CENBOL

CENBOL or CENtrifugal pressure supported BOundary Layer refers to the region of an accretion flow around a black hole. The basic idea is that since the centrifugal force l²/r³ increases very rapidly compared to the gravitational force (which goes as 1/r² )as the distance r decreases, the matter feels increasing centrifugal force as it approaches a black hole. Thus the matter initially slows down, typically through a shock transition and then accelerates again to become a supersonic flow [http://adsabs.harvard.edu/abs/1990ttaf.book.....C] .

The importance of CENBOL is that it literally behaves like a boundary layer of a black hole. This region which is the located between the shock and the innermost sonic point of an accretion flow becomes hot due to sudden reduction of the radial kinetic energy and it puffs up. In certain sense it behaves like a thick accretion disk [http://adsabs.harvard.edu/abs/1980A%26A....88...23P] , except that it also has radial velocity. Because it is hot, it inverse Comptonizes soft photons or seed photons [http://adsabs.harvard.edu/abs/1995ApJ...455..623C] and produce very high energy hard photons. Just as a boundary layer, it also produces jets and outflows.

In presence of radiative or thermal cooling effects, the CENBOL may start to oscillate and then the number of intercepted soft photons would be modulated. As a result, the number of hard photons are also modulated and one observes what is known as Quasi-Periodic Oscillations or QPOs