Bicoid regulation of hunchback
These two papers[1] [2] argue that bcd is a direct activator of hb, although be aware that this does not rule out downstream events feeding back to further activate  hb. (In fact, given the complexity of the embryonic gene network, it is likely that both mechanisms have a role.)


Struhl G, Struhl K, Macdonald PM. 1989. The gradient morphogen bicoid is a concentration-dependent transcriptional activator. Cell, 57(7): 1259-1273, doi:10.1016/0092-8674(89)90062-7.
Driever W, Nüsslein-Volhard C. 1989. The bicoid protin is a positive regulator of hunchback transcription in the early Drosophila embryo. Nature, 337: 138-143.