13.2 Accretion disks and jets

Accretion disks and jets are key players in astrophysical systems, from stars to galaxies. They form when matter falls into a massive object's gravity well, creating swirling disks that heat up and emit energy.

Jets, powerful streams of matter and energy, often accompany accretion disks. They can stretch for light-years, shaping their surroundings. Understanding these phenomena helps us grasp how cosmic objects grow and influence their environments.

Accretion Disk Dynamics

Formation and Structure of Accretion Disks

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• Accretion disk formation occurs when matter falls into a gravitational well of a massive object
• Infalling material possesses angular momentum causing it to orbit rather than directly impact the central object
• Disk-like structure forms as particles collide and settle into circular orbits
• Accretion disks range in size from planetary to galactic scales
• Composition varies depending on the source (gas, dust, plasma)
• Temperature gradients exist within the disk, hotter near the center and cooler at the outer edges
• Viscosity plays a crucial role in angular momentum transport within the disk
  • Allows matter to spiral inward while conserving angular momentum
  • Leads to heating of the disk through friction

Black Hole Accretion and Instabilities

• Black hole accretion involves matter falling into the gravitational field of a black hole
• Innermost stable circular orbit (ISCO) marks the closest stable orbit around a black hole
• Material crossing the ISCO rapidly falls into the black hole
• Accretion efficiency depends on the black hole's spin and mass
• Magnetorotational instability (MRI) drives turbulence in accretion disks
  • Arises from the interaction between weak magnetic fields and differential rotation
  • Enhances angular momentum transport and accretion rates
  • Leads to more efficient heating and higher luminosities
• Magnetic field amplification occurs through dynamo processes in the disk
  • Turbulent motions stretch and twist magnetic field lines
  • Amplified fields can drive outflows and jets

Disk Evolution and Energy Release

• Accretion disks evolve over time as matter is accreted onto the central object
• Disk thickness varies with distance from the center and accretion rate
• Thin disks form under low accretion rates, while thick disks occur at high rates
• Energy release in accretion disks comes from gravitational potential energy conversion
• Disk luminosity often exceeds that of the central object (stars, compact objects)
• Timescales for disk evolution depend on the system size and accretion rate
  • Can range from days for stellar-mass objects to millions of years for supermassive black holes

Jet Phenomena

Astrophysical Jet Formation and Propagation

• Astrophysical jets consist of highly collimated outflows of matter and energy
• Jets form in various systems (protostars, X-ray binaries, active galactic nuclei)
• Jet launching mechanisms involve magnetic fields and rotation of the central object or disk
• Blandford-Znajek process extracts energy from rotating black holes to power jets
• Blandford-Payne mechanism accelerates material from the disk surface along magnetic field lines
• Jets can extend over vast distances, from light-years to millions of light-years
• Jet composition includes electrons, positrons, and in some cases, atomic nuclei
• Shock waves form as jets interact with the surrounding medium
  • Internal shocks within the jet due to velocity differences
  • External shocks where the jet impacts the ambient medium

Relativistic Jets and Their Properties

• Relativistic jets move at speeds close to the speed of light
• Special relativistic effects become important (time dilation, length contraction)
• Apparent superluminal motion observed due to relativistic beaming
• Lorentz factors in jets can exceed 100 in some cases
• Doppler boosting enhances the observed brightness of jets pointing towards Earth
• Synchrotron radiation dominates the emission from relativistic jets
  • Produces power-law spectra across a wide range of frequencies
  • Polarization of synchrotron emission provides information about magnetic field structure

Active Galactic Nuclei and Their Jets

• Active galactic nuclei (AGN) represent the most powerful continuous energy sources in the universe
• Powered by supermassive black holes at the centers of galaxies
• AGN classification scheme includes radio-loud and radio-quiet sources
• Radio-loud AGN produce powerful jets (radio galaxies, quasars, blazars)
• Jet power in AGN can exceed the luminosity of entire galaxies
• AGN jets influence galaxy evolution through feedback processes
  • Heat the surrounding gas, preventing star formation
  • Distribute heavy elements throughout the intergalactic medium
• Variability in AGN jets provides insights into central engine processes
  • Timescales range from minutes to years depending on the wavelength and source

Accretion Disk Emissions

Radiative Processes in Accretion Disks

• Accretion disks emit radiation across the electromagnetic spectrum
• Thermal emission dominates in optically thick regions of the disk
  • Follows a multi-temperature blackbody spectrum
  • Peak temperature depends on the central object mass and accretion rate
• Non-thermal processes contribute in optically thin regions
  • Synchrotron emission from relativistic electrons in magnetic fields
  • Inverse Compton scattering of low-energy photons by hot electrons
• X-ray emission from inner regions of disks around compact objects
  • Reflection spectrum produced by X-rays interacting with disk material
  • Iron K-alpha line serves as a probe of strong gravity near black holes
• Polarization of emitted radiation provides information about disk geometry and magnetic fields
• Spectral energy distribution (SED) of accretion disks varies with system properties
  • AGN disks peak in UV/optical (big blue bump)
  • X-ray binaries show state transitions with changing spectral characteristics

Disk Winds and Outflows

• Disk winds represent outflows of material from the surface of accretion disks
• Driven by various mechanisms including radiation pressure and magnetic fields
• Line-driven winds important in disks around luminous objects (O stars, AGN)
• Magnetically driven winds launch material along open field lines
• Thermal winds arise from X-ray heating of the disk surface
• Winds can carry away significant mass and angular momentum from the disk
• Observational signatures of disk winds include
  • Blueshifted absorption lines in UV and X-ray spectra
  • P Cygni profiles in emission lines
• Winds interact with the surrounding environment, potentially triggering star formation or quenching it
• Relationship between winds and jets not fully understood
  • May represent different manifestations of the same outflow process
  • Relative importance depends on system properties and viewing angle