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.
