Blazar

A blazar is a very bright object in space. It comes from the center of a galaxy, called an active galactic nucleus. What makes a blazar special is that it shoots out a powerful stream of fast-moving particles, called a relativistic jet, almost straight toward us. Because of this, blazars look much brighter than they really are.

Blazars give off energy all across the electromagnetic spectrum and can produce very high-energy gamma ray light. They change in brightness very quickly, sometimes in just a few hours or days. Blazars are divided into two main types: BL Lac objects and flat-spectrum radio quasars. Scientists study blazars to learn more about the huge black holes at the centers of galaxies, the disks of material around them, and the powerful jets they create. This research helps us understand the most energetic places in the universe.

Structure

Blazars are powered by material falling into a very large black hole at the center of a galaxy. As this material falls in, it forms a hot disk that gives off huge amounts of energy. This energy creates light and particles.

From this center, powerful streams of energy shoot out in opposite directions. When one of these streams points toward Earth, the blazar looks much brighter. These streams can stretch for great distances and give off energy across many types of light, from radio waves to very high-energy gamma rays.

Relativistic beaming

Main article: Relativistic beaming

Blazars look brighter because of special effects in their fast-moving jets. These jets can move very close to the speed of light, making them seem brighter to us on Earth.

The jets give off energy through a process called synchrotron emission. How bright they look depends on things like magnetic fields in the jet and how the particles move.

When we look at these jets from far away, three special effects change how bright they seem. First, the direction of the jet can appear to change. Second, time seems to speed up, so bursts of energy look like they happen more often. Third, the angle of the jet can make it look dimmer or brighter.

For example, a jet aimed just 5 degrees away from us can look 70 times brighter, and if it’s aimed straight at us, it can look 600 times brighter! Because of these effects, jets moving away from us look dimmer, making pairs of jets look uneven. This helps explain why blazars look different from other similar objects.

Discovery

Many bright blazars were first thought to be unusual stars because they changed in brightness over days or years.

The development of radio astronomy in the 1950s helped scientists find bright radio sources in the sky. This led to the discovery of quasars, and many early quasars turned out to be blazars. In 1968, scientists linked the "variable star" BL Lacertae to a strong radio source. Though it looked like a star, BL Lacertae was actually a powerful object. By 1972, scientists grouped these special objects together and called them BL Lacertae-type objects, or "BL Lac" objects.

By 2015, over three thousand of these objects had been found. One well-known blazar, 3C 273, is 2.5 billion light years away, and the closest BL Lac object is found in the galaxy Centaurus A.

Current view

Blazars are a special kind of very bright object in space, called active galactic nuclei. They have fast-moving streams of energy pointing almost directly at us. These streams make blazars look much brighter than they really are.

Blazars can change their brightness very quickly and often shine in many different kinds of light. Some well-known blazars include 3C 454.3, 3C 273, BL Lacertae, and Markarian 421. In 2018, scientists used a special detector in Antarctica to find that a high-energy particle came from a blazar named TXS 0506+056. This blazar is 3.7 billion light-years away. This was the first time this kind of detector was used to find an object in space.