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The energy of a test particle orbiting a Schwarzschild black hole isquantized owing to the quantization of the angular momentum. For smalleststable circular orbit, the excitation energy is found to resemble closely theexpression for the temperature of the Hawking radiation. This result isconsistent with the Unruh effect for orbiting test particle. The predictedenergy quantization might be observable by studies of the red-shifted 21-cmline of neutral hydrogen orbiting a primordial black hole with mass of theorder of that of Earth.

Energy quantization for matter orbiting black hole and Hawking radiation