Astrophysical jet

Astrophysical jet
source
Name	Astrophysical jet
Caption	A Hubble Space Telescope image of Herbig-Haro object HH 24, showing jets from a young stellar object.
Field	High-energy astrophysics

Astrophysical jet. These are highly collimated beams of plasma and associated radiation that are ejected at relativistic or near-relativistic speeds from the immediate vicinity of certain astronomical objects. They are a ubiquitous phenomenon, observed across a vast range of scales and power outputs, from forming protostars to the most energetic active galactic nuclei. The study of these jets is central to understanding accretion processes, relativistic physics, and the feedback mechanisms that influence galactic evolution.

Formation mechanisms

The primary engine for jet formation is believed to be the combination of a central accretion disk and a rotating compact object, such as a black hole, neutron star, or young star. Strong magnetic fields threading the disk are twisted by differential rotation, a process described by models like the Blandford–Znajek process for black holes and the Blandford–Payne process for hydromagnetic winds from disks. These models convert the gravitational potential energy of infalling material and the rotational energy of the central object into directed kinetic energy, launching and collimating the outflow. In stellar contexts, such as Herbig-Haro objects, jets are driven by magneto-centrifugal forces from the protostar and its inner disk.

Observational characteristics

Astrophysical jets are observed across the entire electromagnetic spectrum, from radio waves to gamma-rays. In the radio band, observations with instruments like the Very Large Array often reveal synchrotron radiation from relativistic electrons in knotty, extended structures. Optical telescopes, such as the Hubble Space Telescope, capture ionized gas tracers like H-alpha emission in stellar jets. High-energy emission from blazars and microquasars is monitored by facilities like the Fermi Gamma-ray Space Telescope and the Chandra X-ray Observatory. Key observable properties include superluminal motion, high polarization, and complex time variability, which provide clues about the jet's velocity, composition, and internal shocks.

Astrophysical contexts

Jets are observed in diverse astrophysical systems. In active galactic nuclei, powered by supermassive black holes like that in M87, they can extend for hundreds of kiloparsecs. Quasars and radio galaxies, such as Cygnus A, often exhibit giant double-lobed structures created by terminated jets. On stellar scales, microquasars like SS433 and X-ray binaries host jets from stellar-mass black holes or neutron stars. Protostellar jets, associated with objects like HL Tau, are lower-power outflows critical to the star formation process. Even some cataclysmic variable stars and pulsars, like the Crab Pulsar, produce collimated outflows.

Physical processes and models

The internal physics of jets involves complex magnetohydrodynamic processes. Particle acceleration is thought to occur at shock fronts within the jet via mechanisms like the Fermi process. The composition of jets, whether electron-positron pair or electron-proton dominated, remains an open question addressed by studies of sources like the Galactic center source Sgr A*. Stability and collimation over vast distances are explained by models involving magnetic tension and confinement by external pressure from the interstellar medium or intracluster medium. Numerical simulations performed on facilities like the NASA Pleiades supercomputer are crucial for testing these theories.

Impact on surrounding environment

Jets are a major agent of feedback, transferring energy and momentum into their surroundings. In galaxy clusters like the Perseus Cluster, jets from the central galaxy, often a BCG like NGC 1275, inflate giant cavities in the hot intracluster medium, suppressing cooling flows and star formation. Protostellar jets drive powerful molecular outflows, dispersing the natal cloud and regulating the efficiency of star formation in regions like the Orion Nebula. The interaction of jet termination shocks with the ambient medium produces extended radio lobes and hotspots, and can trigger star formation in rare cases, as studied in objects like Minkowski's Object.

Category:Astrophysical jets Category:Plasma physics Category:High-energy astrophysics