A blazar is an active galactic nucleus whose relativistic jet is aimed almost directly at Earth. The alignment beams its radiation toward us, making it appear far brighter and more variable than the same object viewed from the side.

What is the difference between a blazar and a quasar?

Physically nothing — both are supermassive black holes accreting matter and launching jets. The difference is viewing angle: we see a blazar down the jet, and a quasar from a wider angle.

How can blazar jets appear to move faster than light?

They do not. Because the emitting material moves toward us at nearly the speed of light, it nearly keeps pace with its own light, compressing the apparent timeline. The projection onto the sky then makes the motion look superluminal.

Why do blazars matter for neutrino astronomy?

In 2017 the IceCube detector traced a high-energy cosmic neutrino to the flaring blazar TXS 0506+056 — the first identified extragalactic neutrino source, and strong evidence that blazar jets accelerate protons to extreme energies.

什么是Blazar？

A blazar is not a different kind of object from a quasar — it is the same thing viewed from a different seat. Its relativistic jet happens to point within a few degrees of Earth, and that geometry alone multiplies its apparent brightness by hundreds, turning a distant galaxy into one of the most violent-looking sources in the sky.

The mechanism is relativistic beaming. Plasma streaming outward at 99.5% of the speed of light radiates its energy forward into a narrow cone, so an observer sitting inside that cone receives emission boosted by the Doppler factor cubed. The same jet seen from the side would be unremarkable. This model is rendered from inside the cone, and the boost is computed from the viewing geometry rather than dialled in — orbit around to the side and the jet dims exactly as the physics says it should.

The same geometry produces one of astronomy's best optical illusions. Because the emitting knots are chasing their own light toward us, radio interferometry regularly clocks them moving across the sky at apparent speeds of several times c. Nothing is exceeding the speed of light; the projection is lying, and the amount it lies by tells you the jet's true speed and angle. Blazars also flare faster than anything else of their size, sometimes doubling in brightness within hours, which constrains the emitting region to a volume no larger than the Solar System.

They come in two flavours: BL Lacertae objects, whose spectra are nearly featureless, and flat-spectrum radio quasars, which show strong broad emission lines. In 2017 the blazar TXS 0506+056 flared in gamma rays at the same moment IceCube detected a high-energy neutrino from the same patch of sky — the first time a single object was identified as a source of cosmic neutrinos, and a landmark for multi-messenger astronomy.