Markarian 421

Markarian 421 is a nearby blazar, an active galaxy whose relativistic jet points nearly toward Earth. Its intense, rapidly variable emission makes it a prominent source of gamma rays.

Blazar: An active galactic nucleus whose relativistic jet points close to an observer’s line of sight. Markarian 421 is classified as a blazar because its jet points nearly toward Earth.
Very-high-energy gamma-ray astronomy: The study of gamma rays with energies above roughly 100 gigaelectronvolts. Markarian 421 is a foundational target in this branch of astronomy.
Markarian 501: A nearby BL Lac object and blazar known for strong very-high-energy gamma-ray emission. This neighboring TeV blazar provides a close comparison for jet behavior and spectra.
Active galactic nucleus: A galaxy’s luminous central region powered by matter accreting onto a supermassive black hole. Markarian 421’s jet is powered by its active galactic nucleus.
Relativistic jet: A narrow outflow of plasma moving at speeds close to the speed of light from an active galactic nucleus. This galaxy’s jet dominates its observed emission through relativistic beaming.
Whipple Observatory: An astronomical observatory in Arizona known for pioneering ground-based gamma-ray astronomy. Its telescope detected Markarian 421 at very-high-energy gamma rays in 1992.
3C 279: A distant, luminous flat-spectrum radio quasar with a relativistic jet pointed toward Earth. Unlike Markarian 421, it is a quasar whose spectrum includes strong emission lines.
BL Lac object: A blazar whose optical spectrum has weak or absent emission lines. Markarian 421 belongs to this blazar subclass.
Synchrotron radiation: Radiation emitted by charged particles accelerated along curved paths, especially within magnetic fields. It produces the lower-energy component of Markarian 421’s broad emission spectrum.
Imaging atmospheric Cherenkov technique: A method that detects gamma rays through Cherenkov light from air showers they initiate. It enabled ground-based observations of Markarian 421’s highest-energy emission.

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