the triggered stations, which is related to the broadness of the shower front.

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As concerns DG showers, the neutrino selection is performed with a multivariate analysis 120 by using different observables depending on the zenith angle, which are combined together 121 in a Fisher discriminant. Assuming a differential flux ϕ = k × E −2 , the upper limit at 90% 122 C.L. is set through the normalization k and is shown in Fig. 3. The integrated limit to k is Limits for the particular case of the active galaxy Centaurus A, a potential source of UHECRs, are shown in Fig. 9, together with constraints from other experiments. CenA at a declination δ ⇠ −43 is observed ⇠ 7% (⇠ 29%) of one sidereal day in the range of zenith angles corresponding to ES (DG) events. The predicted fluxes for two theoretical models of UHE ⌫-production -in the jets [47] and close to the core of Centaurus A [48] -are also shown. We expect ⇠ 0.7 events from Cen A for the flux model in [47] and ⇠ 0.025 events for the model in [48]. However, there are significant uncertainties in this model that stem from the fact that the neutrino flux is normalized to the UHECR proton flux assumed to originate from CenA, which is uncertain.

Discussion and Conclusions
The search for point sources of neutrinos with data from the Surface Detector Array of the Pierre Auger Observatory relies on selecting showers with large zenith angles in three di↵erent angular ranges where searches with di↵erent sensitivities are performed. The sensitivity of the Observatory to transient sources of UHE neutrinos is demonstrated using the e↵ective -13 -