The Sharpest View of Blazar Jets through Space and Millimeter VLBI Observations

Mass accretion onto the supermassive black holes that power AGN leads to the formation of highly collimated relativistic jets that radiate across the entire electromagnetic spectrum, from radio to γ-rays. Over the past decades, AGN jets have been repeatedly studied through centimeter VLBI observations, but the resolution provided is just a step behind the necessary to resolve the compact regions in the vicinity of the central black hole where jets are formed, collimated, and accelerated. This limitation has recently been overcome by two technological improvements. On one side, the space VLBI mission RadioAstron has allowed us to increase the virtual size of our VLBI telescopes to as large as the distance to the Moon, achieving angular resolutions as small as 20 microarcseconds. On the other side, the participation of ALMA in millimeter VLBI arrays, such as the EHT and GMVA, has not only allowed the EHT to capture the first image of a black hole, but also holds the potential to actually address for the first time the fundamental questions of how gravity works in the strong-field regime near the event horizon, how accretion leads to the formation of relativistic jets, and what are the sites and mechanisms for the production of high energy emission in blazars. The latest observations providing the sharpest view of blazar jets will be reviewed, including the study of the blazar OJ287, one of the best candidates for hosting a sub-pc supermassive binary black hole system.