Relativistic jet
A relativistic jet is a narrow, collimated stream of plasma—electrons, protons, and positrons— ejected perpendicular to the accretion disk of a compact object (black hole or neutron star) at speeds approaching the speed of light. These are among the most energetic phenomena in the universe, visible billions of light-years away andsometimes span millions of light-years.
Jet Formation
The exact mechanism behind the formation of relativistic jets is still debated, but it involves the interaction between the intense magnetic field of the accretion disk and the rotating plasma. The most widely accepted models suggest that magnetic fields twisted by the rotation of the black hole or the disk extract energy and collimate the plasma into bipolar jets (in both directions perpendicular to the disk). The Blandford–Znajek process, which extracts rotational energy from a rotating (Kerr) black hole, is one of the favored mechanisms.
Jets and Blazars
When a relativistic jet points directly toward Earth, the object is observed as a blazar—one of the brightest and most variable sources in the sky. The jet’s radiation, amplified by the relativistic Doppler effect, can completely overwhelm the light from the host galaxy. Blazars are important sources of high-energy gamma rays, cosmic rays, and potentially high-energy neutrinos.
Importance in Astronomy
Relativistic jets are natural particle accelerators on a cosmic scale. They contribute to high-energy cosmic radiation and play a role in heating the intergalactic medium. In radio galaxies, jets can extend over several million light-years, sculpting the surrounding intergalactic medium and influencing galaxy formation within clusters.
Concrete Example
The jet from the galaxy M87, emanating from its supermassive black hole (M87*), &squo;&extends for about 5,000 light-years and has been observed in detail by the Hubble Space Telescope. Its inner region, very close to the black hole, was imaged by theEvent Horizon Telescope at the same time as the first photo of the black hole in 2019, showing the direct link between the accretion disk and the jet ejection.
Frequently Asked Questions
Are relativistic jets dangerous to nearby stellar systems?
A relativistic jet pointed toward Earth would be catastrophic, but the odds are extremely low. The jets from known galactic black holes are not directed toward us. Gamma-ray bursts (GRBs), which are thought to be the manifestation ofa jet directed toward us during a hypernova, could theoretically affect nearby stellar systems, but events at a dangerous distance are extremely rare.
What is the difference between a jet and a stellar wind?
A stellar wind is a stream of particles emitted in all directions by a star. A relativistic jet is a narrow, highly collimated stream emitted in a preferred direction perpendicular to the accretion disk. Jets have velocities and energies far greater than those of ordinary stellar winds.
Can a jet be observed in real time?
On human timescales, jets appear static. But observations taken at intervals of a few years reveal apparent superluminal motions in certain jets— a geometric illusion caused by the near-light speed of the plasmas combined with a favorable viewing angle, not a violation of relativity.