Multi-messenger Astrophysics with the Pierre Auger Observatory
MMulti-messenger Astrophysics with the Pierre AugerObservatory
Michael Schimp for the Pierre Auger Collaboration Bergische Universit¨at Wuppertal, Gaußstr. 20, 42119 Wuppertal, Germany Observatorio Pierre Auger, Av. San Mart´ın Norte 304, 5613 Malarg¨ue, Argentina(Full author list: http: // / archive / authors 2019 02.html)E-mail: auger [email protected](Received January 27, 2021)While the Pierre Auger Observatory is a very successful instrument for ultra-high energy cosmicray (UHECR) detection, it is increasingly used as part of various types of multi-messenger searches,in which it contributes with searches for air showers induced by atomic nuclei, neutrons, photons,and neutrinos. We present an overview of the multi-messenger activities of the Pierre Auger Ob-servatory. The overview includes: searches for ultra-high energy photons and neutrinos detected bythe Pierre Auger Observatory in coincidence with gravitational wave events detected by LIGO andVirgo; searches for correlations of the arrival directions of UHECRs detected by the Pierre AugerObservatory and high-energy neutrinos detected by IceCube and ANTARES; searches for Galacticneutrons; the multi-messenger campaign “Deeper, Wider, Faster”, aiming for common observationsof a variety of complementary instruments. We discuss the motivations, methods and results of thesesearches. KEYWORDS: multi-messenger astrophysics, Pierre Auger Observatory, neutrinos, photons,ultra-high energy cosmic rays, neutrons
1. Introduction
Similar to multi-wavelength observations in photon-based astronomy that have substantially broad-ened the general astrophysical understanding since the 1950s, multi-messenger astrophysics has al-ready provided unique astrophysical insights that otherwise would not have been possible. An earlyexample is the solar storm of 1859, also known as the Carrington Event, establishing the exis-tence of solar flares. It was visible as very bright white light (messenger: photons) close to a setof sunspots [1, 2] and additionally in the form of unusually strong auroras (messenger: cosmic rays,inducing the auroras) that have been observed even at low latitudes, for example below 9 ◦ N in Colom-bia [3].The most remarkable recent discoveries in multi-messenger astrophysics were made in the con-text of the detection of gravitational waves (GWs) and gamma-rays from a binary neutron star (BNS)merger [4], triggering a large search campaign for photons across a very wide range in the electro-magnetic (EM) spectrum as well as searches for other messengers [5], consequently leading to theobservation of a kilonova as a counterpart of the BNS merger [6]. Another so-far unique astrophys-ical multi-messenger observation is the detection of a high energy neutrino from the distant blazarTXS-0506 +
056 in coincidence with a flare of high energy photons in 2017 [7, 8]. Investigations ad-ditionally revealed a period of significantly enhanced neutrino emission from this source in 2014 and2015 [9], corroborating the assumption of TXS-0506 +
056 being a high-energy neutrino source.The Pierre Auger Observatory is the largest cosmic ray detector in the world, regularly used fordetections of extensive air showers (EASs) induced by ultra-high-energy cosmic-rays (UHECRs), a r X i v : . [ a s t r o - ph . H E ] J a n tomic nuclei roughly on an EeV energy scale. As these are of extragalactic origin [10], they travellong distances through magnetic fields from their sources to the Earth, changing their directions, andtherefore can not in general be used as precise pointers indicating their origin.Assuming that other messengers are created at the acceleration sites of UHECRs, or during theirpropagation, multi-messenger astrophysical observations are a promising approach to answer thelong-standing questions regarding the sites and mechanisms of ultra-high-energy particle accelera-tion in the universe. In the following sections, we review various contributions of the Pierre AugerObservatory to multi-messenger astrophysical observations.
2. Ultra-High Energy Neutrino and Photon Follow-Up Searches of LIGO / VirgoGravitational Wave Events
Searches for ultra-high energy (UHE; > . ff erently in the atmosphere.Neutrinos interact deeper in the atmosphere than the other particles, leading to more EM and hadronicparticles reaching the Pierre Auger Surface Detector (SD) than in the case of UHECR-induced EASs,eventually leaving longer-lasting light signals in the photomultiplier tubes (PMTs). This distinctionis more precise and e ffi cient for inclined showers, leading to a severely varying sensitivity acrossthe sky [12]. Photon-induced EASs are distinguished from others based on several measures such assteeper lateral distribution functions, deeper shower maxima, and smaller footprints.A multi-messenger search performed with photons and neutrinos at the Pierre Auger Observatoryis the follow-up of gravitational wave (GW) events detected by the LIGO and Virgo (LV) observato-ries. These were caused by the coalescence and merger of compact binaries, mostly of binary blackholes (BBHs) but also of a binary neutron star (BNS).Before VHEPA 2019, during the first two observational runs LV, called O1 and O2, GW event in-formation was sent in form of alerts only to parties that signed a memorandum of understanding withthe LV collaborations, one of which was the Pierre Auger Collaboration. Each such alert containedthe following estimated parameters if applicable: • Time of merger • Masses of merged objects and remnant • Distance of emitting system • Sky localization probability distributionThe neutrino follow-up searches were performed by applying the default neutrino search insidethe 90% C.L. most probable localization region in the sky during a time range from 500 s before until1 day after the merger. No neutrino candidates have been found for any of the GW events during O1and O2.For each of the BBH mergers, the exposure is used to calculate limits on the UHE neutrino fluenceas a function of declination of the true source, which is often known with a very limited precision(10s of degrees). As an example, Fig. 1 shows the results for GW150914 [15], the first GW eventfrom a compact binary coalescence that has ever been detected [16].One of the GW events in O2, GW170817, has been associated with the coalescence of two objectswith masses in the typical neutron star mass range ( ∼ M (cid:12) ) [4]. Follow-up observations of thisevent with photons yielded signatures of a kilonova caused by a BNS merger at the inferred locationof the GW event via detections in a very large wavelength range [5, 6]: Less than two seconds afterthe merger, a short GRB was observed, whereas at various lower photon energies, light curves of the ig. 1. Black lines represent the 90% C.L. limit on the UHE neutrino fluence from GW150914 as a func-tion of the source declination. Declinations of the 90% C.L. sky localization of the source are highlighted inblue. [15] source have been recorded for several weeks. Therefore, in agreement with IceCube, the time rangeof the search for ultra-high energy neutrinos with the Pierre Auger Observatory from this source wasextended until 14 days after the merger. Fig. 2 shows the fluence limits (90% C.L.) for the time rangesof ±
500 s around the merger, and 0 to 14 days after the merger, respectively [17]. As the UHE neutrinosensitivity of the Pierre Auger Observatory is varying across the sky, and the BNS merger occurredat a time and location with a large sensitivity, the fluence limits for the ±
500 s time range are muchmore competitive than for the 14 day time window, where any short-term sensitivity enhancementsare averaged out due to the moving field of view of the observatory.The photon follow-up searches are unpublished yet. In addition to the process described for neu-trinos, only GW events that are relatively close by or well localized will be taken into account in orderto prevent false-positive detections.
3. Searches for Correlations between Ultra-High Energy Cosmic Rays andHigh-Energy Neutrinos
UHECRs are subject to deflection in magnetic fields due to their charge, which makes find-ing their sources di ffi cult. However, as the deflection decreases with energy, correlations betweenparticularly high-energy UHECRs and high-energy neutrinos can be expected. To search for thesecorrelations, UHECRs with energies (cid:38)
50 EeV detected by the Pierre Auger and the Telescope Arrayobservatories, and high-energy neutrinos detected by IceCube, were analyzed in a joint work of thethree collaborations [18].Two analyses of the correlations between these UHECRs and high-energy neutrinos have beenapplied: a cross-correlation analysis and a stacking likelihood analysis. The neutrino sample contains ig. 2.
90% C.L. limits on the UHE neutrino spectral fluence from GW170817 as a function of energy areshown as black angular lines. Model UHE neutrino spectral fluences are represented by the colored smoothlines with the denoted o ff -axis angles of the merger system. [17] track-like and cascade-like events which are analyzed separately. Taking an isotropic UHECR fluxas the null hypothesis, for track-like neutrino events, both analyses yielded no significant results.However, for cascade-like neutrino events, both analyses yielded significant excesses of correlations.The most significant excess in the cross-correlation analysis was found for a maximum angularseparation of 22 ◦ , with a post-trial p -value of 5 . · − . Fig. 3 shows the excess of pairs for the cross-correlation analysis as a function of the maximum angular separation [18]. In the stacking likelihoodanalysis, the most significant excess was found for a UHECR deflection of ◦ E / , with a post-trial p -value of 2 . · − . Assuming, alternatively, a null hypothesis of an isotropic high-energy neutrinoflux, very similar levels of confidence for the excesses were found [18].In [19], a study published after VHEPA 2019, including also the ANTARES neutrino telescope,deviations from the null hypotheses were found to be much weaker than in [18], indicating that thecorrelations searched for are not very strong. This can be explained by several factors, e.g. uncer-tainties of the magnetic fields responsible for UHECR deflection, and the fact that UHECRs detectedat Earth come from not more than a few 100 Mpc away, while possibly a large fraction of the de-tected neutrinos originates from much further distances. For these neutrinos, a correlation would beunexpected, diluting the overall e ff ect. ig. 3. Relative excesses of pairs as a function of the maximum angular separation are shown for the cross-correlation analysis performed with cascade-like events. Hatched areas indicate expected fluctuations fromisotropically distributed UHECRs. [18]
4. Search for a flux of Ultra-high Energy Neutrons from the Galaxy
Individual neutron-induced EASs are indistinguishable from those induced by protons. However,as neutrons travel in straight lines, a flux of neutrons could be detected via an excess of the numberof EASs from the directions of their sources. As the neutron decay length is ∼ . E / EeV [20],neutrons with energies of a few EeV can reach the Earth from the entire Galaxy but not from muchfurther away. Therefore, 11 classes of sources in the Galaxy have been used as combined target setsfor neutron searches with the Pierre Auger Observatory [21].The chosen target sets are the Galactic center and plane as well as known photon source classeslike pulsars and X-ray binaries, amounting several hundred sources in total. The choice of photonsources as probable neutron sources is motivated by the fact that both messengers are produced inphoto-hadronic interaction scenarios.The searches in the first 9.75 years of data taken by the observatory have yielded no significantexcess of the number of EASs from the target sets with respect to the neutron-free expectation. How-ever, from the sensitivity to the target sources, 95% C.L. upper limits on the energy flux in neutronsof 0.1 eV cm − s − (Galactic center) through 0.56 eV cm − s − (Galactic plane) have been deducedfor the di ff erent target sets. In all cases, these limits already exclude energy fluxes on the level of themeasured TeV photon energy flux from the target sets [21, 22].At energies of a few EeV, the UHECR composition is consistent with a large proton fraction [23].The strong limit on the neutron flux – in particular that it is much lower than the TeV photon flux –leads to the exclusion of an E − Fermi-acceleration of protons up to energies of several EeV. Ref. [21]also provides an interpretation in terms of an e ff ective neutron-to-proton luminosity ratio from theGalactic plane. For this, several theoretical assumptions regarding the proton emission and neutronproduction e ffi ciency are made and related to the neutron flux limits, finally yielding a 95% C.L.upper limit for this ratio of 0 . . Ultra-High Energy Cosmic Rays for the Deeper, Wider, Faster program The Deeper, Wider, Faster (DWF) program is a multi-instrument multi-messenger observationproject. With more than 30 associated observatories involved, the purpose of DWF is the simultane-ous and common observation of sky regions with a large number of instruments at the same time,combining their complementary sensitivities. This allows for simultaneous sensitive and wide-fieldmeasurements of multiple messengers in a variety of energy ranges [24].The observations aim for transient sources, such as fast radio bursts, which last less than a secondand are therefore barely possible to follow up with instruments that are pointing in other directionsand would first need to be adjusted to the region of interest. Furthermore, the simultaneous commonobservation allows to observe transient sources directly before their enhanced emission, which isnaturally missed by said instruments that need to adjust their pointing. At high energies, DWF is alsoused to search for transients lasting seconds to hours. Candidate identification is possible in secondsto minutes and a fast respond time of a few minutes allows for further follow-up observations with ashort delay.The Pierre Auger Observatory is contributing to DWF by sharing all detected events in the field ofview of DWF. The large field of view of the Pierre Auger Observatory lets it contribute to DWF duringa large fraction of the observation time. So far, no significant coincidences of UHECR events withother events in DWF have been found. However, the possibility of detecting such coincidences makesDWF an interesting approach for multi-messenger observations with unprecedented combinations ofmessengers, including UHECRs.
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