  We give a rigorous definition of the Unruh state in the setting of massless
Dirac fields on slowly rotating Kerr spacetimes. In the black hole exterior
region, we show that it is asymptotically thermal at Hawking temperature on the
past event horizon. Furthermore, we demonstrate that in the union of the
exterior and interior regions, the Unruh state is pure and Hadamard. The main
ingredients are the H\"afner-Nicolas scattering theory, new microlocal
estimates for characteristic Cauchy problems and criteria on the level of
square-integrable solutions.
