How it Works
The simulation shows a jet of plasma blobs ejected from an AGN core at relativistic speed β = √(1-1/γ²). The jet is inclined at angle θ to the line of sight. Each blob moves at speed β·c in 3D, but we observe the projected 2D position on the sky.
Due to relativistic time compression: when a blob moves toward the observer, successive light signals have increasingly less travel time, making the blob appear to move faster than it actually does. This produces the superluminal apparent speed β_app = β·sin θ/(1−β·cos θ) in the approaching jet.
Frequently Asked Questions
What is relativistic Doppler boosting?
Relativistic Doppler boosting amplifies the observed flux from a relativistic jet pointed toward the observer. The Doppler factor δ = 1/(γ(1−β·cos θ)) can be very large for small angles, boosting the flux by δ^(3+α).
What is superluminal motion in radio jets?
Superluminal motion is an optical illusion where jet components appear to move faster than light when the jet points close to the line of sight. The apparent speed β_app = β·sin θ/(1−β·cos θ) can exceed c.
What is the Lorentz factor γ of an AGN jet?
AGN jets have Lorentz factors γ typically 5–50 (β = 0.98 to 0.9998). Extreme blazars have γ > 50, making them enormously bright due to Doppler boosting.
What causes AGN jets to form?
AGN jets are launched by the spinning supermassive black hole at the galaxy center, powered by the Blandford-Znajek mechanism where magnetic field lines extract rotational energy. The jet is collimated by magnetic pressure and hoop stress.
What is a blazar?
A blazar is an AGN whose relativistic jet points almost directly toward Earth. The extreme Doppler boosting makes blazars very bright and variable across all wavelengths. BL Lacertae objects and flat-spectrum radio quasars (FSRQs) are the two main classes.
What is the counter-jet dimming?
The counter-jet (moving away) is Doppler de-boosted, appearing much fainter. The brightness ratio between approaching and receding jet is [(1+β·cosθ)/(1−β·cosθ)]^(3+α), which can be thousands for highly relativistic jets.
What is the optimal angle for superluminal motion?
Maximum apparent speed β_app = γβ occurs at θ_opt = arcsin(1/γ) ≈ arctan(1/(γβ)). For γ=10 this is about 6°. At this angle the apparent speed equals γ times the speed of light.
What is VLBI and how does it measure jet motion?
Very Long Baseline Interferometry (VLBI) combines radio telescopes across Earth to achieve milli-arcsecond resolution. This allows direct imaging of AGN jets and measuring their proper motion between observations years apart, revealing superluminal motion.
What is the spectral index α of a radio jet?
The spectral index α describes frequency dependence: S ∝ ν^(-α). Optically thin synchrotron has α ≈ 0.5-0.8. Doppler boosted flux scales as δ^(3+α) for a blob or δ^(2+α) for a continuous jet.
Why do some jets show a one-sided morphology?
Relativistic Doppler boosting brightens the approaching jet enormously while dimming the counter-jet below detection. For β=0.9 and θ=30°, the jet-to-counter-jet ratio exceeds 1000:1, making the counter-jet effectively invisible.