Project start: 1.10.2025.

While photons from the radio to the X-ray band have a long tradition of observations, the highest energy photons and other astrophysical messengers (cosmic rays, neutrinos and gravitational waves) enter the picture only recently thanks to the technology advances in high-energy astrophysics, neutrinos and gravitational waves detectors. In 2017, the first neutrino connected to the flare of an active galactic nucleus was detected, as well as the first gravitational wave connected to an astrophysical source. In 2019, the discovery that gamma-ray bursts can also emit very-high-energy gamma rays boosted even further high energy studies in astronomy. Since then, efforts are focused in the detection of other multimessenger events in order to constrain the existing theoretical models and shed light on the emission mechanism of the most violent and powerful events in our Universe. Our team has a long experience in very-high-energy astronomy  within the MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov), LST (Large-Sized Telescopes), CTAO (Cherenkov Telescopes Array Observatory) and SWGO (Southern Wide-field Gamma-ray Observatory) international collaborations and is strongly connected to other international teams in all the energy bands. Furthermore, we are leading the Research Unit Rijeka of the Einstein Telescope collaboration for Gravitational Waves detection. We aim to observe the next multimessenger events and provide theoretical explanations of the high energy processes causing them. With the proposed investigation at the cutting edge of multimessenger astrophysics, we will confirm and advance the important contribution of Uni Rijeka to those international collaborations and be an active part of the multimessenger era of astronomy.