  We discuss the Gupta-Bleuler quantization of the free electromagnetic field
outside static black holes in the Boulware vacuum. We use a gauge which reduces
to the Feynman gauge in Minkowski spacetime. We also discuss its relation with
gauges used previously. Then we apply the low-energy sector of this field
theory to investigate some low-energy phenomena. First, we discuss the response
rate of a static charge outside the Schwarzschild black hole in four
dimensions. Next, motivated by string physics, we compute the absorption cross
sections of low-energy plane waves for the Schwarzschild and extreme
Reissner-Nordstr\"om black holes in arbitrary dimensions higher than three.
