Blazar

A blazar is an active galactic nucleus whose relativistic jet points nearly toward Earth. Relativistic beaming makes it exceptionally bright and variable across the electromagnetic spectrum.

Relativistic beaming: The concentration of radiation into a narrow direction when its source moves at a substantial fraction of light speed. The jet’s motion toward Earth amplifies its observed radiation.
Supermassive black hole: A black hole with a mass millions to billions of times that of the Sun. Its accretion system powers the central engine associated with blazar jets.
Seyfert galaxy: A nearby galaxy whose active nucleus has strong emission lines and a bright, compact central source. Its nucleus is generally viewed away from the jet axis, unlike a blazar.
Very-high-energy gamma-ray astronomy: The study of gamma rays with energies above roughly 100 gigaelectronvolts. Blazars are prominent sources for testing how jets produce the highest-energy photons.
Hadronic model: A model in which energetic protons or heavier nuclei drive high-energy emission through interactions and secondary particles. It competes with electron-centered explanations for some blazar gamma rays and neutrinos.
Relativistic jet: A narrow outflow of plasma launched from the vicinity of a compact astrophysical object at relativistic speeds. Its orientation toward Earth is what makes an active nucleus a blazar.
Accretion disk: A rotating structure of gas and dust spiraling toward a massive central object. It feeds the black hole and shapes the environment around the jet’s launch region.
Radio galaxy: A galaxy with an active nucleus that emits powerful radio jets, often observed from the side. Its jet orientation exposes extended structure rather than blazar-like beaming.
Neutrino astronomy: The use of cosmic neutrinos to study energetic astrophysical sources and processes. A possible neutrino association with a blazar can constrain hadronic jet models.
Leptonic model: A model in which relativistic electrons produce high-energy radiation through inverse Compton scattering. It explains much blazar emission but does not alone settle the origin of associated neutrinos.

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