CGRO Special Exhibit
Blazars: a New Type of Powerful Quasar
Quasars are the extremely bright cores in a small fraction of very distant galaxies. They are also visible at radio or X-ray energies. This bright emission, billions of times stronger than our Sun, is likely produced by a supermassive black hole pulling in copious amounts of interstellar gas. Along with gamma-ray bursts, quasars are among the most distant and powerful objects that we know of in the universe.
At the time Compton was launched, quasars were not well understood. The only quasar seen in gamma-rays before Compton was one called 3C 273, detected in 1976 by ESA's COS-B satellite. When Compton's EGRET pointed at 3C 273 in 1991, it found another quasar in the same field of view. This other one, named 3C 279, was many times brighter than 3C 273. It just so happened that 3C 279 was undergoing a flare, and it was one of the brightest sources of high-energy gamma-rays in the entire sky at the time.
Quasars visible at gamma-ray energies are all members of the class "blazar". The Third EGRET Catalog contains 66 high-confidence and 27 lower-confidence blazars. Blazars represent the largest class among identified, non-transient gamma-ray sources.
Matter falling into a massive black hole forms a jet of material shooting upward. If the black hole is oriented so the jet is pointed toward earth, we see a bright source of gamma-rays. (Credit: NASA/Honeywell Max Q Digital Group, Dana Berry)
As with quasars, each blazar likely harbors a central supermassive black hole with a pair of relativistic jets emanating in opposite directions from near its poles. The bright, highly variable emission characteristic of the blazar phenomenon can be seen when the observer views a blazar almost along one of the jets -- that is, down the barrel of the gun. Blazars are seen across the electromagnetic spectrum: in radio, infrared, optical, ultraviolet, X-rays, and gamma-rays.
Many of the "unidentified" EGRET sources at high Galactic latitudes may be blazars. A multitude of even more distant blazars may account for the diffuse, isotropic, high-energy, gamma-ray background that was first observed by NASA's SAS-2 (Small Astronomy Satellite) in the 1970s and subsequently confirmed by EGRET. The Fermi Gamma-Ray Space Telescope, launched on June 11, 2008, is studying blazars in-depth to understand the mechanism responsible for creating black hole jets.
Published: May 2000
Text Reviewed: September 2018