High Energy Astrophysics Space-Ground Complementarity in the Multimessenger Era

On August 17, 2017, a new era of astronomy was inaugurated by the detection of a short gamma-ray burst accompanying the gravitational wave GW170817 detected by LIGO-VIRGO. This event has been a milestone towards the "multi-messenger" astronomy. Similarly, a breakthrough in High-Energy Neutrino astronomy has just started, with the recent advances in large volume ice and water detectors and initial discoveries pinpointing the High-Energy-Neutrino/Blazars nature. In these cases, among many others, planned or serendipity common observations from ground and space based observatories has been key to achieve a breakthrough in our understanding of the Universe and its evolution. In fact, one of the most important observational challenges of our time is to establish the link between the discoveries of these so called "New astronomies" and the electromagnetic Universe. As the same violent phenomena that generate gravitational waves and/or high-energy neutrinos are also the source of high-energy photons, gamma-ray astronomy has been - and will remain - pivotal in connecting new and "classic" astronomies. A textbook example of the breakthrough achievable with Ground-Space common observations is the 1.74s delay between the GRB170817A detected by FERMI and INTEGRAL, after a travel time of $\sim$130 million years, and the GW arrival time detected by the LIGO-VIRGO interferometers: this single event place a strict limits between the speed of light and gravitational waves in the Universe with an unprecedented accuracy better that one part over ten to the fifteen.