CENBOL

CENBOL, an acronym for "CENtrifugal pressure supported BOundary Layer," is an astronomical model proposed by Sandip Chakrabarti and his colleagues to describe a specific region within the accretion flow surrounding a black hole. In this model, as matter spirals inward, the centrifugal force—which intensifies more sharply than gravity with decreasing distance—causes the flow to decelerate, often through a shock. The material then re-accelerates into a supersonic state. The CENBOL region itself lies between this shock and the innermost sonic point of the accretion flow. It becomes extremely hot because its radial kinetic energy is abruptly reduced, causing the gas to puff up as it resists gravitational pull. In many ways, it resembles a thick accretion disk—either an ion-pressure-supported torus at low accretion rates or a radiation-pressure-supported torus at high rates—but unlike earlier thick-disk models, it retains a radial velocity. The heat in CENBOL allows electrons to transfer thermal energy to photons, a process known as inverse Comptonization: low-energy X-rays (seed photons) are scattered into high-energy X-rays (hard photons). Additionally, like a boundary layer, it generates jets and outflows. The observed spectrum of a black hole accretion disk combines emission from a standard Keplerian disk—producing a multi-color blackbody spectrum—with a power-law component originating from CENBOL. When the Keplerian component has a high accretion rate, it can cool CENBOL, leading to a spectrum dominated by low-energy X-rays, a state called the "soft state." Conversely, if the Keplerian rate is low relative to the low-angular-momentum component, CENBOL persists, and high-energy X-rays dominate, resulting in the "hard state." Under radiative or thermal cooling, CENBOL may oscillate, particularly when the infall and cooling timescales are similar. This modulates the number of intercepted low-energy photons and, consequently, the high-energy photons, producing quasi-periodic oscillations (QPOs) in black hole candidates.