Multimessenger astrophysics with active galactic nuclei: observational clues of gamma-neutrino connection in blazars

The clear detection of an isotropic flux of very high energy neutrinos by the IceCube experiment has opened the quest for the identification of their possibile electro-magnetic (EM) counterparts. Active Galactic Nuclei (AGNs) of the blazar category are considered as the main cosmic neutrino source candidates, although contribution from other types of active galaxies and Galactic sources is expected. Observation of transient EM emission, spatially and temporally compatible with the IceCube neutrinos, is crucial to identify their possible counterpart. Since April 2016, the IceCube Collaboration alerts the astronomical community whenever a high-energy single neutrino-induced event – HESE – is detected. Thanks to the IceCube real time system and the fast space- and ground-based observations which followed the alert, a neutrino event recorded on Sept. 22nd, 2017, has been significantly associated for the first time to a quasi-simultaneous gamma-ray flare observed by Fermi-LAT, AGILE and the MAGIC experiment from the AGN blazar TXS 0506+056. In this talk, we will review this multi-messenger result, mainly focussing on the MAGIC detection, and we will present some new results about possible transient gamma-ray sources observed by AGILE, coincident in time and space with public IceCube HESE neutrino events.