relativistic beaming

Relativistic beaming is an effect in which charged particles moving at a significant fraction of the speed of light will emit electromagnetic radiation in a narrow beam in the direction of motion – the faster the particles, the narrow beam. This explain why a jet from an active galactic nucleus (AGN) may appear anomalously bright if it is pointing directly at us, or quite faint if pointed away from us. Since the relativistic beaming effect increases with jet speed, the brightest AGN jets represent the fastest known outflows in the universe.