Black holes radiate.
But recently it is shown they even more radiate!
In fact when their horizon fluctuates, they radiate on two or three frequencies that are heavily resonated, like a quantum amplifier.
These lines are at the range of frequency sensitivity that INTEGRAL may become able to find them. Perhaps we have observed them and do not recognize them as black hole QAE lines.
These lines could be foud in evenly or unevenly spaced fashion. in fact, Yakov Bekenstein and Viatcheslav Mukhanov predicted these lines (if are more than two) are exactly evenly spaced. With the support of theories that predict the spectrum of area scaling with the square-root of integers, we predicted they must be found in an unevenly-spaced fashion.
Details can be found in my recent Physical Reviews D paper...
This figure illustrates a quantum black hole horizon in the vicinity of a null boundary (the black sphere). In other words, you see a black hole as the interior black sphere underneath a discrete shell (the outer shell) that represents the hole's horizon area fluctuations.
The spectrum of a black hole radiation should be discrete, intense, and narrow line on top of weak the Hawking radiation.
The search for these lines in observational data is continued...