Black hole accretion flows: from nearby stellar binaries to quasars at cosmic dawn

Accretion onto black holes transforms the darkest objects in the universe into the brightest. I will review what we know about the emission from the accretion flow, starting with the stellar mass black holes in binary systems in our own galaxy. Scaling up to the supermassive black holes in active galaxies and quasars reveals both similarities and differences. One of the key similarities is the 'changing look' phenomena in active galaxies, where the UV continuum associated with an optically thick accretion flow drops over a few months/years, triggering the disappearance of the characteristic broad emission lines, analogous to the state transition in binaries. One of the key differences is the nature of the accretion flow above the transition luminosity, where the stellar mass black holes look like standard discs and have variability timescales like standard discs while the supermassive ones do not. Only at the highest luminosities (extreme narrow line Seyfert 1 galaxies and the weak line quasars) does the accretion flow match to the disc models. I will speculate on the physics underlying all the behavior, and give a united picture of the accretion flow.