Active galactic nuclei in multi-messenger era

 Feeding on the surrounding gas at the centers of active galactic nuclei (AGN), supermassive black holes often produce collimated outflows, known as relativistic jets, whose tightly collimated beams of radiation pierce through the Universe and reach us from billions of years in the past, offering a unique opportunity to study the Universe when it was an order of magnitude younger than today. An exciting discovery made by space- and ground-based high-energy missions is the detection of gamma-rays from over 5000 AGN. Despite the intensive study of AGN, the location and origin of gamma-rays remains a mystery. Multi-frequency observations offer an unparalleled opportunity to probe and pinpoint the high-energy emission.  I will present some exciting results of our recent study using multi-frequency and multi-messenger  observations and our plans for the future high-energy missions.