  The energy of a test particle orbiting a Schwarzschild black hole is
quantized owing to the quantization of the angular momentum. For smallest
stable circular orbit, the excitation energy is found to resemble closely the
expression for the temperature of the Hawking radiation. This result is
consistent with the Unruh effect for orbiting test particle. The predicted
energy quantization might be observable by studies of the red-shifted 21-cm
line of neutral hydrogen orbiting a primordial black hole with mass of the
order of that of Earth.
