Advection Dominated Accretion FlowEdit

Advection Dominated Accretion Flow (ADAF) refers to a class of accretion solutions around compact objects where the flow is hot, optically thin, geometrically thick, and radiatively inefficient. In these systems most of the gravitational energy released by infalling matter is carried inward with the gas as entropy and heat, rather than being radiated away locally. This leads to very low radiative efficiencies at accretion rates well below the Eddington limit, which helps explain the faintness of many active galactic nuclei and quiescent X-ray binaries. The concept arose from attempts to understand why some accreting black holes and neutron stars do not shine as brightly as standard thin-disk models would predict for their inferred accretion rates. ADAFs are often discussed alongside variants that emphasize winds, convection, or jets, and they have become a central part of the broader picture of low-luminosity accretion physics accretion disk black hole.

The physical picture of an ADAF rests on a two-temperature, optically thin plasma in which viscous heating preferentially raises ions to extreme temperatures. Because the density is low, Coulomb coupling between ions and electrons is inefficient at spreading this heat to electrons, so electrons remain relatively cool and radiative cooling is ineffective. The result is a hot, swollen inflow that carries a substantial fraction of the released energy into the central object by advection rather than radiating it away. The flow tends to be quasi-spherical near the central region and geometrically thick, with sub-Keplerian rotation and significant vertical support. Radiative processes such as synchrotron emission, bremsstrahlung, and Compton upscattering contribute to the observed spectrum, but their overall luminosity remains small compared to the accretion power inferred from the mass supply, especially at low dotM relative to the Eddington rate synchrotron radiation bremsstrahlung Compton scattering.

Governing physics and models - Two-temperature plasmas: In ADAFs, the ion temperature can approach the virial value (on the order of 10^11 K near the black hole), while electrons are cooler but still hot enough to emit X-rays via Comptonization and bremsstrahlung. The decoupled heating and cooling channels play a central role in reducing radiative efficiency. See two-temperature plasma for related concepts. - Viscosity and angular momentum transport: The transport of angular momentum is commonly parameterized by the alpha prescription, originally developed for thin disks but adapted to hot, thick flows. The resulting dynamics produce slow, global inflow with substantial energy stored in the gas. See alpha-disk for background on the viscosity concept and its role in accretion theory. - Advection-dominated energy balance: A key feature is that the advective transport term q_adv can dominate over radiative cooling q_rad, so a large fraction of the viscously dissipated energy is advected inward rather than emitted. This is a defining aspect of the ADAF regime and underpins its low radiative efficiency. - Variants and competing models: ADAFs have inspired related ideas, including convection-dominated accretion flows (CDAFs) in which convection redistributes energy and angular momentum, and adiabatic inflow-outflow solutions (ADIOS) in which substantial mass is lost to winds. These models are discussed as alternative or complementary pictures to the original ADAF framework and are explored to reconcile theory with observations. See convection-dominated accretion flow and adiabatic inflow-outflow solution for details. - Numerical simulations: Magnetohydrodynamic (MHD) simulations, including general-relativistic implementations, investigate how magnetic fields and instabilities like the magnetorotational instability (MRI) shape hot, thick flows, outflows, and variability. Results often emphasize the presence and strength of winds or jets and the interplay between inflow and outflow in determining the effective accretion rate onto the central object. See magnetohydrodynamics and MRI for related topics.

Observational contexts and evidence - Low-luminosity active galactic nuclei: ADAFs are invoked to explain faint nuclei in galaxies where the inferred accretion rate is well below the Eddington rate, yet the central engine remains active. Notable examples include certain nearby galactic nuclei and radiatively inefficient systems. See low-luminosity active galactic nucleus for a broader review. - Sgr A* and other galactic center sources: The extremely low luminosity of the supermassive black hole at the center of our galaxy, Sagittarius A* Sagittarius A*, is often cited as a natural setting for ADAF-style accretion, though alternative explanations exist and the precise structure of the flow is still debated. - M87 and radio-loud nuclei: Jet-producing systems like M87 provide observational context in which hot, radiatively inefficient flows and outflows can be studied together with jet phenomena. ADAF-like models can be part of the interpretive toolkit for such sources, especially in the low-luminosity regime. See M87 for related information. - X-ray binaries in quiescence: Stellar-mass black holes accreting at very low rates may host ADAF-like inner flows, contributing to their faint X-ray signatures and spectral shapes in quiescence. See X-ray binary for a broader class of systems where accretion physics is studied.

Debates and open questions - How much mass is lost to winds and outflows?: In ADIOS and related models, outflows can significantly reduce the actual mass reaching the central object, altering the effective accretion rate as a function of radius. Observational tests and simulations probe the presence and strength of such winds, but results remain a topic of active discussion. - The role of convection and the CDAF picture: Some simulations and analytic arguments suggest convection can redistribute energy and angular momentum, producing different density and velocity profiles than a pure ADAF. The extent to which convection dominates versus advection alone is debated, and observational discriminants are sought. - Outflows versus jet-dominated emission: In some systems, the radiative output may be dominated by jets or outflows rather than the inner accretion flow itself. This complicates the inference of the accretion mode from the spectrum and requires joint modeling of inflow and jet physics. - Variability and spectral constraints: The timescales and spectral evolution in ADAF scenarios have to be reconciled with multiwavelength variability observed in LLAGNs and X-ray binaries. Some sources challenge the simplest ADAF expectations, leading researchers to consider hybrid models or time-dependent behavior. - Connection to the standard thin disk regime: ADAFs describe the low-accretion-rate end of the spectrum. Understanding the transition between ADAF-like flows and standard radiatively efficient thin disks at higher accretion rates, as well as the nature of the transition region, remains an ongoing area of study.

See also - accretion disk - black hole - Sagittarius A* - M87 - X-ray binary - radiative efficiency - fundamental plane of black hole activity - convection-dominated accretion flow - adiabatic inflow-outflow solution - two-temperature plasma - alpha-disk - magnetohydrodynamics - MRI