Multi-Messenger studies with the Pierre Auger Observatory
MMulti-Messenger studies with thePierre Auger Observatory
Lukas Zehrer 𝑎, ∗ for the Pierre Auger Collaboration 𝑏, † 𝑎 University of Nova Gorica,Vipavska 13, 5000 Nova Gorica, Slovenia 𝑏 Observatorio Pierre Auger,Av. San Martiń Norte 304, 5613 Malargüe, Argentina
E-mail: [email protected]
Over the past decade the multi-messenger astrophysics has emerged as a distinct discipline,providing unique insights into the properties of high-energy phenomena in the Universe. ThePierre Auger Observatory, located in Malargüe, Argentina, is the world’s largest cosmic raydetector sensitive to photons, neutrinos, and hadrons at ultra-high energies. Using its data, stringentlimits on photon and neutrino fluxes at EeV energies have been obtained. The collaboration usesthe excellent angular resolution and the neutrino identification capabilities of the Observatoryfor follow-up studies of events detected in gravitational waves or other messengers, throughcooperation with global multi-messenger networks. We present a science motivation togetherwith an overview of the multi-messenger capabilities and results of the Pierre Auger Observatory. ∗ Speaker † Full author list available at: © Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/ a r X i v : . [ a s t r o - ph . H E ] F e b ulti-Messenger studies with the Pierre Auger Observatory Lukas Zehrer
1. Introduction
The earliest known messengers taking part in the strong interaction at the very high energies(VHE, 100 GeV to 100 TeV) are called cosmic rays (CRs), which were detected in 1912 by VictorHess with his balloon experiment. For the messengers of the electromagnetic (EM) force at theseenergies the first photons 𝛾 were reported in [1], whilst the messengers that undergo the weakinteraction, neutrinos 𝜈 , were first reported in [2, 3]. In 2016 the first signals from the mergingof two black hole (BH) systems, GW150914 [4], were reported. The detection of gravitationalwaves (GWs) from the merging of binary systems has not only marked the birth of gravitationalwave astronomy, but enabled to combine the messenger particles of all four of nature’s fundamentalforces and thereby completed multi-messenger (MM) astrophysics.The Pierre Auger Observatory [5] was designed with a main purpose to study the properties ofCRs with unprecedented accuracy up to the highest energies. As it is shown in the presented resultsand plots in the following sections, the Pierre Auger collaboration is also able to investigate othermessengers, like neutrinos, photons and galactic neutrons, is complementary to other observatoriesfor those messengers, and has the capabilities to set the most stringent limits in certain ranges ofenergy.
2. Neutrinos
Concerning their physical properties, neutrinos are the ideal messenger to study the highenergy universe, and searches at ultra-high energy (UHE; > 0.1 EeV) for these particles have beenundertaken several times by the Pierre Auger Observatory [6, 7].The search for neutrino candidate events is performed separately for Earth-skimming (ES,zenith angle range 90 ◦ ≤ 𝜃 ≤ ◦ ) and down-going low (DGL, 60 ◦ ≤ 𝜃 ≤ ◦ ) and high (DGH,75 ◦ ≤ 𝜃 ≤ ◦ ) neutrino-induced showers [8, 9]. The ES is the most effective channel for neutrinodetection, since the target mass provided by the Earth is large compared to the atmosphere and alsobecause the threshold energy is low.The discrimination from other primary particles is based on the fact that neutrinos can poten-tially interact at any point in the atmosphere for any zenith angle, which means they are, in contrastto all other particles, able to interact very deep in the atmosphere. This leads to different signalsin the Pierre Auger Surface Detector (SD) array, than in the case of UHECR-induced extensive airshowers (EASs) [6]. The implications of the non-observation of UHE neutrinos are upper limits on diffuse fluxes,shown in figure 1 a), and upper limits on flux from point-like sources of neutrinos, as can be seenin fig. 1 b).The limit on diffuse fluxes is moving toward a physics scenario, which disfavours protonprimary and strong evolution of the sources with redshift. In turn, a scenario of a mixed or heavycomposition with weak evolution becomes more likely, which implies a suppressed neutrino flux.It can also be seen, that the differential upper limit on the diffuse neutrino flux of the Pierre AugerObservatory is comparable at EeV energies to the limits given from IceCube.2 ulti-Messenger studies with the Pierre Auger Observatory
Lukas Zehrera) b) E (eV) E d N / d E ( G e V c m s s r ) Single flavor e : : =1:1:1Waxman-Bahcall bound (2015)90% CL limitAuger (2019)90% CL limitAuger (2019) 90% CL limit Auger(Earth-Skimming)90% CL limitIceCube (2018) 90% CL limitANITA I+II+III (2018) AGN (Murase 2014)Pulsars SFR evolution (Fang 2014)Cosmogenic: p SFR (Aloisio 2015)Cosmogenic: p, Fermi-LAT, E min =3×10 eV (Ahlers 2010)Cosmogenic: p, FRII & SFR source evol. (Kampert 2012)Cosmogenic: p or mixed, SFR & GRB (Kotera 2010)Cosmogenic: Fe, FRII & SFR source evol. (Kampert 2012)
90 75 60 45 30 15 0 15 30 45 60 75 90
Declination (deg) E d N / d E ( G e V c m s ) Single flavor e : : =1:1:1 Northern skySouthern sky 10 eV < E <2×10 eV10 eV E eV 10 eV E eV E <10 eV Upper limits 90% CL
Auger 2018Auger Earth-Skimming Auger DGH 75 o < <90 o Auger DGL 60 o < <75 o IceCube 2017ANTARES 2017
Figure 1: a)
Upper limit on diffuse fluxes, compared to the differential limits obtained by other experimentsand astrophysical and cosmogenical neutrino models, showing in red the integral and differential upper limitfor the normalisation constant assuming a 𝐸 − energy flux for a single flavour for all the three neutrinochannels. The dashed red line is the Earth skimming channel alone, which contributes the strongest tothe limit [7]. b) Auger 90% CL upper limit on the fluxes of point-like sources as a function of equatorialdeclination obtained from the non-observation of ES and DGH neutrino candidates. Note the different energyranges where the limits of each observatory apply. See [6] and references therein.
Neutrinos from point-like sources across the sky, with peak sensitivities at declination around − ◦ and + ◦ , can be detected with the SD array. Since no neutrinos have been identified, upperlimits on the neutrino flux from point-like steady sources have been calculated as a function ofdeclination (see fig. 1 b)). In the plot, upper limits from the Pierre Auger Observatory for a singleflavour point-like flux of UHE neutrinos is shown as a function of declination together with IceCubeand ANTARES upper limits at lower energies. The Auger default neutrino search was applied around ±
500 s and until 1 day after the mergerinside a 90% C.L. of most likely source location in the sky. The results for GW150914, the firstGW event from a compact binary coalescence ever detected, are shown in fig. 2 a). The optimaldeclination positions 𝛿 (cid:39) − ◦ and 𝛿 (cid:39) ◦ , near the extremes in declination of the ES band, canbe seen.The field of view of the observatory at any given instant for each of the ES, DGH and DGLchannels is limited to the bands corresponding to the zenith angle range of the channel, as displayedin fig. 2 b) [10].As can be seen in fig. 3 a), Auger provides the most stringent upper limit to the neutrino fluenceat 90% C.L. in the range from 100 PeV to 25 EeV to the GW event 170817, complementary toIceCube and ANTARES [11]. This GW event was produced by the merging of two neutron stars(NSs). In a time interval of ±
500 s around the NS merger its location was in the optimal Auger ESanalysis window (93.3 ◦ < 𝜃 < . ◦ ).In fig. 3 b) the Auger field of view for ES and DG neutrino channels is shown for the eventGW170817, altogether with the merger progenitor NGC4993 sky location, and the sky positions of3 ulti-Messenger studies with the Pierre Auger Observatory Lukas Zehrera) b)
Figure 2: a)
In black the upper limit to the neutrino spectral fluence in the 100 PeV to 25 EeV range asa function of the equatorial source declination 𝛿 , for the detection of the BH merger GW150914 is shown.The blue band is the 90% C.L. sky localization of the reconstructed source [12]. b) Field of view of theObservatory at the instant of detection of the BH coalescence event GW150914 by LIGO, with band limitsto separate the neutrino search into ES, DGH, and DGL channels, together with the 90% C.L. region of thereconstructed position of the BH merger as obtained by LIGO observations (in black contours) [10]. a) b) E F [ G e V c m ] ±500 sec time-windowAuger IceCube
ANTARES
Kimura et al.EE optimistic048 Kimura et al.EE moderate0 4 Kimura et al.prompt0
GW170817 Neutrino limits (fluence per flavor: x + x ) E/GeV E F [ G e V c m ]
14 day time-window Auger
IceCube
ANTARES
Fang &Metzger30 daysFang &Metzger3 days
Figure 3: a) Top : 90% C.L. upper limits on the neutrino spectral fluence during a ±
500 s window as afunction of energy in black angular lines.
Bottom : Idem for a 14-day window following the GW trigger.The coloured smooth lines represent a model of UHE neutrino spectral fluences [11]. b) The sky locationin equatorial coordinates of the GW170817 event (red contour) and its progenitor NGC4993, altogether withthe Auger sensitive sky areas for neutrino detection at the time of the merger [11]. neutrino candidate events that have been detected within 500 s of the merger.
3. UHE Photons
Photons at UHEs are considered the key to reveal the origin of the most energetic cosmicrays. A flux of UHE photons is expected to be produced from the interaction of nuclei, that canpropagate over several Mpc, with the cosmic microwave background (CMB). Photons can also4 ulti-Messenger studies with the Pierre Auger Observatory
Lukas Zehrer
Figure 4:
Limits on the photon flux at 95% C.L. of the Pierre Auger Observatory, compared to modelpredictions and to the limits by other experiments [13]. help to distinguish between scenarios of CR production via astrophysical particle acceleration("bottom-up") vs. exotic models ("top-down").Primary photons produce mostly EM showers with minor photo-nuclear or muon-pair pro-duction. With respect to protons and nuclei, EASs induced by photons have a deeper showerdevelopment and a smaller muon content. As a consequence, they differ also in other observablecharacteristics such as a steeper lateral distribution function, a smaller number of triggered stations,a slower rise of the signal in the SD stations on the ground, and a broader time front.
With the Pierre Auger Observatory stringent limits are set on the diffuse flux of UHE photons.Above 10 EeV, SD data collected between 2004 and mid-2018 with an exposure of 40,000 km sr yrare used, and below 1 EeV unprecedented separation power between primary photons and hadronscan be achieved by combining observables from low-energy enhancements of the Pierre AugerObservatory. Altogether, the range of photon searches at the Pierre Auger Observatory extends toabout three decades in energy.Various variables with separation power are combined in a multivariate analysis (MVA) usinga Boosted Decision Tree (BDT). Above 10 EeV, in a zenith angle range of 30 ◦ < 𝜃 < ◦ , 11 eventswere found to be above the threshold, two of them at an energy above 20 EeV, but conservativelyan upper limit on the photon flux at 95% C.L. was determined. The resulting upper limits onthe integral photon flux for the thresholds of 0.2, 0.3, 0.5, and 1 EeV at 95% C.L. can be seen infig. 4 [13]. The Pierre Auger Observatory is thus the most sensitive air-shower detector for primaryphotons with energies above ∼
4. Galactic Neutrons
Neutron-induced EASs cannot be distinguished from the ones induced by charged CRs on thebasis of the shower development. Nevertheless, due to their non-deflected paths, it is in principlepossible to determine sources of neutrons by identifying an excess from given directions, or byexploiting potential correlations in time and direction. Since the mean travel distance for relativistic5 ulti-Messenger studies with the Pierre Auger Observatory
Lukas Zehrerneutrons with energies 𝐸 𝑛 of a few EeV is 9.2 kpc · 𝐸 𝑛 / EeV, the distance from Earth to the Galacticcenter is about 8.3 kpc, and the radius of the Galaxy is approximately 15 kpc, neutrons at theseUHEs can reach Earth from the entire Galaxy but not from much further away [10].The choice of photon sources as probable neutron sources is motivated by the fact that bothmessengers are produced in photo-hadronic interaction scenarios. No significant excess of a neutronflux has been found in the searches from any class of candidate sources, however, strong limits at95% C.L. upper limits on the energy flux in neutrons have been deduced, thereby setting strongconstraints on UHE proton production in our galaxy [10].
5. Outlook
References [1] N. R. Ikhsanov,
Astrophys. Space Sci. (1991) 297-311.[2] R. M. Bionta et al. , Phys. Rev. Lett. (1987) 1494.[3] K. Hirata et al. , Phys. Rev. Lett. (1987) 1490.[4] B. P. Abbott et al. , Phys. Rev. Lett. (2016) 061102.[5] A. Aab et al. (The Pierre Auger Collaboration),
Nucl. Instrum. Meth. A (2015) 172.[6] A. Aab et al. (The Pierre Auger Collaboration),
JCAP (2019) 004 [arXiv:1906.07419].[7] A. Aab et al. (The Pierre Auger Collaboration), JCAP (2019) 022 [arXiv:1906.07422].[8] A. Aab et al. (The Pierre Auger Collaboration), Phys. Rev. D (2015) 092008.[9] P. Abreu et al. (The Pierre Auger Collaboration), Phys. Rev. D (2011) 122005.[10] A. Aab et al. (The Pierre Auger Collaboration), Front. Astron. Space Sci. (2019).[11] A. Albert et al. (the ANTARES, IceCube, and Pierre Auger collaborations), Astrophys. J. (2017) L35.[12] A. Aab et al. (The Pierre Auger Collaboration), Phys. Rev. D (2016) 122007.[13] J. Rautenberg for the Pierre Auger Collab., PoS
ICRC2019
398 [arXiv:1909.09073].[14] A. Aab et al.et al.