Blazars - a New Type of Powerful Quasar
Quasars are the extremely bright cores in a small fraction of very
distant galaxies. They are
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
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 across the electromagnetic spectrum:
in radio, infrared, optical, ultraviolet, X-rays, and gamma-rays.
|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.||
IMAGE CREDIT: NASA/Honeywell Max Q Digital Group, Dana
Many of the "unidentified" EGRET sources at high Galactic latitudes
may be blazars. A multitude 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 GLAST mission, launched on June 11, 2008, will study blazars in-depth to understand the mechanism
responsible for creating black hole jets.