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The physicist John Wheeler once famously said “black holes have no hair”, meaning that a black hole can be characterised by a few numbers like its mass, spin and (not relevant for astrophysical black holes) charge. However black holes end up having pretty complex “hair" when they manage to gravitationally capture material in a process called accretion, and channel that energy into other forms that can drastically affect their environments. The most dramatic examples of this energy release are the enormous magnetic plasma structures we call ‘jets’, which can extend well beyond their host galaxy. These jets are also of great interest for astroparticle physics as the suspected source of ultra-high energy cosmic rays, neutrinos, gamma-rays, and possibly also Galactic cosmic rays from smaller, stellar mass black holes that also launch jets. I am involved in two seemingly very different projects that can study black holes and their consequences: the Event Horizon Telescope (EHT) and the Cherenkov Telescope Array (CTA), that observe at very different scales of the problem. I will discuss how the methods we use to interpret EHT results, particularly to better characterise the "hairy" astrophysics, have direct consequences for our understanding of high-energy particle acceleration. I will also discuss the outlook and need for continued coordinated multi-facility approaches to develop a complete physical picture of black hole accretion.
giulia.pagliaroli@gssi.it