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 massive, compact astronomical object so dense that its gravity prevents anything from escaping, even light. Albert Einstein's theory of general relativity predicts that a sufficiently compact mass will form a black hole. Th ...

Centrifugal force dominated boundary layer

Because

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

l²/r³ increases very rapidly compared to the

gravitational force Newton's law of universal gravitation describes gravity as a force by stating that every particle attracts every other particle in the universe with a force that is proportional to the product of their masses and inversely proportional to the sq ...

(which goes as 1/r²) as the distance r decreases, 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. 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 Boundary (thermodynamic), bounding surface formed by the fluid flowing along the surface. The fluid's interaction with the wall induces ...

of a

. This region is located between the

shock Shock may refer to: Common uses Healthcare * Acute stress reaction, also known as psychological or mental shock ** Shell shock, soldiers' reaction to battle trauma * Circulatory shock, a medical emergency ** Cardiogenic shock, resulting from ...

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 form of energy that it possesses due to its motion. In classical mechanics, the kinetic energy of a non-rotating object of mass ''m'' traveling at a speed ''v'' is \fracmv^2.Resnick, Rober ...

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 (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

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 most frequently a star. Friction, uneven irradiance, magnetohydrodynamic effects, and ...

is in reality partly coming from a Keplerian disk (viscous Shakura and

Sunyaev Rashid Alievich Sunyaev (, ; born 1 March 1943 in Tashkent, USSR) is a German, Soviet, and Russian astrophysicist of Tatars, Tatar descent. He got his MS degree from the Moscow Institute of Physics and Technology (MIPT) in 1966. He became a pro ...

(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, doi-access=free , 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