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Old research pages
I'm keeping these old research pages here for the benefit of anyone
who might arrive here from an external page. They are no longer
updated. Please visit my new web site at
http://sipapu.astro.illinois.edu/~ricker/.
Eventually some content from these pages will migrate to the new site.
The Cosmic X-Ray Background Radiation
The cosmic X-ray background (XRB), like the
cosmic
microwave background (CMB), is radiation which originates at high redshift
and has an extremely uniform distribution of intensity across the sky, which
is to say it is extremely isotropic.
As is the case with the CMB, small deviations from isotropy
(anisotropies) exist on a variety of angular scales, and the pattern of
these anisotropies can tell us interesting
things about the cosmological parameters
describing our universe, the pattern of large-scale structure, and the
nature of dark matter.
However, the two background radiations differ in important ways.
The CMB is known to be of truly diffuse origin, whereas the
X-ray background is now thought to come from myriad individual sources.
CMB radiation is a `relic'
from a time when the Universe was hot and dense enough for baryonic matter
(made of protons, neutrons, and electrons) and
radiation to exist in a state of thermal equilibrium with each other.
Because of this equilibrium, radiation pressure kept the baryonic matter from
collapsing under its own gravity to form bound objects like stars and
galaxies, instead forcing it to remain relatively uniform and diffuse.
The baryons existed in the form of ionized atomic nuclei (mostly hydrogen
and helium) and free electrons, which were kept separated from the ions
by the high temperature. Free electrons scatter radiation very
efficiently, so keeping the atoms ionized also kept the Universe opaque.
As the Universe expanded, it cooled, and eventually (about 500,000 years
after the Big Bang) the radiation and baryons grew too `cool' (about
3,000 degrees Kelvin) for the electrons to remain free. The electrons
and ions `recombined' to form atoms, and the Universe became transparent
to the radiation. The CMB is that radiation. We observe it today to have
a temperature of about 3 degrees above absolute zero because the Universe
has expanded by a factor of 1,000 since recombination, diluting and cooling
the radiation.
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Photon spectrum of the XRB as measured by several instruments
on HEAO 1
(Gruber et al. 1999).
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The XRB, by contrast, appears to have originated in many
individual sources which formed after recombination
(see Fabian & Barcons 1992
for a review).
Its spectrum is similar to that which would be produced by ionized gas
at a temperature of nearly half a billion degrees (40 keV), and on
large angular scales it appears quite diffuse.
However, when patches of sky without bright, nearby X-ray sources are
observed for a long time at high angular resolution, the
XRB appears to break up into a large number of faint
point sources
(Hasinger et al. 1998).
(The absence of the CMB spectral distortions which would be expected
from scattering of CMB photons from electrons at a temperature of half a
billion degrees also strongly constrains the diffuse source model
[Wright et al. 1994].)
The exact nature of these sources is unknown at present, but
they are most likely some type of
active galactic nucleus (AGN) at high redshift (2-3, or about 12 billion
light-years away)
(Comastri et al. 1995;
Gilli, Risaliti, & Salvati 1999).
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ROSAT
observation of the XRB in the direction of the Lockman Hole, a region of
low Galactic X-ray absorption in the constellation Ursa Major.
More than 60% of the extragalactic background above 1
keV can be resolved into over 100 discrete X-ray sources.
(Max Planck Institute)
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AGN are thought to be galaxies with massive
black holes at their centers, accreting interstellar matter
and releasing in the form of radiation the energy
the matter gains by falling in the black holes' gravitational fields.
AGN are the best candidates for the source of the X-ray background because
at low redshift (nearby), where there are fewer of them and they are
bright enough for us to examine their individual high-energy spectra in detail,
we see that they produce radiation which is almost as hard as the
X-ray background itself.
They are also the brightest non-transient sources of hard X-rays in the
Universe.
X-ray binary systems in
starburst galaxies are another possible XRB source, but they probably do
not produce an X-ray spectrum which is hard enough to explain the background
(Ricker & Meszaros 1993).
The new X-ray telescopes Chandra and XMM
are helping to resolve this mystery as they bring high angular resolution
and high-energy X-ray sensitivity to bear together
on the XRB for the first time.
Current work in this area seeks, among other things, to understand the
nature of the objects which produce the XRB, to determine what can be
learned about the large-scale structure of the Universe from their
spatial distribution, and to determine what role the XRB played during
the period of galaxy formation.
Publications
References
- Comastri, A., Setti, G., Zamorani, G., & Hasinger, G.
A & A 296 1 (1995)
- Fabian, A. C., & Barcons, X. ARA & A 30 429 (1992)
- Gilli, R., Risaliti, G., & Salvati, M. A & A 347
424 (1999)
- Gruber, D. E., Matteson, J. L., Peterson, L. E., &
Jung, G. V. Ap. J. 520 124 (1999)
- Hasinger, G., Burg, R., Giacconi, R., Schmidt, M., Trumper, J.,
& Zamorani, G. A & A 329 482 (1998)
- Wright, E. L., Mather, J. C., Fixsen, D. J., et al.
Ap. J. 420 450 (1994)
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