The new Messengers ___________________________________________
The results of gravitational wave observations already testify to the variety and wealth of astrophysical information contained in these detections, and have enabled us to glimpse the population of intermediate-mass black holes.
The observation of photonic counterparts of gravitational waves during the coalescence of a neutron star binary system has demonstrated the power of multi-messenger observations to study the mechanisms involved in violent phenomena such as short γ bursts, to study the structure of jets, to constrain the equation of state of dense matter, to independently determine the Hubble constant, to understand the role of these phenomena in the nucleosynthesis of heavy elements and to constrain dark matter candidates. Future observing campaigns will enable us to systematically probe these aspects and better understand the formation and evolution of compact objects.
High-energy neutrinos, because they interact very little with matter and are therefore not deviated from their initial trajectory, shed unique light on the sources of cosmic radiation and can reveal deeply buried objects. IceCube’s first detection of high-energy cosmic neutrinos was a major breakthrough for neutrino astronomy. The arrival directions of these neutrinos are consistent with a uniform distribution across the sky. Such levels of neutrino emission have been predicted for several categories of source, from gamma-ray bursts to blazars to starburst galaxies. The recent discovery of neutrino emission simultaneously with a gamma-ray flare in the direction of the TXS 0656+056 blazar is the first plausible identification of a very high-energy neutrino source, and indicates that neutrinos can be produced in the jets of active galactic nuclei. Identifying neutrino source populations will be a key objective in the coming years.