The Diffuse High-Energy Background
The sky at an energy of 0.75 keV
(Image produced from the ROSAT all-sky survey)
When we look up on a clear night, we see the beautiful twinkling of
stars and planets amid a black sky. If we could see the same sight
with X-ray or gamma ray eyes, we'd still see bright points,
but the sky would no longer be dark. Instead, it would glow faintly. This
is the diffuse high-energy background: X-ray and gamma-ray light
from all over the sky. By looking at different wavelengths of
X-rays and gamma rays, which correspond to different energies,
we can find out what causes this amazing background glow.
X-ray Diffuse Background
At low X-ray energies (about 1/4 keV),
the sky glows with radiation from hot gas filling some of the space between
the stars. This gas has a temperature of about 1 million degrees and
is heated in two ways: by
supernovae, which leave shining remnants of hot gas
behind; and by the hot winds of massive young stars, which heat
surrounding gas and form stellar wind bubbles.
At higher X-ray energies (above 1/2 keV), the source of the diffuse
background changes considerably. While emission from supernova
remnants and stellar wind bubbles is still visible, it is less dominant
than at lower energies. Much of the background radiation becomes
isotropic (that is, it looks the same in all directions). Scientists
believe the radiation comes from outside our Milky Way Galaxy, since
radiation from within the Galaxy would be brighter in some places and
dimmer in others, due to our galaxy's shape.
Above 1 keV, most of the "diffuse" background is not truly
diffuse in origin at all, but comes from many distant extragalactic objects. We know this from "deep" observations of the
diffuse X-ray background. In
a "deep" observation means that a detector looks at a given point in space
for a very long time. Using the deepest
over 60 % of the 1 - 2 keV diffuse background has been resolved into very
distant, separate sources, typically
Gamma-ray Diffuse Background
While there are many individual point sources of gamma rays, there
is also a significant amount of gamma-ray light from gas in our own
Milky Way Galaxy. Gamma-ray light from this gas stretches out
in a band across the sky, comprising much of the diffuse gamma-ray
background. The remaining gamma-ray background light we see comes
from far outside our galaxy: it's the faint glow of the rest of
the Universe, covering all the sky beyond the Milky Way.
This map of the sky in gamma-ray light is based
on data taken with the EGRET instrument (in this case, showing
light with Energy > 100 MeV). Aside from
the obvious bright point sources, the strong emission
along the center of this image is the Milky Way.
Beyond our galaxy, a much fainter, extragalactic emission can
be seen (blue areas in this image).
Within our galaxy, there are actually several different types of
diffuse gamma-ray background radiation. The image above shows one
type. Here, the yellow and red band through the center of the image
is the high-energy glow caused by cosmic rays interacting with
interstellar gas. The gas, heated by the cosmic rays, gives off
gamma rays with energies > 100 MeV. Similarly glowing gas has been
seen in the nearby galaxy known as the Large Magellanic Cloud.
Another type of diffuse gamma-ray emission in our galaxy is pictured
in the image below. This image shows the central region of our Milky Way
Galaxy, using data from the Compton Gamma Ray Observatory's
A view of the galactic bulge of the Milky Way in the light of radioactive
In this COMPTEL image, only one type of gamma-ray light is shown:
the light given off by a radioactive form of Aluminum, called
26Al. Scientists theorized that this form of aluminum
was created by novae or in massive stars, and therefore they expected
a lot of it near the galactic center (where the concentration of massive
stars would be highest). They also expected 26Al to
be distributed smoothly and symmetrically on each side of the galactic
center, since that's generally how the stars were distributed, too.
Interestingly, the 26Al radiation detected by
COMPTEL has a more complicated structure. The emission is very localized or
"clumpy" and not as bright towards the center of the Galaxy
as expected. Explaining these results will require both new theoretical
insights and more sensitive observations from future gamma-ray
Yet another source of diffuse gamma-ray radiation in our galaxy is
the violent interaction of matter and anti-matter. Certain galactic
phenomena produce positrons, the anti-matter equivalents of electrons.
Positrons have a positive electric charge and are made of anti-matter.
Electrons have a negative charge and are made of matter. When positrons
and electrons come into contact, they destroy each other violently,
an event called "pair annihilation." Pair annihilation releases a
burst of gamma-ray energy of exactly 511 keV. These gamma rays
are part of the diffuse gamma-ray background in our galaxy.
Many other, less violent processes also produce gamma rays at specific
energies. Taken together, these all contribute to the diffuse gamma-ray
background in our galaxy. For example, the image below shows
an area in the constellation Orion where gamma rays were detected in
a range of energies between 3 and 7 MeV. Just as 511 keV gamma rays
are the "signature" of pair-annihilation, the gamma rays in this picture
are also "signatures" of certain events. In this case, though, the
specific gamma-ray energies represent more mundane things, like the
presence of carbon and oxygen in the interstellar gas of the Orion region.
An image of the Orion region
taken by the COMPTEL instrument on CGRO.
The diffuse gamma-ray emission from outside our galaxy may be due
to the combined light of far-away individual objects. The objects
are most likely active galaxies (AGN), a type of galaxy that emits
unusual amounts of gamma-ray and X-ray radiation. These are very
similar to quasars, the main source of the X-ray background
outside our galaxy.
The sensitive study of the diffuse gamma-ray background
is a field which is only beginning to be realized with current
gamma-ray instruments. Future observations will provide important
insights into one of the most fundamental questions astronomy
can address - how did the matter we see all around us evolve?