Astronomers think that this is because of their orientation. We see an active galaxy as a blazar if one of its jets is pointing towards us, as though we're looking down the barrel of a gun. The jet sends out a narrow beam of radiation whose brightness can change rapidly as it shifts slightly or its supply of material from the black hole changes. This special alignment also means we are assailed with ferociously energetic radiation. In 1992, the orbiting Compton Gamma Ray Observatory picked up high-energy gamma rays from Markarian 421.
Astrophysicists believe they are cooked up in the jet by superfast particles.
As electrons and protons spiral around the jet's strong magnetic fields, they emit powerful radiation. They could also be colliding with ordinary photons, boosting them to ultra-high energies. But it wasn't until March 1997 that astronomers saw what a blazar can do when it really flexes its muscles. They watched astonished as Markarian 501 flared up from one of the puniest gamma-ray sources in the sky to upstage even the Crab Nebula, the debris of an exploded star in our own galactic backyard, which is the brightest steady gamma source in the sky. These emit light, and because they move faster than the speed of light in air their emissions pile up into blue flashes known as Cerenkov radiation-just as sound waves from supersonic aircraft pile up into a sonic boom.