The recent LHAASO observations have unveiled the presence of extremely powerful accelerators in the Galactic Plane, emitting radiation with energy well above 100 TeV, possibly responsible for the cosmic-ray (CR) spectrum up to 1 PeV, and beyond. The localization of such sources will allow to identify the PeVatrons once the physical mechanism at the origin of the observed radiation will be firmly established. To this extent, neutrinos constitute unambiguous probes of proton acceleration and in situ interaction. Furthermore, the simultaneous observation of gravitational waves (GWs) might bring a deeper knowledge about the mechanism operating the inner engine, yielding clear information on the nature of the source.
These arguments show the importance of tackling simultaneously the wealth of information that arise from the different messengers, namely CRs, neutrinos, photons and GWs. The combined study of the Universe with all the aforementioned probes offers unique opportunities, as demonstrated by the observation of the merging of two neutron stars in 2017. The prompt finding of a GW and of a Gamma-Ray Burst (GRB) was followed by the most extensive worldwide observational campaign, using about 70 observatories on all continents and in space. This key event in science was possible thanks to the fast dissemination of information among possible partners. In such a framework, we plan to combine measurements from several observatories (LHAASO, ANTARES, KM3NeT, LIGO-Virgo) in synergical energy ranges, and perform tailored data analyses, supplemented by a solid interpretation of the physical mechanism at their origin. In particular, we will explore the 12 recently detected LHAASO extremely energetic sources, in order to shed light on the nature of such ground-breaking emission.
High-energy astrophysics is now studied using at least three experimental branches: the electromagnetic radiation, from radio to gamma rays; charged CRs and nu's, detected with experimental methods developed in particle physics; and GWs, observed using laser interferometers. The observation of the electromagnetic counterpart of GW transient GW170817 demonstrated the potential in extracting astrophysical information from multimessenger discoveries.
The success of the observational campaign for this event, which marked the start of multimessenger astronomy with GWs, shows the importance of coordinated follow-up observations and of the strategies to carry out them. The network of experiments sending/receiving alerts is composed by space experiments, ground-based telescopes for different radiation wavelengths, CR experiments, neutrino telescopes, GW interferometers. The rapid provision of alerts for cosmic neutrino or gamma rays will enable both ground and space based observatories with limited solid angle visibility to quickly point in the correct direction of the sky. A fast follow-up is crucial to identify any multi-messenger and multi-wavelength counterparts of cosmic short-lived phenomena such as GRBs, blazar flares, supernova shock breakout, and other still unknown phenomena. The positive results of IceCube on TXS 0506+056 [1] is a breakthrough showing the power of this method [2-5].
The LHAASO observatory [6], which is currently taking data even though it is still under construction, is expected to be the most sensitive extended air shower array, operating combined studies between CRs and gamma rays beyond 100 GeV. Recent LHAASO observations have unveiled 12 extreme accelerators in our Galaxy [7-9]. In particular, among these sources, stellar clusters have emerged, e.g. the LHAASO J2032+4102 being positionally coincident with the Cygnus Cocoon, which might possibly suggest the capability of massive stars to operate as PeVatrons. The Cygnus region has also been suggested as a candidate neutrino source [10], hence a spatially coincident detection of neutrinos will be crucial to confirm the hadronic nature of the observed radiation. In addition to MSCs, other candidates for addressing the origin of the extremely energetic gamma rays measured by LHAASO are pulsar wind nebulae and SNRs. We plan to investigate the physics of CR acceleration and escape in these sources, by providing interpretation to LHAASO public data on selected SNRs, MSCs and other interesting sources, in terms of morphological and spectral features of the recently announced PeVatrons. This will involve modelling of magnetic turbulence, density and shock features at the source site, yielding predictions on neutrino fluxes arising from pp collisions. These fluxes will be compared to the estimated sensitivity of neutrino detectors. As a result of such a comparison, we intend to be proponents of combined data analyses on promising neutrino sources. The same sources will also be investigated in the time domain in terms of GW emissions from extreme cosmic transients. The GW detections might unambiguously identify the formation and evolution of such common sources, and allow to carry out searches that can connect high energy emission to the nature of the progenitor.
Therefore, we will set the grounding for joint analyses of LHAASO, GW interferometers and neutrino telescopes. Selected event samples will be requested to each Collaboration, masking critical information (e.g., randomising event coordinates while conserving instrumental effects like data gaps as well as background rates versus zenith angle). The instrument response function of each detector will be constructed. The sensitivity of combined analyses (i.e. searching for neutrino/GW counterparts to gamma-ray PeVatrons) will be evaluated based upon a combined maximum likelihood technique. Our ultimate goal is to unveil astrophysical CR sources thanks to the unprecedented sensitivity of LHAASO, and to investigate the still unknown energy transition between Galactic and extragalactic accelerators.
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