This image shows a "direct" picture of the first four years of EGRET
data. The intensity scale is logarithmic. To generate this picture,
the sky is divided up into 1/2 degree squares. The number of photons
which arrived from directions within each square is counted, and then
plotted as an image. The dark blue dots in this picture each represent one
photon, green and yellow about 10-20 photons, and
red areas correspond to about 200 photons. The "graininess" or "noise"
caused by the small number of photons in most squares makes it necessary
to perform some added processing on this picture to see what the
underlying gamma-ray brightness is.
The bright horizontal band
along the center is the gamma-ray emission from the Milky Way
galaxy, and is caused by high-energy cosmic rays interacting with
interstellar matter and light. Many of the photons away from the Galactic
plane are coming from outside the Milky Way, and form a uniform gamma-ray
glow over the sky. Most of what is seen in this picture can be accounted
for by known gamma-ray sources, and theoretical models of the Galactic
and extra-Galactic gamma-ray emission. The recent discovery shows that
there is an extra component, the "halo", above what is predicted.
An annotated version of this figure
is also available.
Contact: Kathy Barton
(909) 787-5326
e-mail: barton@ucrac1.ucr.edu
