But there are more different kinds of oscillations observed than theory had dreamed of. An especially important and unusual fast vibration is one at 67 cycles per second which Dr. Morgan discovered to be present in many observations, always at the same frequency. This oscillation appears to be related to the time it would take matter to circulate around the black hole at the closest radius that it could do so for more than an orbit. Inside it would quickly disappear into the black hole. In the simplest picture the period is just that of the matter orbiting the black hole.
Theories prefer resonant vibrations of the matter orbiting the hole.
The mass of the black hole would then be lower, but still very large among the known black hole candidates. When the source has large oscillations the average energy of the X-rays also oscillates. For black hole candidates, one type of spectrum, like that from a ten million degree oven, is believed to come from disk material spiraling gradually in. The other type of spectrum might come out of a billion degree plasma that is puffed up above the disk. It is as if the disk material suddenly disappears.