In astronomy, CENBOL (derived from "CENtrifugal pressure supported BOundary Layer) is a model developed by the astrophysicist

Sandip Chakrabarti Sandip Chakrabarti is an Indian astrophysicist. He developed a computer model to show how life on earth could have originated in outer space. Education He started his education in Lalit Mohan Shyam Mohini High School in Malda. After finish ...

and collaborators to explain the region of an accretion flow around a

black hole A black hole is a region of spacetime where gravitation, gravity is so strong that nothing, including light or other Electromagnetic radiation, electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts t ...

Centrifugal force dominated boundary layer

Because

centrifugal force In Newtonian mechanics, the centrifugal force is an inertial force (also called a "fictitious" or "pseudo" force) that appears to act on all objects when viewed in a rotating frame of reference. It is directed away from an axis which is paralle ...

l²/r³ increases very rapidly compared to the

gravitational force In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the strong ...

(which goes as 1/r²) as the distance r decreases, matter feels increasing centrifugal force as it approaches a

. Thus the matter initially slows down, typically through a shock transition, and then accelerates again to become a supersonic flow. The importance of CENBOL is that it behaves like a

boundary layer In physics and fluid mechanics, a boundary layer is the thin layer of fluid in the immediate vicinity of a bounding surface formed by the fluid flowing along the surface. The fluid's interaction with the wall induces a no-slip boundary condi ...

of a

. This region is located between the

shock Shock may refer to: Common uses Collective noun *Shock, a historic commercial term for a group of 60, see English numerals#Special names * Stook, or shock of grain, stacked sheaves Healthcare * Shock (circulatory), circulatory medical emergen ...

and the innermost sonic point of an accretion flow. CENBOL becomes hot due to sudden reduction of radial

kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its accele ...

and the flow is puffed up, since hot gas can fight against gravitational. In a certain sense it behaves like a thick accretion disk (ion pressure supported torus if the accretion rate is low; or radiation pressure supported torus, if the accretion rate is high), except that it also has radial velocity, which original models of thick disk did not have. Because it is hot, the electrons transfer their thermal energy to the photons. In other words CENBOL inverse Comptonizes low energy

X-rays An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 Picometre, picometers to 10 Nanometre, nanometers, corresponding to frequency, ...

or soft photons (also called seed photons) and produces very high energy

(also called hard photons). Just as a boundary layer, it also produces jets and outflows. The observed spectrum of a black hole

accretion disk An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction, uneven irradiance, magnetohydrodynamic effects, and other fo ...

is in reality partly coming from a Keplerian disk (viscous Shakura and Sunyaev (1973) type disk) in the form of multi-colour black body emission. But the power-law component primarily comes from the CENBOL. In presence of high accretion rates in the Keplerian component, the CENBOL can be cooled down so that the spectrum may be totally dominated by the low energy X-rays. The spectrum goes to a 'so-called' soft state. When the Keplerian rate is not high compared to the low-angular momentum component, the CENBOL survives and the spectrum is dominated by the high energy X-rays. Then it is said to be in a so-called 'hard-state'. In the presence of radiative or thermal cooling effects, CENBOL may start to oscillate, especially when the infall time scale and the cooling time scale are comparable.{{Cite journal, last1=Garain, first1=S. K., last2=Ghosh, first2=H., last3=Chakrabarti, first3=S. K., year=2014 , title=Quasi-periodic oscillations in a radiative transonic flow: results of a coupled Monte Carlo-TVD simulation, journal=Monthly Notices of the Royal Astronomical Society, volume=437, issue=2, pages=1329, bibcode=2014MNRAS.437.1329G, doi=10.1093/mnras/stt1969, arxiv = 1310.6493 , s2cid=118597088 In this case, number of intercepted low energy photons would be modulated. As a result, the number of high energy photons are also modulated. This effect produces what is known as

quasi-periodic oscillations In X-ray astronomy, quasi-periodic oscillation (QPO) is the manner in which the X-ray light from an astronomical object flickers about certain frequencies. In these situations, the X-rays are emitted near the inner edge of an accretion disk in w ...

(or QPOs) in black hole candidates.

References

Black holes