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The X- Ray Riddle: Cosmic background radiation is s
X-ray riddle: cosmic background is still unexplained.
Corey S. Powell
Even the most contentious people usually agree that the night sky is
dark. Don't try arguing the point with an astronomer, however. In
1962 researchers discovered that when seen through instruments
sensitive to X rays, the sky glows with a bright and oddly uniform
intensity.
This pervasive radiation, rather unpoetically known as the diffuse
X-ray background, has eluded easy explanation. Roughly 25 to 30
percent of the background has been attributed to quasars, tiny cosmic
powerhouses that supposedly lie in the center of some galaxies. The
origin of the rest has been a persistent mystery.
Two recent discoveries--one based on spanking new data, the other on a
reevaluation of older findings--may put part of the mystery to rest.
The results come from the Roentgen Satellite, or ROSAT, an X-ray
astronomy instrument that is the fruit of a collaboration between the
governments of Germany, Britain and the U.S. Since its launch some
nine months ago, ROSAT has been mapping the sky with a sensitivity and
resolution that far surpass previous X-ray instruments.
One of the goals of the mapping has been to resolve the X-ray
background into discrete components and determine their nature. On
January 15 at the annual meeting of the American Astronomical Society
in Philadelphia, Joachim Truemper and Guenther Hasinger of the Max
Planck Institute for Extraterrestrial Physics told scientists that
ROSAT had revealed far more quasars than could be detected
before--about 100 quasars per square degree. This implies the
presence of about four million quasars over the whole sky, enough to
account for 40 percent of the X-ray background, Hasinger estimates.
Even more intriguing, ROSAT has detected extended blobs of X-ray
emission that lie between the discrete X-ray sources. Hasinger and
Truemper propose that these blobs are actually clusters of quasars so
distant that ROSAT cannot separate the emissions of individual ones.
Assuming the universe is about 13 billion years old, these clusters
appear to be some eight to 10 billion light-years distant and 15 to 30
million light-years across.
This is troubling news for cosmologists. They already have their
hands full explaining how clumpy structures of galaxies could have
evolved from a presumably smooth big bang. The existence of large,
organized clusters of quasars "would pose an even bigger problem for
big bang cosmology," Hasinger notes.
Yet quasars still can explain only about 50 percent of the X-ray
background. At the Philadelphia meeting, Antonella Fruscione, Richard
Griffiths and John MacKenty of the Space Telescope Science Institute
in Baltimore presented a possible source for some of the missing X
rays. Combining old data from NASA's Einstein X-ray Telescope and the
Infrared Astronomical Satellite with infrared measurements of their
own, they suggest that 30 percent or more of the background comes from
so-called starburst galaxies in which stars are forming at an
extraordinary rate, triggered perhaps by a close encounter with a
galactic neighbor.
Some massive newborn double stars in these galaxies rapidly evolve
into X-ray binaries, systems in which one object--either a neutron
star or a black hole--slowly consumes its companion, releasing
torrents of X rays. As a result, starburst galaxies would be powerful
X-ray emitters, and large numbers of these objects spread around the
sky could produce much of the observed background.
There are still problems to overcome before the riddle of the X-ray
background is solved, warned Stephen S. Holt of the Goddard Space
Flight Center. Holt, the U.S. project scientist for ROSAT, points out
that the spectrum of the background at very short wavelengths is
fundamentally unlike that of quasars, and so quasars may have little
to do with this component. Starburst galaxies "are a better way to
fill in the X-ray background" at these wavelengths, he thinks. The
ROSAT results are as yet inadequate to distinguish whether the
observed blobs are clusters of quasars or clusters of starburst
galaxies.
Or the blobs may be something else entirely. "Hasinger sees clumping,
but the clumps are no necessarily composed of condensed objects," Holt
says. In fact, the spectrum of the X-ray background closely resembles
that of a thin, hot gas. Another satellite, COBE (the Cosmic
Background Explorer), found no evidence of such a gas, but if the gas
is concentrated into small clumps, COBE would not be able to detect
it.
Could it be that the blobs observed by ROSAT are such hot gas clumps
and not really quasar clusters after all? "It's totally ad hoc," Holt
chuckles, "but it is possible."
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