11.3 Accretion disks

Accretion disks are cosmic powerhouses, converting gravitational energy into intense radiation. These swirling structures of matter form around stars, black holes, and other massive objects, shaping the evolution of galaxies and planetary systems.

In high-energy density physics, accretion disks serve as natural laboratories for extreme conditions. They showcase complex interplay between gravity, magnetism, and radiation, helping scientists understand phenomena like jets, winds, and high-energy emissions in space.

Fundamentals of accretion disks

• Accretion disks play a crucial role in high-energy astrophysical phenomena, serving as efficient mechanisms for converting gravitational potential energy into radiation
• In the context of High Energy Density Physics, accretion disks provide natural laboratories for studying extreme physical conditions, including high temperatures, densities, and magnetic fields

Definition and formation

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Top images from around the web for Definition and formation

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• Accretion disks form when matter with angular momentum falls towards a central gravitating object (stars, black holes, neutron stars)
• Conservation of angular momentum causes infalling material to orbit the central object, forming a flattened, disk-like structure
• Viscous forces within the disk cause angular momentum transport outward, allowing matter to spiral inward
• Disk formation occurs in various astrophysical scenarios (protoplanetary systems, close binary stars, active galactic nuclei)

Accretion disk structure

• Radial structure consists of inner, middle, and outer regions with distinct physical properties
• Vertical structure includes a dense midplane and less dense upper layers
• Temperature gradient decreases from the inner to outer regions of the disk
• Pressure support maintains the vertical structure against the central object's gravity
• Opacity variations throughout the disk affect radiation transport and energy dissipation

Angular momentum transport

• Efficient angular momentum transport enables continued accretion of matter onto the central object
• Molecular viscosity proves insufficient to explain observed accretion rates
• Turbulence plays a crucial role in enhancing effective viscosity within the disk
• Magnetorotational instability (MRI) serves as a primary mechanism for generating turbulence
• Gravitational torques can contribute to angular momentum transport in some disk types (self-gravitating disks)

Physical processes in accretion

• Accretion disks involve complex interplay of various physical processes, including hydrodynamics, thermodynamics, and radiative transfer
• Understanding these processes helps explain observed high-energy phenomena in astrophysical systems and informs laboratory experiments in High Energy Density Physics

Mass transfer mechanisms

• Roche lobe overflow occurs in binary systems when one star fills its gravitational equipotential surface
• Stellar winds from companion stars can be captured by the compact object's gravitational field
• Bondi-Hoyle-Lyttleton accretion describes spherical accretion in the absence of angular momentum
• Tidal disruption events involve stars torn apart by tidal forces near supermassive black holes
• Disk instabilities can lead to episodic mass transfer in some systems (dwarf novae)

Energy dissipation

• Viscous dissipation converts gravitational potential energy into heat as matter spirals inward
• Compressional heating occurs as material becomes more compressed near the central object
• Magnetic reconnection releases energy stored in magnetic fields, contributing to disk heating
• Shock heating can occur in regions of supersonic flow (disk-stream impact in cataclysmic variables)
• Radiative cooling balances energy input, maintaining the disk's thermal equilibrium

Radiation emission

• Thermal emission dominates in optically thick regions, approximating blackbody radiation
• Bremsstrahlung (free-free emission) occurs in hot, ionized plasma within the disk
• Synchrotron radiation arises from relativistic electrons spiraling in magnetic fields
• Compton scattering modifies the emergent spectrum, especially in hot, optically thin regions
• Line emission provides diagnostic information about disk composition and physical conditions

Types of accretion disks

• Various accretion disk types exist, characterized by different physical properties and dominant processes
• Understanding disk types helps explain diverse observational phenomena in High Energy Density Physics

Thin vs thick disks

• Thin disks have vertical scale height much smaller than radius (H << R)
• Thick disks possess substantial vertical extent, with H ~ R
• Thin disks efficiently radiate away heat, maintaining low temperatures
• Thick disks retain significant thermal energy, leading to high temperatures and pressures
• Transition between thin and thick disks depends on accretion rate and radiative efficiency

Standard vs advection-dominated

• Standard disks efficiently radiate away locally generated energy
• Advection-dominated accretion flows (ADAFs) retain most of their energy, advecting it inward
• ADAFs occur at very low or very high accretion rates, where radiative cooling becomes inefficient
• Standard disks typically appear in X-ray binaries in high/soft states
• ADAFs explain low-luminosity active galactic nuclei and X-ray binaries in low/hard states

Magnetically arrested disks

• Form when magnetic flux accumulates near the central object, impeding accretion
• Strong magnetic fields disrupt the inner regions of the accretion flow
• Efficient extraction of rotational energy from the central object via magnetic fields
• Associated with powerful relativistic jets in active galactic nuclei
• Provide a mechanism for explaining very high energy radiation from some accreting systems

Accretion disk models

• Theoretical models of accretion disks aim to explain observed phenomena and predict disk behavior
• These models inform experimental designs in High Energy Density Physics to study analogous processes

Alpha disk model

• Introduced by Shakura and Sunyaev to parameterize turbulent viscosity
• Assumes turbulent stress proportional to total pressure: $\tau = \alpha P$
• Alpha parameter (α) encapsulates unknown physics of angular momentum transport
• Typically, α ranges from 0.01 to 0.1 based on observations and numerical simulations
• Provides a simple framework for studying disk structure and evolution

Shakura-Sunyaev model

• Describes geometrically thin, optically thick accretion disks
• Assumes local thermal equilibrium and neglects self-gravity
• Divides disk into inner, middle, and outer regions based on dominant pressure and opacity sources
• Predicts radial dependencies of disk properties (temperature, density, pressure)
• Successfully explains many observed features of accretion disks in X-ray binaries and AGN

Magnetorotational instability

• Fundamental mechanism driving turbulence in weakly magnetized, differentially rotating disks
• Discovered by Balbus and Hawley in the context of accretion disks
• Amplifies initial magnetic field perturbations, leading to self-sustained turbulence
• Provides physical basis for anomalous viscosity assumed in alpha disk models
• Explains angular momentum transport rates inferred from observations

Observational signatures

• Accretion disks produce distinct observational features across the electromagnetic spectrum
• These signatures allow astronomers to probe physical conditions in high-energy astrophysical environments

Spectral characteristics

• Multicolor blackbody spectrum from optically thick regions of the disk
• Power-law component in X-rays from Comptonization in hot corona
• Reflection features including fluorescent iron line and Compton hump
• Broad emission lines in optical/UV from photoionized gas in the disk
• Synchrotron emission in radio from relativistic electrons in jets or corona

Variability and oscillations

• Flickering on short timescales due to turbulence in the accretion flow
• Quasi-periodic oscillations (QPOs) linked to orbital motion or disk instabilities
• State transitions in X-ray binaries associated with changes in disk structure
• Outbursts in dwarf novae and soft X-ray transients due to disk instabilities
• Long-term variability in AGN potentially related to changes in accretion rate

Jets and outflows

• Collimated relativistic jets observed in some accreting systems (microquasars, AGN)
• Disk winds detected through blueshifted absorption lines in UV and X-ray spectra
• Correlation between accretion state and jet production in X-ray binaries
• Evidence for jet-disk coupling in AGN through observed correlations
• Outflows play crucial role in feedback processes, influencing galaxy evolution

Accretion in astrophysical systems

• Accretion disks manifest in various astrophysical contexts, each with unique characteristics
• Studying these systems provides insights into extreme physical conditions relevant to High Energy Density Physics

Active galactic nuclei

• Supermassive black holes at galactic centers accrete matter, powering AGN
• Accretion rates span wide range, from low-luminosity AGN to bright quasars
• Unified model explains diverse AGN types through differences in viewing angle and accretion rate
• Relativistic jets in radio-loud AGN likely powered by disk-black hole interactions
• AGN feedback influences galaxy evolution through energy and momentum input

X-ray binaries

• Compact object (neutron star or black hole) accretes matter from companion star
• Exhibit distinct spectral states (low/hard, high/soft) associated with different disk configurations
• Transient behavior in some systems due to disk instabilities (soft X-ray transients)
• Provide laboratories for studying accretion physics on human timescales
• Allow probing of strong-field gravity effects near compact objects

Protoplanetary disks

• Form around young stars as part of the star and planet formation process
• Serve as sites of planet formation through dust grain growth and gravitational instabilities
• Exhibit complex structures (gaps, rings, spirals) potentially caused by forming planets
• Evolve over time through accretion onto the central star and photoevaporation
• Provide insights into the formation of our own solar system

High-energy phenomena

• Accretion disks generate various high-energy phenomena of interest to High Energy Density Physics
• These processes involve extreme physical conditions difficult to reproduce in terrestrial laboratories

Disk coronae

• Hot, tenuous plasma above and below the disk, analogous to the solar corona
• Temperatures reach millions of degrees Kelvin, much hotter than the disk itself
• Produce hard X-ray emission through Comptonization of soft photons from the disk
• Magnetic reconnection likely plays a key role in heating the corona
• Serve as launching sites for winds and jets in some models

Relativistic effects

• Strong gravity near compact objects leads to observable relativistic effects
• Gravitational redshift affects emission from inner disk regions
• Frame-dragging by rotating black holes influences disk and jet dynamics
• Relativistic Doppler boosting enhances emission from approaching side of the disk
• Extreme cases lead to formation of photon orbits and ergosphere near black holes

Comptonization processes

• Inverse Compton scattering of low-energy photons by hot electrons
• Thermal Comptonization in hot coronae produces power-law X-ray spectra
• Bulk Comptonization can occur in regions with large-scale motions (jets, outflows)
• Comptonization parameters (optical depth, electron temperature) inferred from spectral shape
• Compton cooling important in regulating temperature of hot plasmas in accretion flows

Numerical simulations

• Computational modeling plays crucial role in understanding complex accretion disk physics
• Simulations bridge theory and observations, informing both laboratory experiments and astrophysical interpretations

Magnetohydrodynamic simulations

• Solve equations of ideal or resistive MHD to model disk dynamics
• Capture development and saturation of magnetorotational instability
• Reveal complex magnetic field structures and their role in angular momentum transport
• Allow study of disk winds and jet formation in magnetized accretion flows
• Incorporate additional physics (radiation, relativistic effects) for more realistic models

General relativistic simulations

• Account for effects of strong gravity near black holes and neutron stars
• Use various formulations (GRMHD, GRRMHD) to include magnetic fields and radiation
• Reveal behavior of accretion flows in extreme environments (near ISCO, ergosphere)
• Model formation and propagation of relativistic jets from disk-black hole systems
• Provide templates for interpreting observations of black hole shadows (M87, Sgr A*)

Radiation transfer modeling

• Compute observable spectra and images from simulated accretion flows
• Account for various emission processes (thermal, non-thermal) and radiative transfer effects
• Include special and general relativistic effects on photon propagation
• Allow direct comparison between theoretical models and observations
• Essential for interpreting data from current and future X-ray observatories

Accretion disk instabilities

• Instabilities in accretion disks lead to observable phenomena and affect disk evolution
• Understanding these instabilities informs both astrophysical observations and laboratory plasma experiments

Thermal instability

• Occurs when cooling rate increases with decreasing temperature or vice versa
• Can lead to limit cycle behavior in accretion disks (dwarf nova outbursts)
• Triggered by changes in opacity as disk temperature crosses ionization thresholds
• Produces S-shaped equilibrium curves in temperature-surface density plane
• Explains outburst behavior in cataclysmic variables and soft X-ray transients

Viscous instability

• Arises when viscosity increases faster than linear with surface density
• Can cause disk to break up into rings or lead to episodic accretion
• Often coupled with thermal instability in realistic disk models
• Plays role in explaining long-term variability in some accreting systems
• Can influence evolution of protoplanetary disks and planet formation

Gravitational instability

• Occurs when disk's self-gravity becomes significant compared to central object's gravity
• Characterized by Toomre Q parameter: $Q = \frac{c_s \kappa}{\pi G \Sigma}$
• Can lead to formation of spiral arms and clumps in massive disks
• Important in context of planet formation in protoplanetary disks
• May play role in feeding supermassive black holes in galactic nuclei

Observational techniques

• Various observational methods are employed to study accretion disks across the electromagnetic spectrum
• These techniques provide data for testing theoretical models and informing laboratory experiments in High Energy Density Physics

Spectroscopy of accretion disks

• Measures disk emission across wide range of wavelengths (radio to gamma-rays)
• Reveals information about disk temperature, composition, and velocity structure
• X-ray spectroscopy probes inner disk regions and hot coronae
• Optical/UV spectroscopy provides information on outer disk and broad line regions
• Infrared spectroscopy useful for studying dust in protoplanetary disks

Timing analysis

• Studies variability of disk emission on various timescales
• Power spectral analysis reveals characteristic frequencies in the system
• Search for quasi-periodic oscillations provides information on disk dynamics
• Eclipse mapping in binary systems allows reconstruction of disk structure
• Reverberation mapping probes geometry of AGN accretion flows

Multiwavelength observations

• Combine data from multiple wavelength regimes to build comprehensive picture
• Simultaneous observations reveal connections between different emission components
• Long-term monitoring captures disk evolution and state transitions
• Coordinated campaigns provide insights into disk-jet connections
• Multiwavelength SED modeling constrains physical parameters of accretion flows