Schematic of the model for BA formation. The white cloud on top of the compact object is the illuminating X-ray source (e.g. the base of the jet or the corona). It emits both soft (cyan) and hard (black) X-ray photons. The former heats up the disc chromospheric region and gives rise to emission lines, while the later contributes to heating up deeper layers of the accretion disc. This produces a stellar-like temperature profile, with a relatively cold region on the surface producing the observed BAs. Note that the accretion disc also exhibits a radial gradient of temperature, so only the outermost rings develop such a region.